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[[Image:GPCR1.png|thumb|A [[opioid receptor|Mu-opioid G-protein-coupled receptor]] with its agonist]]
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[[File:7TM4 (GPCR).png|thumb|The seven-transmembrane α-helix structure of a G-protein-coupled receptor]]
[[Image:7TM4.png|thumb|300px|Figure 1. The seven transmembrane α-helix structure of a G-protein-coupled receptor.]]
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'''G protein coupled receptors''' ('''GPCRs'''), also known as '''seven-transmembrane domain receptors''', '''7TM receptors''', '''heptahelical receptors''', '''serpentine receptor''', and '''G protein-linked receptors''' ('''GPLR'''), constitute a large [[protein]] family of [[membrane receptor|receptors]] that sense [[molecule]]s outside the [[Cell (biology)|cell]] and activate inside [[signal transduction]] pathways and, ultimately, cellular responses. They are called transmembrane receptors because they pass through the cell membrane, and they are called seven-transmembrane receptors because they pass through the cell membrane seven times.
   
'''G-protein-coupled receptors''' ('''GPCRs'''), also known as '''seven transmembrane receptors''', '''7TM receptors''', '''heptahelical receptors''', and '''G-protein-linked receptors''' ('''GPLR'''), are a large [[protein]] family of [[transmembrane receptor]]s that sense [[molecule]]s outside the [[Cell (biology)|cell]] and activate inside [[signal transduction]] pathways and, ultimately, cellular responses. G-protein-coupled receptors are found only in [[eukaryote]]s, including yeast, plants, choanoflagellates,<ref name="pmid12869759">{{cite journal | author = King N, Hittinger CT, Carroll SB | title = Evolution of key cell signaling and adhesion protein families predates animal origins | journal = Science | volume = 301 | issue = 5631 | pages = 361-3 | year = 2003 | pmid = 12869759 | doi = 10.1126/science.1083853 | issn = }}</ref> and animals. The [[Ligand (biochemistry)|ligands]] that bind and activate these receptors include light-sensitive compounds, [[odor]]s, [[pheromone]]s, [[hormone]]s, and [[neurotransmitter]]s, and vary in size from small molecules to [[peptide]]s to large [[protein]]s. G-protein-coupled receptors are involved in many diseases, but are also the target of around half of all modern medicinal drugs.<ref>{{cite journal | first=David | last=Filmore | pages=24-28 | title=It's a GPCR world | journal=Modern Drug Discovery | volume=2004 | year=2004 | issue=November | publisher=American Chemical Society | url=http://pubs.acs.org/subscribe/journals/mdd/v07/i11/html/1104feature_filmore.html}}</ref>
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G protein-coupled receptors are found only in [[eukaryote]]s, including [[yeast]], [[choanoflagellate]]s,<ref name="pmid12869759">{{cite journal | author = King N, Hittinger CT, Carroll SB | title = Evolution of key cell signaling and adhesion protein families predates animal origins | journal = Science | volume = 301 | issue = 5631 | pages = 361–3 | year = 2003 | pmid = 12869759 | doi = 10.1126/science.1083853 }}</ref> and animals. The [[Ligand (biochemistry)|ligands]] that bind and activate these receptors include light-sensitive compounds, [[odor]]s, [[pheromone]]s, [[hormone]]s, and [[neurotransmitter]]s, and vary in size from small molecules to [[peptide]]s to large [[protein]]s. G protein-coupled receptors are involved in many diseases, and are also the target of approximately 40% of all modern medicinal drugs.<ref>{{cite journal | author= Filmore D | pages=24–28 | title=It's a GPCR world | journal = Modern Drug Discovery | volume=2004 | year=2004 | issue=November | publisher=American Chemical Society | url = http://pubs.acs.org/subscribe/journals/mdd/v07/i11/html/1104feature_filmore.html }}</ref><ref name="pmid17139284">{{cite journal | author = Overington JP, Al-Lazikani B, Hopkins AL | title = How many drug targets are there? | journal = Nat Rev Drug Discov | volume = 5 | issue = 12 | pages = 993–6 | year = 2006 | month = December | pmid = 17139284 | doi = 10.1038/nrd2199 }}</ref> The 2012 [[Nobel prize in chemistry]] was awarded to [[Brian Kobilka]] and [[Robert Lefkowitz]] for their work that was "crucial for understanding how G-protein–coupled receptors function."<ref name="Nobel committee,2012">{{cite news|last=Royal Swedish Academy of Sciences|title=The Nobel Prize in Chemistry 2012 Robert J. Lefkowitz, Brian K. Kobilka|url=http://www.nobelprize.org/nobel_prizes/chemistry/laureates/2012/press.html|accessdate=10 October 2012|date=10 October 2012}}</ref>
   
==Classification==
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There are two principal signal transduction pathways involving the G protein-coupled receptors: the [[Cyclic adenosine monophosphate|cAMP]] signal pathway and the [[phosphatidylinositol]] signal pathway.<ref name="Gilman_1987">{{cite journal | author = Gilman AG | title = G proteins: transducers of receptor-generated signals | journal = Annu. Rev. Biochem. | volume = 56 | issue = | pages = 615–49 | year = 1987 | pmid = 3113327 | doi = 10.1146/annurev.bi.56.070187.003151 }}</ref> When a ligand binds to the GPCR it causes a conformational change in the GPCR, which allows it to act as a [[guanine nucleotide exchange factor]] (GEF). The GPCR can then activate an associated [[G-protein]] by exchanging its bound [[guanosine diphosphate|GDP]] for a [[guanosine triphosphate|GTP]]. The G-protein's α subunit, together with the bound GTP, can then dissociate from the β and γ subunits to further affect intracellular signaling proteins or target functional proteins directly depending on the α subunit type ([[Gαs|G<sub>αs</sub>]], [[Gαi|G<sub>αi/o</sub>]], [[Gαq|G<sub>αq/11</sub>]], [[G12/G13 alpha subunits|G<sub>α12/13</sub>]]).<ref name="Wettschureck_2005">{{cite journal | author = Wettschureck N, Offermanns S | title = Mammalian G proteins and their cell type specific functions | journal = Physiol. Rev. | volume = 85 | issue = 4 | pages = 1159–204 | year = 2005 | month = October | pmid = 16183910 | doi = 10.1152/physrev.00003.2005 }}</ref>{{rp|1160}}
GPCRs can be grouped into 6 classes based on sequence homology and functional similarity:<ref>{{cite journal | author=Attwood TK, Findlay JB | title=Fingerprinting G-protein-coupled receptors | journal=Protein Eng | year=1994 | volume=7 | issue=2 | pages=195-203 | pmid=8170923 | url=http://peds.oxfordjournals.org/cgi/reprint/7/2/195}}</ref><ref>{{cite journal | author=Kolakowski LF Jr | title=GCRDb: a G-protein-coupled receptor database | journal=Receptors Channels | year=1994 | volume=2 | issue=1 | pages=1-7 | pmid=8081729}}</ref><ref>{{cite journal | author=Foord SM, Bonner TI, Neubig RR, Rosser EM, Pin JP, Davenport AP, Spedding M, Harmar AJ | title=International Union of Pharmacology. XLVI. G protein-coupled receptor list | journal=Pharmacol Rev | year= 2005 | volume=57 | issue=2 | pages=279-88 | pmid= 15914470 | doi=doi:10.1124/pr.57.2.5}}</ref><ref>[http://www.ebi.ac.uk/interpro/ISearch?query=gpcr InterPro]</ref>
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* Class A (or 1) ([[Rhodopsin-like receptors|Rhodopsin]]-like)
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== Classification ==
* Class B (or 2) ([[Secretin receptor family]])
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[[Image:GPCR classification.JPG|right|thumb|300px| Classification Scheme of GPCRs. Class A (Rhodopsin-like), Class B (Secretin-like),Class C (Glutamate Receptor-like), Others (Adhesion (33), Frizzled (11), Taste type-2 (25), unclassified (23)).<ref name="pmid16753280">{{cite journal | author = Bjarnadóttir TK, Gloriam DE, Hellstrand SH, Kristiansson H, Fredriksson R, Schiöth HB | title = Comprehensive repertoire and phylogenetic analysis of the G protein-coupled receptors in human and mouse | journal = Genomics | volume = 88 | issue = 3 | pages = 263–73 | year = 2006 | month = September | pmid = 16753280 | doi = 10.1016/j.ygeno.2006.04.001 }}</ref>]]
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The exact size of the GPCR [[Superfamily (molecular biology)|superfamily]] is unknown but nearly 800 different [[human]] [[genes]] (or ≈4% of the entire [[Protein biosynthesis|protein-coding]] [[genome]]) have been predicted from genome [[sequence analysis]]. Although numerous classification schemes have been proposed, the superfamily is classically divided into three main classes (A, B, and C) with no detectable shared [[sequence homology]] between classes. The largest class by far is class A, which accounts for nearly 85% of the GPCR genes. Of class A GPCRs, over half of these are predicted to encode [[olfactory receptors]] while the remaining receptors are [[ligand (biochemistry)|liganded]] by known [[endogenous]] [[Chemical compound|compounds]] or are classified as [[orphan receptor]]s. Despite the lack of sequence homology between classes, all GPCRs share a common [[Protein tertiary structure|structure]] and mechanism of [[signal transduction]].
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In all, GPCRs can be grouped into 6 classes based on sequence homology and functional similarity:<ref>{{cite journal | author=Attwood TK, Findlay JB | title=Fingerprinting G-protein-coupled receptors | journal=Protein Eng | year=1994 | volume=7 | issue=2 | pages=195–203 | pmid=8170923 | url=http://peds.oxfordjournals.org/cgi/reprint/7/2/195 | doi=10.1093/protein/7.2.195}}</ref><ref>{{cite journal | author=Kolakowski LF Jr | title=GCRDb: a G-protein-coupled receptor database | journal=Receptors Channels | year=1994 | volume=2 | issue=1 | pages=1–7 | pmid=8081729}}</ref><ref>{{cite journal | author=Foord SM, Bonner TI, Neubig RR, Rosser EM, Pin JP, Davenport AP, Spedding M, Harmar AJ | title=International Union of Pharmacology. XLVI. G protein-coupled receptor list | journal=Pharmacol Rev | year= 2005 | volume=57 | issue=2 | pages=279–88 | pmid= 15914470 | doi=10.1124/pr.57.2.5}}</ref><ref>[http://www.ebi.ac.uk/interpro/ISearch?query=gpcr InterPro]</ref>
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* Class A (or 1) ([[Rhodopsin-like receptors|Rhodopsin-like]])
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* Class B (or 2) ([[Secretin receptor family]])
 
* [[Class C GPCR|Class C]] (or 3) ([[Metabotropic glutamate receptor|Metabotropic glutamate]]/pheromone)
 
* [[Class C GPCR|Class C]] (or 3) ([[Metabotropic glutamate receptor|Metabotropic glutamate]]/pheromone)
 
* Class D (or 4) ([[Fungal mating pheromone receptors]])
 
* Class D (or 4) ([[Fungal mating pheromone receptors]])
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* Class F (or 6) ([[Frizzled]]/[[Smoothened]])
 
* Class F (or 6) ([[Frizzled]]/[[Smoothened]])
   
The very large rhodopsin A group has been further subdivided into 19 subgroups (A1-A19).<ref>{{cite journal | author=Joost P, Methner A | title=Phylogenetic analysis of 277 human G-protein-coupled receptors as a tool for the prediction of orphan receptor ligands | journal=Genome Biol | year= 2002 | volume=3 | issue=11 | pages=research0063.1-0063.16 | pmid= 12429062 | doi=10.1186/gb-2002-3-11-research0063}}</ref> More recently, an alternative classification system called GRAFS ([[Metabotropic glutamate receptor|Glutamate]], [[Rhodopsin]], Adhesion, [[Frizzled]]/[[Taste receptor|Taste2]], [[Secretin receptor| Secretin]]) has been proposed.<ref>{{cite journal | author=Bjarnadottir TK, Gloriam DE, Hellstrand SH, Kristiansson H, Fredriksson R, Schioth HB | title=Comprehensive repertoire and phylogenetic analysis of the G protein-coupled receptors in human and mouse | journal= Genomics | year= 2006 | volume=88 | issue=3 | pages=263-73 | pmid=16753280 | doi=10.1016/j.ygeno.2006.04.001}}</ref>
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The very large rhodopsin A group has been further subdivided into 19 subgroups ([[Rhodopsin-like_receptors#Classes|A1-A19]]).<ref>{{cite journal | author=Joost P, Methner A | title=Phylogenetic analysis of 277 human G-protein-coupled receptors as a tool for the prediction of orphan receptor ligands | journal=Genome Biol | year= 2002 | volume=3 | issue=11 | pages=research0063.1–0063.16 | pmid= 12429062 | doi=10.1186/gb-2002-3-11-research0063 | pmc=133447}}</ref> More recently, an alternative classification system called GRAFS ([[Metabotropic glutamate receptor|Glutamate]], [[Rhodopsin]], Adhesion, [[Frizzled]]/[[Taste receptor|Taste2]], [[Secretin receptor|Secretin]]) has been proposed.<ref>{{cite journal | author=Bjarnadottir TK, Gloriam DE, Hellstrand SH, Kristiansson H, Fredriksson R, Schioth HB | title=Comprehensive repertoire and phylogenetic analysis of the G protein-coupled receptors in human and mouse | journal= Genomics | year= 2006 | volume=88 | issue=3 | pages=263–73 | pmid=16753280 | doi=10.1016/j.ygeno.2006.04.001}}</ref>
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The human genome encodes thousands of G protein-coupled receptors,<ref>{{cite journal | title=The G protein-coupled receptor repertoires of human and mouse| author=Vassilatis DK, Hohmann JG, Zeng H, Li F, Ranchalis JE et al| journal=Proc Natl Acad Sci USA| year=2003| volume=100| pages=4903–4908| url=http://www.pnas.org/content/100/8/4903.full | doi=10.1073/pnas.0230374100 | pmid=12679517 | issue=8 | pmc=153653}}</ref> about 350 of which detect hormones, growth factors, and other endogenous ligands. Approximately 150 of the GPCRs found in the human genome have unknown functions.
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Some web-servers<ref>{{cite journal | author = Xiao X, Wang P, Chou KC | year = 2009 | title = A cellular automaton image approach for predicting G-protein-coupled receptor functional classes | url =http://icpr.jci.jx.cn/bioinfo/GPCR-CA | journal = Journal of Computational Chemistry | volume = 30 | issue = 9| pages = 1414–1423 |pmid=19037861 | doi = 10.1002/jcc.21163 }}</ref> and bioinformatics prediction methods<ref name="pmid19364489">{{cite journal | author = Qiu JD, Huang JH, Liang RP, Lu XQ | title = Prediction of G-protein-coupled receptor classes based on the concept of Chou's [[pseudo amino acid composition]]: an approach from discrete wavelet transform | journal = Anal. Biochem. | volume = 390 | issue = 1 | pages = 68–73 | year = 2009 | month = July | pmid = 19364489 | doi = 10.1016/j.ab.2009.04.009 | url = }}</ref><ref name="pmid19594431">{{cite journal | author = Gu Q, Ding YS, Zhang TL| title = Prediction of G-Protein-Coupled Receptor Classes in Low Homology Using Chou's [[pseudo amino acid composition]] with Approximate Entropy and Hydrophobicity Patterns | journal = Protein Pept. Lett. | volume = 17 | issue = 5 | pages = 559–67 | year = 2010 | month = May | pmid = 19594431 | doi = 10.2174/092986610791112693| url = }}</ref> have been used for predicting the classification of GPCRs according to their amino acid sequence alone, by means of the [[pseudo amino acid composition]] approach.
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== Physiological roles ==
   
==Physiological roles==
 
 
GPCRs are involved in a wide variety of physiological processes. Some examples of their physiological roles include:
 
GPCRs are involved in a wide variety of physiological processes. Some examples of their physiological roles include:
   
# the visual sense: the [[opsin]]s use a photoisomerization reaction to translate electromagnetic radiation into cellular signals. [[Rhodopsin]], for example, uses the conversion of ''11-cis''-retinal to ''all-trans''-retinal for this purpose
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# The visual sense: the [[opsin]]s use a photoisomerization reaction to translate electromagnetic radiation into cellular signals. [[Rhodopsin]], for example, uses the conversion of ''11-cis''-retinal to ''all-trans''-retinal for this purpose
# the sense of smell: receptors of the [[olfactory epithelium]] bind odorants (olfactory receptors) and pheromones (vomeronasal receptors)
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# The sense of smell: receptors of the [[olfactory epithelium]] bind odorants (olfactory receptors) and pheromones (vomeronasal receptors)
# behavioral and mood regulation: receptors in the [[mammal]]ian [[brain]] bind several different [[neurotransmitter]]s, including [[serotonin]], [[dopamine]], [[GABA]], and [[glutamate]]
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# Behavioral and mood regulation: receptors in the [[mammal]]ian [[brain]] bind several different [[neurotransmitter]]s, including [[serotonin]], [[dopamine]], [[Gamma-aminobutyric acid|GABA]], and [[glutamate]]
# regulation of [[immune system]] activity and [[inflammation]]: [[chemokine]] receptors bind ligands that mediate intercellular communication between cells of the immune system; receptors such as [[histamine]] receptors bind [[inflammatory mediators]] and engage target cell types in the [[Inflammation|inflammatory response]]
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# Regulation of [[immune system]] activity and [[inflammation]]: [[chemokine]] receptors bind ligands that mediate intercellular communication between cells of the immune system; receptors such as [[histamine receptors]] bind [[inflammatory mediators]] and engage target cell types in the [[Inflammation|inflammatory response]]
# autonomic nervous system transmission: both the [[sympathetic]] and [[parasympathetic]] nervous systems are regulated by GPCR pathways, responsible for control of many automatic functions of the body such as blood pressure, heart rate, and digestive processes
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# Autonomic nervous system transmission: both the [[Sympathetic nervous system|sympathetic]] and [[Parasympathetic nervous system|parasympathetic]] nervous systems are regulated by GPCR pathways, responsible for control of many automatic functions of the body such as blood pressure, heart rate, and digestive processes
# cell density sensing: A novel GPCR role in regulating cell density sensing.
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# Cell density sensing: A novel GPCR role in regulating cell density sensing.
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# Homeostasis modulation (e.g., water balance).<ref name="pmid21802439">{{cite journal | author = Hazell GG, Hindmarch CC, Pope GR, Roper JA, Lightman SL, Murphy D, O'Carroll AM, Lolait SJ | title = G protein-coupled receptors in the hypothalamic paraventricular and supraoptic nuclei - serpentine gateways to neuroendocrine homeostasis | journal = Front Neuroendocrinol | volume = 33| issue = 1| pages = 45–66| year = 2011 | month = July | pmid = 21802439 | doi = 10.1016/j.yfrne.2011.07.002 | pmc=3336209}}</ref>
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# Involved in [[tumor]] growth and [[metastasis]].<ref>{{cite journal | author = Dorsam RT, Gutkind JS.| title = G-protein-coupled receptors and cancer | journal = Nat Rev Cancer| volume = 7| issue = 2| pages = 79–94| year = 2007 | month = Feb | pmid = 17251915| doi = 10.1038/nrc2069| pmc=}}</ref>
   
[[Image:GPCR mechanism.png|right|thumb|400px|Figure 2. G-protein-coupled receptor mechanism.]]
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== Receptor structure ==
   
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GPCRs are [[integral membrane protein]]s that possess seven membrane-spanning domains or [[transmembrane helix|transmembrane helices]]. The extracellular parts of the receptor can be [[Glycosylation|glycosylated]]. These extracellular loops also contain two highly-conserved [[cysteine]] residues that form [[disulfide bond]]s to stabilize the receptor structure. Some seven-transmembrane helix proteins ([[channelrhodopsin]]) that resemble GPCRs may contain ion channels, within their protein.
   
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Similar to GPCRs, the adiponectin receptors 1 and 2 ([[ADIPOR1]] and [[ADIPOR2]]) also possess 7 [[transmembrane domain]]s. However ADIPOR1 and ADIPOR2 are orientated oppositely to GPCRs in the membrane (i.e., cytoplasmic [[N-terminus]], extracellular [[C-terminus]]) and do not associate with [[G protein]]s.<ref name="pmid12802337">{{cite journal | author = Yamauchi T, Kamon J, Ito Y, Tsuchida A, Yokomizo T, Kita S, Sugiyama T, Miyagishi M, Hara K, Tsunoda M, Murakami K, Ohteki T, Uchida S, Takekawa S, Waki H, Tsuno NH, Shibata Y, Terauchi Y, Froguel P, Tobe K, Koyasu S, Taira K, Kitamura T, Shimizu T, Nagai R, Kadowaki T | title = Cloning of adiponectin receptors that mediate antidiabetic metabolic effects | journal = Nature | volume = 423 | issue = 6941 | pages = 762–9 | year = 2003 | month = June | pmid = 12802337 | doi = 10.1038/nature01705 }}</ref>
   
==Receptor structure==
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Early structural models for GPCRs were based on their weak analogy to [[bacteriorhodopsin]], for which a structure had been determined by both electron diffraction ({{PDB|2BRD}}, {{PDB2|1AT9}})<ref name="pmid8676377">{{cite journal | author = Grigorieff N, Ceska TA, Downing KH, Baldwin JM, Henderson R | title = Electron-crystallographic refinement of the structure of bacteriorhodopsin | journal = J. Mol. Biol. | volume = 259 | issue = 3 | pages = 393–421 | year = 1996 | pmid = 8676377 | doi = 10.1006/jmbi.1996.0328 }}</ref><ref name="pmid9296502">{{cite journal | author = Kimura Y, Vassylyev DG, Miyazawa A, Kidera A, Matsushima M, Mitsuoka K, Murata K, Hirai T, Fujiyoshi Y | title = Surface of bacteriorhodopsin revealed by high-resolution electron crystallography | journal = Nature | volume = 389 | issue = 6647 | pages = 206–11 | year = 1997 | pmid = 9296502 | doi = 10.1038/38323 }}</ref> and [[X-ray crystallography|X ray-based crystallography]] ({{PDB2|1AP9}}).<ref name="pmid9287223">{{cite journal | author = Pebay-Peyroula E, Rummel G, Rosenbusch JP, Landau EM | title = X-ray structure of bacteriorhodopsin at 2.5 angstroms from microcrystals grown in lipidic cubic phases | journal = Science | volume = 277 | issue = 5332 | pages = 1676–81 | year = 1997 | pmid = 9287223 | doi = 10.1126/science.277.5332.1676 }}</ref> In 2000, the first crystal structure of a mammalian GPCR, that of bovine [[rhodopsin]] ({{PDB2|1F88}}), was solved.<ref name="pmid10926528">{{cite journal | author = Palczewski K, Kumasaka T, Hori T, Behnke CA, Motoshima H, Fox BA, Trong IL, Teller DC, Okada T, Stenkamp RE, Yamamoto M, Miyano M | title = Crystal structure of rhodopsin: A G protein-coupled receptor | journal = Science | volume = 289 | issue = 5480 | pages = 739–45 | year = 2000 | pmid = 10926528 | doi = 10.1126/science.289.5480.739 }}</ref> While the main feature, the seven transmembrane helices, is conserved, the relative orientation of the helices differ significantly from that of bacteriorhodopsin. In 2007, the first structure of a human GPCR was solved ({{PDB2|2R4R}}, {{PDB2|2R4S}}).<ref name="Rasmussen_2007">{{cite journal | author = Rasmussen SG, Choi HJ, Rosenbaum DM, Kobilka TS, Thian FS, Edwards PC, Burghammer M, Ratnala VR, Sanishvili R, Fischetti RF, Schertler GF, Weis WI, Kobilka BK | title = Crystal structure of the human β<sub>2</sub>-adrenergic G-protein-coupled receptor | journal = Nature | volume = 450 | issue = 7168 | pages = 383–7 | year = 2007 | pmid = 17952055 | doi = 10.1038/nature06325 }}</ref> This was followed immediately by a higher resolution structure of the same receptor ({{PDB2|2RH1}}).<ref name="Cherezov_2007">{{cite journal | author = Cherezov V, Rosenbaum DM, Hanson MA, Rasmussen SG, Thian FS, Kobilka TS, Choi HJ, Kuhn P, Weis WI, Kobilka BK, Stevens RC | title = High-resolution crystal structure of an engineered human β<sub>2</sub>-adrenergic G protein-coupled receptor | journal = Science | volume = 318 | issue = 5854 | pages = 1258–65 | year = 2007 | pmid = 17962520 | doi = 10.1126/science.1150577| pmc = 2583103 }}</ref><ref name="Rosenbaum_2007">{{cite journal | author = Rosenbaum DM, Cherezov V, Hanson MA, Rasmussen SG, Thian FS, Kobilka TS, Choi HJ, Yao XJ, Weis WI, Stevens RC, Kobilka BK | title = GPCR engineering yields high-resolution structural insights into β<sub>2</sub>-adrenergic receptor function | journal = Science | volume = 318 | issue = 5854 | pages = 1266–73 | year = 2007 | pmid = 17962519 | doi = 10.1126/science.1150609 }}</ref> This human [[beta-2 adrenergic receptor|β<sub>2</sub>-adrenergic receptor]] GPCR structure, proved highly similar to the bovine rhodopsin in terms of the relative orientation of the seven-transmembrane helices. However the conformation of the second extracellular loop is entirely different between the two structures. Since this loop constitutes the "lid" that covers the top of the ligand binding site, this conformational difference highlights the difficulties in constructing [[homology modeling|homology model]]s of other GPCRs based only on the rhodopsin structure.
GPCRs are [[integral membrane protein]]s that possess seven membrane-spanning domains or [[transmembrane helix|transmembrane helices]] (Figure 1). The extracellular parts of the receptor can be [[Glycosylation|glycosylated]]. These extracellular loops also contain two highly-conserved [[cysteine]] residues that form [[disulfide bond]]s to stabilize the receptor structure. Some seven transmembrane helix proteins (such as [[channelrhodopsin]]) that resemble GPCRs may contain different functional groups, such as entire ion channels, within their protein.
 
   
Early structural models for GPCRs were based on their weak analogy to [[bacteriorhodopsin]] for which a structure had been determined by both electron diffraction ({{PDB|2BRD}}, {{PDB2|1AT9}})<ref name="pmid8676377">{{cite journal | author = Grigorieff N, Ceska TA, Downing KH, Baldwin JM, Henderson R | title = Electron-crystallographic refinement of the structure of bacteriorhodopsin | journal = J. Mol. Biol. | volume = 259 | issue = 3 | pages = 393–421 | year = 1996 | pmid = 8676377 | doi = 10.1006/jmbi.1996.0328 | issn = }}</ref><ref name="pmid9296502">{{cite journal | author = Kimura Y, Vassylyev DG, Miyazawa A, Kidera A, Matsushima M, Mitsuoka K, Murata K, Hirai T, Fujiyoshi Y | title = Surface of bacteriorhodopsin revealed by high-resolution electron crystallography | journal = Nature | volume = 389 | issue = 6647 | pages = 206–11 | year = 1997 | pmid = 9296502 | doi = 10.1038/38323 | issn = }}</ref> and [[X-ray crystallography|X ray-based crystallography]] ({{PDB2|1AP9}}).<ref name="pmid9287223">{{cite journal | author = Pebay-Peyroula E, Rummel G, Rosenbusch JP, Landau EM | title = X-ray structure of bacteriorhodopsin at 2.5 angstroms from microcrystals grown in lipidic cubic phases | journal = Science | volume = 277 | issue = 5332 | pages = 1676–81 | year = 1997 | pmid = 9287223 | doi = 10.1126/science.277.5332.1676 | issn = }}</ref> In 2000, the first crystal structure of a mammalian GPCR, that of bovine [[rhodopsin]] ({{PDB2|1F88}}), was solved.<ref name="pmid10926528">{{cite journal | author = Palczewski K, Kumasaka T, Hori T, Behnke CA, Motoshima H, Fox BA, Trong IL, Teller DC, Okada T, Stenkamp RE, Yamamoto M, Miyano M | title = Crystal structure of rhodopsin: A G protein-coupled receptor. | journal = Science | volume = 289 | issue = 5480 | pages = 739-45 | year = 2000 | pmid = 10926528 | doi = 10.1126/science.289.5480.739 | issn = }}</ref> While the main feature, the seven transmembrane helices, is conserved, the relative orientation of the helices differ significantly from that of bacteriorhodopsin. In 2007, the first structure of a human GPCR was solved ({{PDB2|2R4R}}, {{PDB2|2R4S}}).<ref name="Rasmussen_2007">{{cite journal | author = Rasmussen SG, Choi HJ, Rosenbaum DM, Kobilka TS, Thian FS, Edwards PC, Burghammer M, Ratnala VR, Sanishvili R, Fischetti RF, Schertler GF, Weis WI, Kobilka BK | title = Crystal structure of the human β<sub>2</sub>-adrenergic G-protein-coupled receptor | journal = Nature | volume = 450 | issue = 7168 | pages = 383–7 | year = 2007 | pmid = 17952055 | doi = 10.1038/nature06325 | issn = }}</ref> This was followed immediately by a higher resolution structure of the same receptor ({{PDB2|2RH1}}).<ref name="Cherezov_2007">{{cite journal | author = Cherezov V, Rosenbaum DM, Hanson MA, Rasmussen SG, Thian FS, Kobilka TS, Choi HJ, Kuhn P, Weis WI, Kobilka BK, Stevens RC | title = High-resolution crystal structure of an engineered human β<sub>2</sub>-adrenergic G protein-coupled receptor | journal = Science | volume = 318 | issue = 5854 | pages = 1258–65 | year = 2007 | pmid = 17962520 | doi = 10.1126/science.1150577 | issn = }}</ref><ref name="Rosenbaum_2007">{{cite journal | author = Rosenbaum DM, Cherezov V, Hanson MA, Rasmussen SG, Thian FS, Kobilka TS, Choi HJ, Yao XJ, Weis WI, Stevens RC, Kobilka BK | title = GPCR engineering yields high-resolution structural insights into β<sub>2</sub>-adrenergic receptor function | journal = Science | volume = 318 | issue = 5854 | pages = 1266–73 | year = 2007 | pmid = 17962519 | doi = 10.1126/science.1150609 | issn = }}</ref> This human [[beta-2 adrenergic receptor|β<sub>2</sub>-adrenergic receptor]] GPCR structure, proved to be highly similar to the bovine rhodopsin in terms of the relative orientation of the seven transmembrane helices. However the conformation of the second extracellular loop is entirely different between the two structures. Since this loop constitutes the "lid" that covers the top of the ligand binding site, this conformational difference highlights the difficulties in constructing homology models of other GPCRs based only on the rhodopsin structure.
+
The structures of activated and/or agonist-bound GPCRs have also been determined.<ref name="pmid21228869">{{cite journal | author = Rasmussen SG, Choi HJ, Fung JJ, Pardon E, Casarosa P, Chae PS, Devree BT, Rosenbaum DM, Thian FS, Kobilka TS, Schnapp A, Konetzki I, Sunahara RK, Gellman SH, Pautsch A, Steyaert J, Weis WI, Kobilka BK | title = Structure of a nanobody-stabilized active state of the β(2) adrenoceptor | journal = Nature | volume = 469 | issue = 7329 | pages = 175–80 | year = 2011 | month = January | pmid = 21228869 | pmc = 3058308 | doi = 10.1038/nature09648 }}</ref><ref name="pmid21228876">{{cite journal | author = Rosenbaum DM, Zhang C, Lyons JA, Holl R, Aragao D, Arlow DH, Rasmussen SG, Choi HJ, Devree BT, Sunahara RK, Chae PS, Gellman SH, Dror RO, Shaw DE, Weis WI, Caffrey M, Gmeiner P, Kobilka BK | title = Structure and function of an irreversible agonist-β(2) adrenoceptor complex | journal = Nature | volume = 469 | issue = 7329 | pages = 236–40 | year = 2011 | month = January | pmid = 21228876 | pmc = 3074335 | doi = 10.1038/nature09665 }}</ref><ref name="pmid21228877">{{cite journal | author = Warne T, Moukhametzianov R, Baker JG, Nehmé R, Edwards PC, Leslie AG, Schertler GF, Tate CG | title = The structural basis for agonist and partial agonist action on a β(1)-adrenergic receptor | journal = Nature | volume = 469 | issue = 7329 | pages = 241–4 | year = 2011 | month = January | pmid = 21228877 | pmc = 3023143 | doi = 10.1038/nature09746 }}</ref><ref name="pmid21393508">{{cite journal | author = Xu F, Wu H, Katritch V, Han GW, Jacobson KA, Gao ZG, Cherezov V, Stevens RC | title = Structure of an agonist-bound human A2A adenosine receptor | journal = Science | volume = 332 | issue = 6027 | pages = 322–7 | year = 2011 | month = April | pmid = 21393508 | doi = 10.1126/science.1202793 | pmc = 3086811 }}</ref> These structures indicate how ligand binding at the extracellular side of a receptor leads to conformational changes in the cytoplasmic side of the receptor. The biggest change is an outward movement of the cytoplasmic part of the 5th and 6th transmembrane helix (TM5 and TM6). The structure of activated beta-2 adrenergic receptor in complex with G<sub>s</sub> confirmed that the binds to a cavity created by this movement.<ref name="pmid21772288">{{cite journal | author = Rasmussen SG, Devree BT, Zou Y, Kruse AC, Chung KY, Kobilka TS, Thian FS, Chae PS, Pardon E, Calinski D, Mathiesen JM, Shah ST, Lyons JA, Caffrey M, Gellman SH, Steyaert J, Skiniotis G, Weis WI, Sunahara RK, Kobilka BK | title = Crystal structure of the β(2) adrenergic receptor-Gs protein complex | journal = Nature | volume = 477| issue = 7366| pages = 549–55| year = 2011 | month = July | pmid = 21772288 | doi = 10.1038/nature10361 | pmc = 3184188 }}</ref>
   
==Mechanism==
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== Structure-function relationships ==
G protein-coupled receptor are activated by an external signal in the form of a ligand or other signal mediator. This creates a conformational change in the receptor, causing activation of a [[G protein]]. Further effect depends on the type of G protein.
 
   
===Ligand binding===
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[[Image:GPCR in membrane.jpg|right|thumb|500px|Two-dimensional schematic of a generic GPCR set in a Lipid Raft. Click the image for higher resolution to see details regarding the locations of important structures.]]
GPCRs include receptors for sensory signal mediators (e.g., [[light]] and [[olfactory]] stimulatory molecules); [[adenosine]], [[bombesin]], [[bradykinin]], [[endothelin]], γ-aminobutyric acid ([[GABA]]), [[melanocortin]]s, [[neuropeptide Y]], [[opioid]] peptides, [[opsin]]s, [[somatostatin]], [[tachykinins]], [[vasoactive intestinal polypeptide]] family, and [[vasopressin]]; [[biogenic amines]] (e.g., [[dopamine]], [[epinephrine]], [[norepinephrine]], [[histamine]], [[glutamate]] ([[metabotropic]] effect), [[glucagon]], [[acetylcholine]] ([[muscarinic]] effect), and [[serotonin]]); [[chemokines]]; [[lipid]] mediators of [[inflammation]] (e.g., [[prostaglandins]], [[prostanoid]]s, [[platelet-activating factor]], and [[leukotrienes]]); and [[peptide hormones]] (e.g., [[calcitonin]], C5a [[anaphylatoxin]], follicle-stimulating hormone ([[FSH]]), gonadotropic-releasing hormone ([[GnRH]]), [[neurokinin]], thyrotropin-releasing hormone ([[TRH]]), and [[oxytocin]]). GPCRs that act as receptors for stimuli that have yet to be identified are known as [[orphan receptor]]s.
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Structurally, GPCRs are characterized by an extracellular [[N-terminus]], followed by seven [[Transmembrane domain|transmembrane]] (7-TM) [[alpha helix|α-helices]] (TM-1 to TM-7) connected by three intracellular (IL-1 to IL-3) and three extracellular loops (EL-1 to EL-3), and finally an intracellular [[C-terminus]]. The GPCR arranges itself into a [[Protein tertiary structure|tertiary structure]] resembling a barrel, with the seven transmembrane helices forming a cavity within the plasma membrane that serves a [[ligand (biochemistry)|ligand]]-binding domain that is often covered by EL-2. Ligands may also bind elsewhere, however, as is the case for bulkier ligands (e.g., [[proteins]] or large [[peptides]]), which instead interact with the extracellular loops, or, as illustrated by the class C [[metabotropic glutamate receptors]] (mGluRs), the N-terminal tail. The class C GPCRs are distinguished by their large N-terminal tail, which also contains a ligand-binding domain. Upon glutamate-binding to an mGluR, the N-terminal tail undergoes a conformational change that leads to its interaction with the residues of the extracellular loops and TM domains. The eventual effect of all three types of [[agonist]]-induced activation is a change in the relative orientations of the TM helices (likened to a twisting motion) leading to a wider intracellular surface and "revelation" of residues of the intracellular helices and TM domains crucial to signal transduction function (i.e., G-protein coupling). [[Inverse agonists]] and [[Receptor antagonist|antagonist]]s may also bind to a number of different sites, but the eventual effect must be prevention of this TM helix reorientation.
   
Whereas, in other types of receptors that have been studied, ligands bind externally to the membrane, the [[Ligand (biochemistry)|ligand]]s of GPCRs typically bind within the transmembrane domain.
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The structure of the N- and C-terminal tails of GPCRs may also serve important functions beyond ligand-binding. In particular, the C-terminus often contains [[serine]] (Ser) or [[threonine]] (Thr) residues that, when [[phosphorylation|phosphorylated]], increase the [[affinity (pharmacology)|affinity]] of the intracellular surface for the binding of scaffolding proteins called β-[[arrestin]]s (β-arr).<ref name="pmid2163110">{{cite journal | author = Lohse MJ, Benovic JL, Codina J, Caron MG, Lefkowitz RJ | title = β-Arrestin: a protein that regulates β-adrenergic receptor function | journal = Science | volume = 248 | issue = 4962 | pages = 1547–1550 | year = 1990 | month = June | pmid = 2163110 | url = | doi = 10.1126/science.2163110 }}</ref> Once bound, β-arrestins both [[sterically]] prevent G-protein coupling and may recruit other proteins leading to the creation of signaling complexes involved in extracellular-signal regulated kinase ([[extracellular signal-regulated kinases|ERK]]) pathway activation or receptor [[endocytosis]] (internalization). As the phosphorylation of these Ser and Thr residues often occurs as a result of GPCR activation, the β-arr-mediated G-protein-decoupling and internalization of GPCRs are important mechanisms of [[Desensitization (medicine)|desensitization]].<ref name="pmid11861753">{{cite journal | author = Luttrell LM, Lefkowitz RJ | title = The role of beta-arrestins in the termination and transduction of G-protein-coupled receptor signals | journal = J. Cell. Sci. | volume = 115 | issue = Pt 3 | pages = 455–65 | year = 2002 | month = February | pmid = 11861753 | doi = | url = }}</ref>
   
===Conformational change===
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A final common structural theme among GPCRs is [[palmitoylation]] of one or more sites of the C-terminal tail or the intracellular loops. Palmitoylation is the covalent modification of [[cysteine]] (Cys) residues via addition of hydrophobic [[acyl|acyl groups]], and has the effect of targeting the receptor to [[cholesterol]]- and [[sphingolipid]]-rich microdomains of the plasma membrane called [[lipid raft]]s. As many of the downstream transducer and effector molecules of GPCRs (including those involved in [[negative feedback]] pathways) are also targeted to lipid rafts, this has the effect of facilitating rapid receptor signaling.
The [[signal transduction|transduction of the signal]] through the membrane by the receptor is not completely understood. It is known that the inactive [[G protein]] is bound to the receptor in its inactive state. Once the ligand is recognized, the receptor shifts [[Chemical conformation|conformation]] and thus mechanically activates the G protein, which detaches from the receptor. The receptor can now either activate another G protein or switch back to its inactive state. This is an overly simplistic explanation, but suffices to convey the overall set of events.
 
   
It is believed that a receptor molecule exists in a conformational equilibrium between active and inactive biophysical states.<ref>{{cite journal | author=Rubenstein, Lester A. and Lanzara, Richard G. | title=Activation of G protein-coupled receptors entails cysteine modulation of agonist binding | journal=Journal of Molecular Structure (Theochem) | year=1998 | volume=430 | pages=57–71 | url=http://cogprints.org/4095/}}</ref> The binding of ligands to the receptor may shift the equilibrium toward the active receptor states.<ref>[http://www.bio-balance.com/Graphics.htm http://www.bio-balance.com/<!-- Bot generated title -->]</ref> Three types of ligands exist: agonists are ligands that shift the equilibrium in favour of active states; [[inverse agonist]]s are ligands that shift the equilibrium in favour of inactive states; and neutral antagonists are ligands that do not affect the equilibrium. It is not yet known how exactly the active and inactive states differ from each other.
+
GPCRs respond to extracellular signals mediated by a huge diversity of agonists, ranging from proteins to [[biogenic amines]] to [[protons]], but all transduce this signal via a mechanism of G-protein coupling. This is made possible by virtue of a [[guanine]]-nucleotide exchange factor ([[guanine nucleotide exchange factor|GEF]]) domain primarily formed by a combination of IL-2 and IL-3 along with adjacent residues of the associated TM helices.
   
===Activation of G protein===
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== Mechanism ==
{{Seealso|G protein}}
 
If a receptor in an active state encounters a [[G protein]], it may activate it (Figure 2, blue protein in part B). Some evidence suggests that receptors and G proteins are actually pre-coupled. For example, binding of G proteins to receptors affects the receptor's affinity for ligands. Activated G proteins are bound to [[Guanosine triphosphate|GTP]].
 
   
Further signal transduction depends on the type of G protein. The enzyme [[adenylate cyclase]] (Figure 2, green protein in panel C) is an example of a cellular protein that can be regulated by a G protein, in this case the G protein [[Gs alpha subunit|G<sub>s</sub>]]. Adenylate cyclase activity is activated when it binds to a subunit of the activated G protein (Figure 2, Panel D). Activation of adenylate cyclase ends when the G protein returns to the [[Guanosine diphosphate|GDP]]-bound state (Figure 2, panels E and A).
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[[Image:GPCR activation.jpg|right|thumb|400px|Cartoon depicting the basic concept of GPCR Conformational Activation. Ligand binding disrupts an ionic lock between the E/DRY motif of TM-3 and acidic residues of TM-6. As a result the GPCR reorganizes to allow activation of G-alpha proteins. The side perspective is a view from above and to the side of the GPCR as it is set in the plasma membrane (the membrane lipids have been omitted for clarity). The intracellular perspective shows the view looking up at the plasma membrane from inside the cell.<ref name="pmid20019124">{{cite journal | author = Millar RP, Newton CL | title = The year in G protein-coupled receptor research | journal = Mol. Endocrinol. | volume = 24 | issue = 1 | pages = 261–74 | year = 2010 | month = January | pmid = 20019124 | doi = 10.1210/me.2009-0473 }}</ref>]]
   
===GPCR signaling without G proteins===
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The G protein-coupled receptor is activated by an external signal in the form of a ligand or other signal mediator. This creates a conformational change in the receptor, causing activation of a [[G protein]]. Further effect depends on the type of G protein.
In the late 1990s, evidence began accumulating to suggest that some GPCRs are able to signal without G proteins. The ERK2 mitogen-activated protein kinase, a key signal transduction mediator downstream of receptor activation in many pathways, has been shown to be activated in response to cAMP-mediated receptor activation in the slime mold ''D. discoideum'' despite the absence of the associated G protein α- and β-subunits.
 
   
In mammalian cells, the much-studied β2-adrenoceptor has been demonstrated to activate the ERK2 pathway after arrestin-mediated uncoupling of G-protein-mediated signaling. It therefore seems likely that some mechanisms previously believed to be purely related to receptor desensitisation are actually examples of receptors switching their signaling pathway rather than simply being switched off.
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=== Ligand binding ===
   
In kidney cells, the [[bradykinin]] B2 receptor has been shown to interact directly with a protein tyrosine phosphatase. The presence of a tyrosine-phosphorylated ITIM (immunoreceptor tyrosine-based inhibitory motif) sequence in the B2 receptor is necessary to mediate this interaction and subsequently the antiproliferative effect of bradykinin.<ref>{{cite journal | author=Duchene J, Schanstra JP, Pecher C, Pizard A, Susini C, Esteve JP, Bascands JL, Girolami JP | title=A novel protein-protein interaction between a G protein-coupled receptor and the phosphatase SHP-2 is involved in bradykinin-induced inhibition of cell proliferation | journal=J Biol Chem | year=2002 | volume=277 | issue=43 | pages=40375-83 | pmid=12177051 | doi=10.1074/jbc.M202744200}}</ref>
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GPCRs include receptors for sensory signal mediators (e.g., light and [[olfactory]] stimulatory molecules); [[adenosine]], [[bombesin]], [[bradykinin]], [[endothelin]], γ-aminobutyric acid ([[Gamma-aminobutyric acid|GABA]]), hepatocyte growth factor ([[hepatocyte growth factor|HGF]]), [[melanocortin]]s, [[neuropeptide Y]], [[opioid]] peptides, [[opsin]]s, [[somatostatin]], [[growth hormone|GH]], [[tachykinins]], members of the [[vasoactive intestinal peptide]] family, and [[vasopressin]]; [[biogenic amine]]s (e.g., [[dopamine]], [[epinephrine]], [[norepinephrine]], [[histamine]], [[glutamate]] ([[metabotropic]] effect), [[glucagon]], [[acetylcholine]] ([[muscarinic]] effect), and [[serotonin]]); [[chemokines]]; [[lipid]] mediators of [[inflammation]] (e.g., [[prostaglandins]], [[prostanoid]]s, [[platelet-activating factor]], and [[leukotrienes]]); and peptide hormones (e.g., [[calcitonin]], C5a [[anaphylatoxin]], follicle-stimulating hormone ([[follicle-stimulating hormone|FSH]]), gonadotropin-releasing hormone ([[gonadotropin-releasing hormone|GnRH]]), [[neurokinin]], thyrotropin-releasing hormone ([[thyrotropin-releasing hormone|TRH]]), [[cannabinoid]]s, and [[oxytocin]]). GPCRs that act as receptors for stimuli that have not yet been identified are known as [[orphan receptor]]s.
  +
  +
Whereas, in other types of receptors that have been studied, wherein ligands bind externally to the membrane, the [[ligand (biochemistry)|ligand]]s of GPCRs typically bind within the transmembrane domain. However, [[protease-activated receptor]]s are activated by cleavage of part of their extracellular domain.<ref name="pmid12970120">{{cite journal | author = Brass LF | title = Thrombin and platelet activation | journal = Chest | volume = 124 | issue = 3 Suppl | pages = 18S–25S | year = 2003 | month = September | pmid = 12970120 | pmc = | doi = 10.1378/chest.124.3_suppl.18S| url = }}</ref>
  +
  +
=== Conformational change ===
  +
  +
[[Image:Beta2Receptor-with-Gs.png|right|thumb|300px|Crystal structure of activated beta-2 adrenergic receptor in complex with G<sub>s</sub>([[w:Protein Data Bank|PDB]] entry [http://www.rcsb.org/pdb/cgi/explore.cgi?pdbId=3SN6 3SN6]). The receptor is colored red, Gα green, Gβ cyan and Gγ yellow. The C-terminus of Gα is located in a cavity created by an outward movement of the cytoplasmic parts of TM5 and 6.]]
  +
  +
The [[signal transduction|transduction of the signal]] through the membrane by the receptor is not completely understood. It is known that the inactive [[G protein]] is bound to the receptor in its inactive state. Once the ligand is recognized, the receptor shifts [[Protein structure|conformation]] and, thus, mechanically activates the G protein, which detaches from the receptor. The receptor can now either activate another G protein or switch back to its inactive state. This is an overly simplistic explanation, but suffices to convey the overall set of events.
  +
  +
It is believed that a receptor molecule exists in a conformational equilibrium between active and inactive biophysical states.<ref>{{cite journal | author=Rubenstein, Lester A. and Lanzara, Richard G. | title=Activation of G protein-coupled receptors entails cysteine modulation of agonist binding | journal=Journal of Molecular Structure (Theochem) | year=1998 | volume=430 | pages=57–71 | url=http://cogprints.org/4095/ | doi=10.1016/S0166-1280(98)90217-2}}</ref> The binding of ligands to the receptor may shift the equilibrium toward the active receptor states.<ref>[http://www.bio-balance.com/Graphics.htm http://www.bio-balance.com/<!-- Bot generated title -->]</ref> Three types of ligands exist: agonists are ligands that shift the equilibrium in favour of active states; [[inverse agonist]]s are ligands that shift the equilibrium in favour of inactive states; and neutral antagonists are ligands that do not affect the equilibrium. It is not yet known how exactly the active and inactive states differ from each other.
  +
  +
=== G-protein activation/deactivation cycle ===
  +
  +
[[File:GPCR cycle.jpg|thumb|500px|Cartoon depicting the Heterotrimeric G-protein activation/deactivation cycle in the context of GPCR signaling]]
  +
{{See also|G protein}}
  +
When the receptor is inactive, the [[Guanine nucleotide exchange factor|GEF]] domain may be bound to an also inactive α-subunit of a [[heterotrimeric G-protein]]. These "G-proteins" are a [[Protein trimer|trimer]] of α, β, and γ subunits (known as Gα, Gβ, and Gγ, respectively) that is rendered inactive when reversibly bound to [[Guanosine diphosphate]] (GDP) (or alternatively, no guanine nucleotide) but active when bound to [[Guanosine triphosphate]] (GTP). Upon receptor activation, the GEF domain, in turn, [[allosterically]] activates the G-protein by facilitating the exchange of a molecule of GDP for GTP at the G-protein's α-subunit. The cell maintains a 10:1 ratio of cytosolic GTP:GDP so exchange for GTP is ensured. At this point, the subunits of the G-protein dissociate from the receptor, as well as each other, to yield a Gα-GTP [[monomer]] and a tightly interacting [[Beta-gamma complex|Gβγ dimer]], which are now free to modulate the activity of other intracellular proteins. The extent to which they may [[diffuse]], however, is limited due to the [[palmitoylation]] of Gα and the presence of a molecule of [[Glycosylphosphatidylinositol]] (GPI) that has been [[covalently]] added to the C-termini of Gγ. The [[phosphatidylinositol]] [[Moiety (chemistry)|moiety]] of the GPI-linkage contains two [[hydrophobic]] [[acyl|acyl groups]] that anchor any GPI-linked proteins (e.g. Gβγ) to the [[plasma membrane]], and also, to some extent, to the local [[lipid raft]]. (Compare this to the effect of palmitoylation on GPCR localization discussed above)
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Because Gα also has slow [[GTP-ase|GTP→GDP hydrolysis]] capability, the inactive form of the α-subunit (Gα-GDP) is eventually regenerated, thus allowing reassociation with a Gβγ dimer to form the "resting" G-protein, which can again bind to a GPCR and await activation. The rate of GTP hydrolysis is often accelerated due to the actions of another family of allosteric modulating proteins called [[Regulator of G protein signalling|Regulators of G-protein Signaling]], or RGS proteins, which are a type of [[GTPase activating protein|GTPase-Activating Protein]], or GAP. In fact, many of the primary [[Effector (biology)|effector]] proteins (e.g. [[adenylate cyclase]]s) that become activated/inactivated upon interaction with Gα-GTP also have GAP activity. Thus, even at this early stage in the process, GPCR-initiated signaling has the capacity for self-termination.
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== GPCR signaling ==
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[[Image:GPCR mechanism.png|right|thumb|300px|G-protein-coupled receptor mechanism]]
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If a receptor in an active state encounters a [[G protein]], it may activate it. Some evidence suggests that receptors and G proteins are actually pre-coupled. For example, binding of G proteins to receptors affects the receptor's affinity for ligands. Activated G proteins are bound to [[Guanosine triphosphate|GTP]].
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Further signal transduction depends on the type of G protein. The enzyme [[adenylate cyclase]] is an example of a cellular protein that can be regulated by a G protein, in this case the G protein [[Gs alpha subunit|G<sub>s</sub>]]. Adenylate cyclase activity is activated when it binds to a subunit of the activated G protein. Activation of adenylate cyclase ends when the G protein returns to the [[Guanosine diphosphate|GDP]]-bound state.
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Adenylate cyclases (of which 9 membrane-bound and one cytosolic forms are known in humans) may also be activated or inhibited in other ways (e.g., Ca2+/[[Calmodulin]] binding), which can modify the activity of these enzymes in an additive or synergistic fashion along with the G proteins.
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The signaling pathways activated through a GPCR are limited by the [[Protein primary structure|primary sequence]] and [[tertiary structure]] of the GPCR itself but ultimately determined by the particular [[Protein conformation|conformation]] stabilized by a particular [[ligand (biochemistry)|ligand]], as well as the availability of [[transducer]] molecules. Currently, GPCRs are considered to utilize two primary types of transducers: [[G-proteins]] and [[arrestin|β-arrestins]]. Because β-arr’s only have high [[affinity (pharmacology)|affinity]] to the [[phosphorylated]] form of most GPCRs (see above or below), the majority of signaling is ultimately dependent upon G-protein activation. However, the possibility for interaction does allow for G-protein independent signaling to occur.
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=== G-protein-dependent signaling ===
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There are three main G-protein-mediated signaling pathways, mediated by four [[Class (biology)|sub-classes]] of G-proteins distinguished from each other by [[sequence homology]] ([[Gαs|G<sub>αs</sub>]], [[Gαi|G<sub>αi/o</sub>]], [[Gαq|G<sub>αq/11</sub>]], and [[G12/G13 alpha subunits|G<sub>α12/13</sub>]]). Each sub-class of G-protein consists of multiple proteins, each the product of multiple [[genes]] and/or [[splice variant|splice variations]] that may imbue them with differences ranging from subtle to distinct with regard to signaling properties, but in general they appear reasonably grouped into four classes. Because the signal transducing properties of the various possible [[Beta-gamma complex|βγ combinations]] do not appear to radically differ from one another, these classes are defined according to the isoform of their α-subunit.<ref name="Wettschureck_2005"/>{{rp|1163}}
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While most GPCRs are capable of activating more than one Gα-subtype, they also show a preference for one subtype over another. When the subtype activated depends on the ligand that is bound to the GPCR, this is called [[functional selectivity]] (also known as agonist-directed trafficking, or conformation specific agonism). However, the binding of any single particular [[agonist]] may also initiate activation of multiple different G-proteins, as it may be capable of stabilizing more than one conformation of the GPCR’s [[Guanine nucleotide exchange factor|GEF]] domain, even over the course of a single interaction. Additionally, a conformation that preferably activates one [[isoform]] of Gα may activate another if the preferred is less available. Furthermore, [[feedback]] pathways may result in [[post-translational modification|receptor modifications]] (e.g. phosphorylation) that alter the G-protein preference. Regardless of these various nuances, the GPCR’s preferred coupling partner is usually defined according to the G-protein most obviously activated by the [[endogenous]] ligand under most [[physiological]] and/or [[experimental]] conditions.
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==== G<sub>α</sub> signaling ====
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# The effector of both the G<sub>αs</sub> and G<sub>αi/o</sub> pathways is the [[Cyclic amp|Cyclic-adenosine monophosphate]] (cAMP) generating enzyme [[Adenylyl cyclase|Adenylate Cyclase]], or AC. While there are ten different AC gene products in mammals, each with subtle differences in [[Tissue (biology)|tissue]] distribution and/or function, all [[catalyze]] the conversion of [[cytosolic]] [[Adenosine Triphosphate]] (ATP) to cAMP, and all are directly stimulated by G-proteins of the G<sub>αs</sub> class. Conversely, interaction with Gα subunits of the G<sub>αi/o</sub> type inhibits AC from generating cAMP. Thus, a GPCR coupled to G<sub>αs</sub> counteracts the actions of a GPCR coupled to G<sub>αi/o</sub>, and vice versa. The level of cytosolic cAMP may then determine the activity of various [[Cyclic nucleotide-gated ion channel|ion channels]] as well as members of the [[Serine/threonine-specific protein kinase|ser/thr specific]] [[Protein Kinase A]] (PKA) family. Thus cAMP is considered a [[Second messenger system|second messenger]] and PKA a secondary [[Effector (biology)|effector]].
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# The effector of the G<sub>αq/11</sub> pathway is [[Phospholipase C|Phospholipase C-β]] (PLCβ), which catalyzes the cleavage of membrane-bound [[Phosphatidylinositol 4,5-bisphosphate|phosphatidylinositol 4,5-biphosphate]] (PIP2) into the second messengers [[Inositol trisphosphate|inositol (1,4,5) trisphosphate]] (IP3) and [[Diglyceride|diacylglycerol]] (DAG). IP3 acts on [[Inositol trisphosphate receptor|IP3 receptors]] found in the membrane of the [[endoplasmic reticulum]] (ER) to elicit [[ca2+|Ca<sup>2+</sup>]] release from the ER, while DAG diffuses along the [[plasma membrane]] where it may activate any membrane localized forms of a second ser/thr kinase called [[Protein Kinase C]] (PKC). Since many isoforms of PKC are also activated by increases in intracellular Ca<sup>2+</sup>, both these pathways can also converge on each other to signal through the same secondary effector. Elevated intracellular Ca<sup>2+</sup> also binds and [[allosterically]] activates proteins called [[Calmodulin]]s, which in turn go on to bind and allosterically activate enzymes such as [[Camk|Ca2+/Calmodulin-dependant Kinases]] (CAMKs).
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# The effectors of the G<sub>α12/13</sub> pathway are three [[RhoGEF domain|RhoGEFs]] ([[p115-RhoGEF]], [[PDZ-RhoGEF]], and [[LARG]]), which, when bound to G<sub>α12/13</sub> allosterically activate the cytosolic [[small GTPase]], [[Rho family of GTPases|Rho]]. Once bound to GTP, Rho can then go on to activate various proteins responsible for [[cytoskeleton]] regulation such as [[Rho-associated protein kinase|Rho-kinase]] (ROCK). Most GPCRs that couple to G<sub>α12/13</sub> also couple to other sub-classes, often G<sub>αq/11</sub>.
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==== Gβγ signaling ====
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The above descriptions ignore the effects of [[Beta-gamma complex|Gβγ]]–signalling, which can also be important, in particular in the case of activated G<sub>αi/o</sub>-coupled GPCRs. The primary effectors of Gβγ are various ion channels, such as [[G protein-coupled inwardly-rectifying potassium channel|G-protein-regulated Inwardly Rectifying K+ channels]] (GIRKs), [[P-type calcium channel|P]]/[[Q-type calcium channel|Q]]- and [[N-type calcium channel|N-]]type [[Voltage-dependent calcium channel|voltage-gated Ca<sup>2+</sup> Channels]], as well as some isoforms of AC and PLC, along with some [[PI3K|Phosphoinositide-3-Kinase]] (PI3K) isoforms.
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=== G-Protein-independent signaling ===
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Although they are classically thought of working only together, GPCRs may signal through G-protein-independent mechanisms, and heterotrimeric G-proteins may play functional roles independent of GPCRs. GPCRs may signal independently through many proteins already mentioned for their roles in G-protein-dependent signaling such as [[Arrestin|β-arrs]], [[G protein-coupled receptor kinase|GRKs]], and [[Src (gene)|Srcs]]. Additionally, further scaffolding proteins involved in [[subcellular localization]] of GPCRs (e.g., [[PDZ (biology)|PDZ-domain]]-containing proteins) may also act as signal transducers. Most often the effector is a member of the [[MAPK]] family.
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==== Examples ====
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In the late 1990s, evidence began accumulating to suggest that some GPCRs are able to signal without G proteins. The [[MAPK1|ERK2]] mitogen-activated protein kinase, a key signal transduction mediator downstream of receptor activation in many pathways, has been shown to be activated in response to cAMP-mediated receptor activation in the [[slime mold]] [[Dictyostelium discoideum|''D. discoideum'']] despite the absence of the associated G protein α- and β-subunits.<ref>{{cite journal |author=Kim JY, Haastert PV, Devreotes PN |title=Social senses: G-protein-coupled receptor signaling pathways in Dictyostelium discoideum |journal=Chem. Biol. |volume=3 |issue=4 |pages=239–43 |year=1996 |month=April |pmid=8807851 |doi= 10.1016/S1074-5521(96)90103-9|url= http://www.cell.com/chemistry-biology/retrieve/pii/S1074552196901039}}</ref>
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In mammalian cells, the much-studied β<sub>2</sub>-adrenoceptor has been demonstrated to activate the ERK2 pathway after arrestin-mediated uncoupling of G-protein-mediated signaling. Therefore it seems likely that some mechanisms previously believed purely related to receptor desensitisation are actually examples of receptors switching their signaling pathway, rather than simply being switched off.
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In kidney cells, the [[bradykinin receptor B2]] has been shown to interact directly with a protein tyrosine phosphatase. The presence of a tyrosine-phosphorylated [[immunoreceptor tyrosine-based inhibitory motif|ITIM]] (immunoreceptor tyrosine-based inhibitory motif) sequence in the B2 receptor is necessary to mediate this interaction and subsequently the antiproliferative effect of bradykinin.<ref>{{cite journal | author=Duchene J, Schanstra JP, Pecher C, Pizard A, Susini C, Esteve JP, Bascands JL, Girolami JP | title=A novel protein-protein interaction between a G protein-coupled receptor and the phosphatase SHP-2 is involved in bradykinin-induced inhibition of cell proliferation | journal=J Biol Chem | year=2002 | volume=277 | issue=43 | pages=40375–83 | pmid=12177051 | doi=10.1074/jbc.M202744200}}</ref>
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==== GPCR-independent signaling by heterotrimeric G-proteins ====
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Although it is a relatively immature area of research, it appears that heterotrimeric G-proteins may also take part in non-GPCR signaling. There is evidence for roles as signal transducers in nearly all other types of receptor-mediated signaling, including [[integrins]], [[receptor tyrosine kinases]] (RTKs), [[cytokine receptors]] ([[JAK-STAT signaling pathway|JAK/STATs]]), as well as modulation of various other "accessory" proteins such as [[Guanine nucleotide exchange factor|GEFs]], [[Guanosine nucleotide dissociation inhibitors|Guanine-nucleotide Dissociation Inhibitors]] (GDIs) and [[protein phosphatases]]. There may even be specific proteins of these classes whose primary function is as part of GPCR-independent pathways, termed [[Activators of G-protein Signalling]] (AGS). Both the ubiquity of these interactions and the importance of Gα vs. Gβγ subunits to these processes are still unclear.
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== Details of cAMP and PIP2 pathways ==
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[[File:Proteinkinase 1.svg|thumb|250px|Activation effects of cAMP on Protein Kinase A]]
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[[File:The effect of Rs and Gs in cAMP signal pathway.jpg|thumb|250px|The effect of Rs and Gs in cAMP signal pathway]]
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[[File:The effect of Ri and Gi in cAMP signal pathway.jpg|thumb|250px|The effect of Ri and Gi in cAMP signal pathway]]
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There are two principal signal transduction pathways involving the [[G protein-coupled receptors|G protein-linked receptors]]: [[Cyclic adenosine monophosphate|cAMP]] signal pathway and [[Phosphatidylinositol]] signal pathway.<ref name="Gilman_1987"/>
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=== cAMP signal pathway ===
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{{Main|cAMP-dependent pathway}}
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The cAMP signal transduction contains 5 main characters: stimulative [[hormone]] receptor (Rs) or inhibitory [[hormone receptor]] (Ri);Stimulative regulative G-protein (Gs) or inhibitory regulative G-protein (Gi);[[Adenylyl cyclase]]; [[Protein Kinase A|Protein Kinase A (PKA)]]; and cAMP [[phosphodiesterase]].
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Stimulative hormone receptor (Rs) is a receptor that can bind with stimulative signal molecules, while inhibitory hormone (Ri) is a receptor that can bind with inhibitory signal molecules.
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Stimulative regulative G-protein is a G protein-linked to stimulative hormone receptor (Rs) and its α subunit upon activation could stimulate the activity of an enzyme or other intracellular metabolism. On the contrary, inhibitory regulative G-protein is linked to an inhibitory hormone receptor and its α subunit upon activation could inhibit the activity of an enzyme or other intracellular metabolism.
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The Adenylyl cyclase is a 12-transmembrane glucoprotein that catalyzes ATP to form cAMP with the help of cofactor Mg<sup>2+</sup> or Mn<sup>2+</sup>. The cAMP produced is a second messenger in cellular metabolism and is an allosteric activator to Protein kinase A.
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Protein kinase A is an important enzyme in cell metabolism due to its ability to regulate cell metabolism by phosphorylating specific committed enzymes in the metabolic pathway. It can also regulate specific gene expression, cellular secretion, and membrane permeability. The protein enzyme contains two catalytic subunits and two regulatory subunits. When there is no cAMP,the complex is inactive. When cAMP binds to the regulatory subunits, their conformation is altered, causing the dissociation of the regulatory subunits, which activates protein kinase A and allows further biological effects.
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cAMP phosphodiesterase is an enzyme that can degrade cAMP to 5'-AMP, which terminates the signal.
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=== Phosphatidylinositol signal pathway ===
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In the [[phosphatidylinositol]] signal pathway, the extracellular signal molecule binds with the G-protein receptor (G<sub>q</sub>) on the cell surface and activates [[phospholipase C]], which is located on the [[cell membrane|plasma membrane]]. The [[lipase]] hydrolyzes [[Phosphatidylinositol 4,5-bisphosphate|phosphatidylinositol 4,5-bisphosphate (PIP2)]] into two second messengers: [[Inositol trisphosphate|Inositol 1,4,5-trisphosphate (IP3)]] and [[Diacylglycerol|Diacylglycerol (DAG)]]. IP3 binds with the receptor in the membrane of the smooth endoplasmic reticulum and mitochondria, help open the Ca<sup>2+</sup> channel. DAG helps activate [[Protein Kinase C|Protein Kinase C (PKC)]], which phosphorylates many other proteins, changing their catalytic activities, leading to cellular responses. The effects of Ca<sup>2+</sup> is also remarkable: it cooperates with DAG in activating PKC and can activate [[Ca2+/calmodulin-dependent protein kinase|CaM kinase]] pathway, in which calcium modulated protein [[calmodulin]] (CaM) binds Ca<sup>2+</sup>, undergoes a change in conformation, and activates CaM kinase II, which has unique ability to increase its binding affinity to CaM by autophosphorylation, making CaM unavailable for the activation of other enzymes. The kinase then phosphorylates target enzymes, regulating their activities. The two signal pathways are connected together by Ca<sup>2+</sup>-CaM, which is also a regulatory subunit of adenylyl cyclase and phosphodiesterase in cAMP signal pathway.
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== Receptor regulation ==
   
==Receptor regulation==
 
 
GPCRs become desensitized when exposed to their ligand for a prolonged period of time. There are two recognized forms of desensitization: 1) [[homologous desensitization]], in which the activated GPCR is downregulated; and 2) [[heterologous desensitization]], wherein the activated GPCR causes downregulation of a different GPCR. The key reaction of this downregulation is the [[phosphorylation]] of the intracellular (or [[cytoplasm]]ic) receptor domain by [[protein kinase]]s.
 
GPCRs become desensitized when exposed to their ligand for a prolonged period of time. There are two recognized forms of desensitization: 1) [[homologous desensitization]], in which the activated GPCR is downregulated; and 2) [[heterologous desensitization]], wherein the activated GPCR causes downregulation of a different GPCR. The key reaction of this downregulation is the [[phosphorylation]] of the intracellular (or [[cytoplasm]]ic) receptor domain by [[protein kinase]]s.
   
===Phosphorylation by cAMP-dependent protein kinases===
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=== Phosphorylation by cAMP-dependent protein kinases ===
Cyclic AMP-dependent protein kinases ([[protein kinase A]]) are activated by the signal chain coming from the G protein (that was activated by the receptor) via [[adenylate cyclase]] and [[cyclic AMP]] (cAMP). In a ''feedback mechanism'', these activated kinases phosphorylate the receptor. The longer the receptor remains active, the more kinases are activated, the more receptors are phosphorylated.
 
   
===Phosphorylation by GRKs===
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Cyclic AMP-dependent protein kinases ([[protein kinase A]]) are activated by the signal chain coming from the G protein (that was activated by the receptor) via [[adenylate cyclase]] and [[cyclic AMP]] (cAMP). In a ''feedback mechanism'', these activated kinases phosphorylate the receptor. The longer the receptor remains active, the more kinases are activated, the more receptors are phosphorylated. In [[beta-2 adrenergic receptor|β<sub>2</sub>-adrenoceptor]]s, this phosphorylation results in the switching of the coupling from the G<sub>s</sub> class of G-protein to the [[Gi alpha subunit|G<sub>i</sub>]] class.<ref name="pmid11053129">{{cite journal | author = Chen-Izu Y, Xiao RP, Izu LT, Cheng H, Kuschel M, Spurgeon H, Lakatta EG | title = G(i)-dependent localization of beta(2)-adrenergic receptor signaling to L-type Ca(2+) channels | journal = Biophys. J. | volume = 79 | issue = 5 | pages = 2547–56 | year = 2000 | month = November | pmid = 11053129 | pmc = 1301137 | doi = 10.1016/S0006-3495(00)76495-2 }}</ref> cAMP-dependent PKA mediated phosphorylation can cause heterologous desensitisation in receptors other than those activated.<ref name="pmid14744258">{{cite journal | author = Tan CM, Brady AE, Nickols HH, Wang Q, Limbird LE | title = Membrane trafficking of G protein-coupled receptors | journal = Annu. Rev. Pharmacol. Toxicol. | volume = 44 | issue = | pages = 559–609 | year = 2004 | pmid = 14744258 | doi = 10.1146/annurev.pharmtox.44.101802.121558 }}</ref>
The [[G protein-coupled receptor kinases]] (GRKs) are protein kinases that phosphorylate only active GPCRs.
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=== Phosphorylation by GRKs ===
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The [[G protein-coupled receptor kinases]] (GRKs) are protein kinases that phosphorylate only{{Citation needed|date=December 2012}} active GPCRs.
   
 
Phosphorylation of the receptor can have two consequences:
 
Phosphorylation of the receptor can have two consequences:
   
# ''Translocation'': The receptor is, along with the part of the membrane it is embedded in, brought to the inside of the cell, where it is dephosphorylated and then brought back. This mechanism is used to regulate long-term exposure, for example, to a hormone.
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# ''Translocation'': The receptor is, along with the part of the membrane it is embedded in, brought to the inside of the cell, where it is dephosphorylated within the acidic vesicular environment<ref name="pmid8995214">{{cite journal | author = Krueger KM, Daaka Y, Pitcher JA, Lefkowitz RJ | title = The role of sequestration in G protein-coupled receptor resensitization. Regulation of β<sub>2</sub>-adrenergic receptor dephosphorylation by vesicular acidification | journal = J. Biol. Chem. | volume = 272 | issue = 1 | pages = 5–8 | year = 1997 | pmid = 8995214 | doi = 10.1074/jbc.272.1.5 }}</ref> and then brought back. This mechanism is used to regulate long-term exposure, for example, to a hormone, by allowing resensitisation to follow desensitisation. Alternatively, the receptor may undergo lysozomal degradation, or remain internalised, where it is thought to participate in the initiation of signalling events, the nature of which depend on the internalised vesicle's subcellular localisation.<ref name="pmid14744258"/>
# ''[[Arrestin]] linking'': The phosphorylated receptor can be linked to ''arrestin'' molecules that prevent it from binding (and activating) G proteins, effectively switching it off for a short period of time. This mechanism is used, for example, with [[rhodopsin]] in [[retina]] cells to compensate for exposure to bright light. In many cases, arrestin binding to the receptor is a prerequisite for translocation.
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# ''[[Arrestin]] linking'': The phosphorylated receptor can be linked to ''arrestin'' molecules that prevent it from binding (and activating) G proteins, effectively switching it off for a short period of time. This mechanism is used, for example, with [[rhodopsin]] in [[retina]] cells to compensate for exposure to bright light. In many cases, arrestin binding to the receptor is a prerequisite for translocation. For example, beta-arrestin bound to β<sub>2</sub>-adrenoreceptors acts as an adaptor for binding with clathrin, and with the beta-subunit of AP2 (clathrin adaptor molecules); thus the arrestin here acts as a scaffold assembling the components needed for clathrin-mediated endocytosis of β<sub>2</sub>-adrenoreceptors.<ref name="pmid10770944">{{cite journal | author = Laporte SA, Oakley RH, Holt JA, Barak LS, Caron MG | title = The interaction of β-arrestin with the AP-2 adaptor is required for the clustering of β<sub>2</sub>-adrenergic receptor into clathrin-coated pits | journal = J. Biol. Chem. | volume = 275 | issue = 30 | pages = 23120–6 | year = 2000 | pmid = 10770944 | doi = 10.1074/jbc.M002581200 }}</ref><ref name="pmid10097102">{{cite journal | author = Laporte SA, Oakley RH, Zhang J, Holt JA, Ferguson SS, Caron MG, Barak LS | title = The beta2-adrenergic receptor/betaarrestin complex recruits the clathrin adaptor AP-2 during endocytosis | journal = Proc. Natl. Acad. Sci. U.S.A. | volume = 96 | issue = 7 | pages = 3712–7 | year = 1999 | pmid = 10097102 | doi = 10.1073/pnas.96.7.3712| pmc = 22359 }}</ref>
   
==Receptor oligomerization==
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=== Mechanisms of GPCR signal termination ===
It is generally accepted that G-protein-coupled receptors can form [[homodimer|homo]]- and/or [[heterodimer]]s and possibly more complex [[oligomeric]] structures, and indeed heterodimerization has been shown to be essential for the function of receptors such as the metabotropic GABA(B) receptors. However, it is presently unproven that true heterodimers exist. Present biochemical and physical techniques lack the resolution to differentiate between distinct homodimers assembled into an oligomer or true 1:1 heterodimers. It is also unclear what the functional significance of [[oligomerization]] might be, although it is thought that the phenomenon may contribute to the pharmacological [[heterogeneity]] of GPCRs in a manner not previously anticipated. This is an actively-studied area in GPCR research.
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As mentioned above, G-proteins may terminate their own activation due to their intrinsic [[GTPase|GTP→GDP hydrolysis]] capability. However, this reaction proceeds at a slow [[rate constant|rate]] (≈.02 times/sec) and thus it would take around 50 seconds for any single G-protein to deactivate if other factors did not come into play. Indeed, there are around 30 [[protein isoform|isoforms]] of [[Regulator of G protein signalling|RGS proteins]] that, when bound to through their [[GTPase activating protein|GAP domain]], accelerate the hydrolysis rate to ≈30 times/sec. This 1500-fold increase in rate allows for the cell to respond to external signals with high speed, as well as spatial [[angular resolution|resolution]] due to limited amount of [[second messenger]] that can be generated and limited distance a G-protein can diffuse in .03 seconds. For the most part, the RGS proteins are [[promiscuous]] in their ability to activate G-proteins, while which RGS is involved in a given signaling pathway seems more determined by the tissue and GPCR involved than anything else. Additionally, RGS proteins have the additional function of increasing the rate of GTP-GDP exchange at GPCRs, (i.e. as a sort of co-GEF) further contributing to the time resolution of GPCR signaling.
   
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In addition, the GPCR may be [[homologous desensitization|desensitized]] itself. This can occur as:
   
== ''Dictyostelium'' ==
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# a direct result of [[receptor theory|ligand occupation]], wherein the change in [[protein conformation|conformation]] allows recruitment of [[G protein-coupled receptor kinase|GPCR-Regulating Kinases]] (GRKs), which go on to [[phosphorylation|phosphorylate]] various [[serine]]/[[threonine]] residues of IL-3 and the [[C-terminal]] tail. Upon GRK phosphorylation, the GPCR's affinity for [[arrestin|β-arrestin]] (β-arrestin-1/2 in most tissues) is increased, at which point β-arrestin may bind and act to both [[sterically]] hinder G-protein coupling as well as initiate the process of [[Receptor-mediated endocytosis|receptor internalization]] through [[clathrin-mediated endocytosis]]. Because only the liganded receptor is desensitized by this mechanism, it is called [[homologous desensitization]]
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# the affinity for β-arr may increased in a ligand occupation and GRK-independent manner through phosphorylation of different ser/thr sites (but also of IL-3 and the C-terminal tail) by PKC and PKA. These phosphorylations are often sufficient to impair G-protein coupling on their own as well.{{Citation needed|date=October 2012}}
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# PKC/PKA may, instead, phosphorylate GRKs, which can also lead to GPCR phosphorylation and β-arrestin binding in an occupation-independent manner. These latter two mechanisms allow for desensitization of one GPCR due to the activities of others, or [[heterologous desensitization]]. GRKs may also have GAP domains and so may contribute to inactivation through non-[[kinase]] mechanisms as well. A combination of these mechanisms may also occur.
   
A novel GPCR containing a lipid kinase domain has recently been identified in ''[[Dictyostelium]]'' that regulates cell density sensing.<ref>{{cite journal | author=Bakthavatsalam D, Brazill D, Gomer RH, Eichinger L, Rivero F, Noegel AA | title=A G protein-coupled receptor with a lipid kinase domain is involved in cell-density sensing | journal=Curr Biol | year=2007 | volume=17 | issue=10 | pages=892-7 | pmid= 17481898 | doi=10.1016/j.cub.2007.04.029}}</ref>
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Once β-arrestin is bound to a GPCR, it undergoes a conformational change allowing it to serve as a scaffolding protein for an adaptor complex termed [[AP2 adaptors|AP-2]], which in turn recruits another protein called [[clathrin]]. If enough receptors in the local area recruit clathrin in this manner, they aggregate and the [[plasma membrane|membrane]] buds inwardly as a result of interactions between the molecules of clathrin, in a process called [[opsonization]]. Once the pit has been pinched off, the [[plasma membrane]] due to the actions of two other proteins called [[amphiphysin]] and [[dynamin]], it is now an [[endocytosis|endocytic]] [[vesicle (biology)|vesicle]]. At this point, the adapter molecules and clathrin have [[dissociated]], and the receptor is either [[protein targeting|trafficked]] back to the plasma membrane or targeted to [[lysosomes]] for [[proteolysis|degradation]].
   
==References==
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At any point in this process, the β-arrestins may also recruit other proteins—such as the [[non-receptor tyrosine kinase]] (nRTK), [[Src (gene)|c-SRC]]—which may activate [[Extracellular signal-regulated kinases|ERK1/2]], or other [[mitogen-activated protein kinase]] (MAPK) signaling through, for example, phosphorylation of the [[Small GTPase|small GTP-ase]], [[Ras subfamily|Ras]], or recruit the proteins of the [[MAPK/ERK pathway|ERK cascade]] directly (i.e., [[Raf-1]], [[mitogen-activated protein kinase kinase|MEK]], ERK-1/2) at which point signaling is initiated due to their close proximity to one another. Another target of c-SRC are the dynamin molecules involved in endocytosis. Dynamins [[polymerization|polymerize]] around the neck of an incoming vesicle, and their phosphorylation by c-SRC provides the energy necessary for the conformational change allowing the final "pinching off" from the membrane.
{{Reflist|2}}
 
   
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=== GPCR cellular regulation ===
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Receptor desensitization is mediated through a combination phosphorylation, β-arr binding, and endocytosis as described above. Downregulation occurs when endocytosed receptor is embedded in an endosome that is trafficked to merge with an organelle called a lysosome. Because lysosomal membranes are rich in proton pumps, their interiors have low pH (≈4.8 vs. the pH≈7.2 cytosol), which acts to denature the GPCRs. Additionally, lysosomes contain many [[degradative enzyme]]s, including proteases, which can function only at such low pH, and so the peptide bonds joining the residues of the GPCR together may be cleaved. Whether or not a given receptor is trafficked to a lysosome, detained in endosomes, or trafficked back to the plasma membrane depends on a variety of factors, including receptor type and magnitude of the signal.
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GPCR regulation is additionally mediated by gene transcription factors. These factors can increase or decrease gene transcription and thus increase or decrease the generation of new receptors (up- or down-regulation) that travel to the cell membrane.
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== Receptor oligomerization ==
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{{Main|GPCR oligomer}}
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G-protein-coupled receptor oligomerisation is a widespread phenomenon. One of the best-studied example is the metabotropic [[GABAB receptor|GABA<sub>B</sub> receptor]]. This so-called constitutive receptor is formed by heterodimerization of [[GABBR1|GABA<sub>B</sub>R1]] and [[GABBR2|GABA<sub>B</sub>R2]] subunits. Expression of the GABA<sub>B</sub>R1 without the GABA<sub>B</sub>R2 in heterologous systems leads to retention of the subunit in the [[endoplasmic reticulum]]. Expression of the GABA<sub>B</sub>R2 subunit alone, meanwhile, leads to surface expression of the subunit, although with no functional activity (''i.e.'', the receptor does not bind agonist and cannot initiate a response following exposure to agonist). Expression of the two subunits together leads to plasma membrane expression of functional receptor. It has been shown that GABA<sub>B</sub>R2 binding to GABA<sub>B</sub>R1 causes masking of a retention signal<ref name="pmid10939334">{{cite journal | author = Margeta-Mitrovic M, Jan YN, Jan LY | title = A trafficking checkpoint controls GABA(B) receptor heterodimerization | journal = Neuron | volume = 27 | issue = 1 | pages = 97–106 | year = 2000 | pmid = 10939334 | doi = 10.1016/S0896-6273(00)00012-X }}</ref> of functional receptors.<ref name="pmid9872316">{{cite journal | author = White JH, Wise A, Main MJ, Green A, Fraser NJ, Disney GH, Barnes AA, Emson P, Foord SM, Marshall FH | title = Heterodimerization is required for the formation of a functional GABA(B) receptor | journal = Nature | volume = 396 | issue = 6712 | pages = 679–82 | year = 1998 | pmid = 9872316 | doi = 10.1038/25354 }}</ref>
  +
  +
== Origin and diversification of the superfamily ==
  +
Signal transduction mediated by the superfamily of GPCRs dates back to the origin of multicellularity. Mammalian-like GPCRs are found in [[fungi]], and have been classified according to the [[GRAFS]] classification system based on GPCR fingerprints.<ref>{{cite journal | author=Krishnan A, Alme´n MS, Fredriksson R, Schiöth HB | title=The Origin of GPCRs: Identification of Mammalian like Rhodopsin, Adhesion, Glutamate and Frizzled GPCRs in Fungi | journal=PLoS ONE | year=2012 | pages=e29817 | pmid= 22238661 | doi=10.1371/journal.pone.0029817 | pmc=3251606 | volume=7 | issue=1 | editor1-last=Xue | editor1-first=Chaoyang }}</ref> Identification of the superfamily members across the [[eukaryotic]] domain, and comparison of the family-specific motifs, have shown that the superfamily of GPCRs have a common origin.<ref>{{cite journal | author=Nordström KJ, Sällman Almén M, Edstam MM, Fredriksson R, Schiöth HB | title=Independent HHsearch, Needleman–Wunsch-Based, and Motif Analyses Reveal the Overall Hierarchy for Most of the G Protein-Coupled Receptor Families | journal=Mol Biol Evol | year=2011 | pages=2471–80| pmid= 21402729 | doi=10.1093/molbev/msr061 | volume=28 | issue=9}}</ref> Characteristic motifs indicate that three of the five GRAFS families, ''Rhodopsin'', [[adhesion-GPCRs|''Adhesion'']], and ''Frizzled'', evolved from the ''[[Dictyostelium]] discoideum'' cAMP receptors before the split of Opisthokonts. Later the ''Secretin'' family evolved from the ''Adhesion'' GPCR receptor family before the split of [[nematode]]s.
  +
  +
== ''Dictyostelium discoideum'' ==
  +
A novel GPCR containing a lipid kinase domain has recently been identified in ''[[Dictyostelium discoideum]]'' that regulates cell density sensing.<ref>{{cite journal | author=Bakthavatsalam D, Brazill D, Gomer RH, Eichinger L, Rivero F, Noegel AA | title=A G protein-coupled receptor with a lipid kinase domain is involved in cell-density sensing | journal=Curr Biol | year=2007 | volume=17 | issue=10 | pages=892–7 | pmid= 17481898 | doi=10.1016/j.cub.2007.04.029}}</ref>
   
 
== See also ==
 
== See also ==
   
 
* [[Orphan receptor]]
 
* [[Orphan receptor]]
  +
* [[Pepducin]]s, a class of drug candidates targeted at GPCRs
  +
* [[G protein-coupled receptors database]]
  +
* [[Metabotropic receptor]]
   
==External links==
+
== References ==
*[http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=153653&blobname=pnas_0230374100v2_1.pdf A phylogenetic tree of all human GPCRs] showing family relationships from Vassilatis, ''et al''.<ref>{{cite journal | author=Vassilatis DK, Hohmann JG, Zeng H, Li F, Ranchalis JE, Mortrud MT, Brown A, Rodriguez SS, Weller JR, Wright AC, Bergmann JE, Gaitanaris GA | title=The G protein-coupled receptor repertoires of human and mouse | journal=Proc Natl Acad Sci U S A | year=2003 | volume=100 | issue=8 | pages=4903-8 | pmid= 12679517 | doi= 10.1073/pnas.0230374100}}</ref>
+
{{Reflist|35em}}
*[http://www.bio-balance.com/Ref.htm Reference for molecular and mathematical models for the initial receptor response]
 
   
*[http://www.iuphar-db.org IUPHAR GPCR Database]
+
==External links==
*[http://www.gpcr.org/7tm/ G Protein-Coupled Receptor Database (GPCRDB)]
 
 
* {{MeshName|G-protein-coupled+receptors}}
 
* {{MeshName|G-protein-coupled+receptors}}
*[[Wikipedia:MeSH D12.776#MeSH D12.776.543.750.100 --- receptors.2C g-protein-coupled]]
+
* [[Wikipedia:MeSH D12.776#MeSH D12.776.543.750.100 --- receptors.2C g-protein-coupled]]
* {{UMichOPM|families|superfamily|6}}
+
* {{cite web | url = http://www.iuphar-db.org/GPCR/ReceptorFamiliesForward | title = GPCR Database | author = | authorlink = | coauthors = | date = | format = | work = IUPHAR Database | publisher = International Union of Basic and Clinical Pharmacology | pages = | language = | archiveurl = | archivedate = | quote = | accessdate = 2008-08-11}}
  +
* {{cite web | url = http://www.gpcr.org/7tm/ | title = GPCRDB: Information system for G protein-coupled receptors (GPCRs) | author = Vriend G, Horn F| authorlink = | coauthors = | date = 2006-06-29 | format = | work = | publisher = Molecular Class-Specific Information System (MCSIS) project | pages = | language = | archiveurl = | archivedate = | quote = | accessdate = 2008-08-11}}
  +
* {{cite web | url = http://www.gproteincoupledreceptors.net | title = G Protein-Coupled Receptors on the NET | author = | authorlink = | coauthors = | date = | format = | work = | publisher = | pages = | language = | archiveurl = | archivedate = | quote = a classification of GPCRs | accessdate = 2010-11-10}}
  +
  +
==Further reading==
  +
* {{cite web | url = http://www.nobelprize.org/nobel_prizes/chemistry/laureates/2012/popular-chemistryprize2012.pdf | title = The Nobel Prize in Chemistry 2012 | accessdate = 2012-10-10}}
  +
* {{cite web | url = http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=153653&blobname=pnas_0230374100v2_1.pdf | title = A phylogenetic tree of all human GPCRs | author = | authorlink = | coauthors = | date = | format = | work = {{cite journal | author=Vassilatis DK, Hohmann JG, Zeng H, Li F, Ranchalis JE, Mortrud MT, Brown A, Rodriguez SS, Weller JR, Wright AC, Bergmann JE, Gaitanaris GA | title=The G protein-coupled receptor repertoires of human and mouse | journal=Proc Natl Acad Sci USA | year=2003 | volume=100 | issue=8 | pages=4903–8 | pmid= 12679517 | doi= 10.1073/pnas.0230374100 | pmc=153653}} | publisher = | pages = | language = | archiveurl = | archivedate = | quote = | accessdate = 2008-08-11}}
  +
* {{cite web | url = http://www.bio-balance.com/Ref.htm | title = GPCR Reference Library | author = | authorlink = | coauthors = | date = | work = | publisher = | pages = | language = | archiveurl = | archivedate = | quote = Reference for molecular and mathematical models for the initial receptor response | accessdate = 2008-08-11}}
  +
* [http://pdbe.org/nobel2012 GPCR structures] in the [[Protein Data Bank|PDB]]
   
 
{{Transmembrane receptors}}
 
{{Transmembrane receptors}}
 
{{G protein-coupled receptors}}
 
{{G protein-coupled receptors}}
   
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[[Category:G protein coupled receptors| ]]
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[[Category:Protein families]]
 
[[Category:Signal transduction]]
 
[[Category:Signal transduction]]
 
[[Category:Integral membrane proteins]]
 
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[[Category:Molecular biology]]
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[[Category:Biochemistry]]
   
 
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File:7TM4 (GPCR).png

G protein coupled receptors (GPCRs), also known as seven-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptor, and G protein-linked receptors (GPLR), constitute a large protein family of receptors that sense molecules outside the cell and activate inside signal transduction pathways and, ultimately, cellular responses. They are called transmembrane receptors because they pass through the cell membrane, and they are called seven-transmembrane receptors because they pass through the cell membrane seven times.

G protein-coupled receptors are found only in eukaryotes, including yeast, choanoflagellates,[1] and animals. The ligands that bind and activate these receptors include light-sensitive compounds, odors, pheromones, hormones, and neurotransmitters, and vary in size from small molecules to peptides to large proteins. G protein-coupled receptors are involved in many diseases, and are also the target of approximately 40% of all modern medicinal drugs.[2][3] The 2012 Nobel prize in chemistry was awarded to Brian Kobilka and Robert Lefkowitz for their work that was "crucial for understanding how G-protein–coupled receptors function."[4]

There are two principal signal transduction pathways involving the G protein-coupled receptors: the cAMP signal pathway and the phosphatidylinositol signal pathway.[5] When a ligand binds to the GPCR it causes a conformational change in the GPCR, which allows it to act as a guanine nucleotide exchange factor (GEF). The GPCR can then activate an associated G-protein by exchanging its bound GDP for a GTP. The G-protein's α subunit, together with the bound GTP, can then dissociate from the β and γ subunits to further affect intracellular signaling proteins or target functional proteins directly depending on the α subunit type (Gαs, Gαi/o, Gαq/11, Gα12/13).[6]:1160

Classification Edit

File:GPCR classification.JPG

The exact size of the GPCR superfamily is unknown but nearly 800 different human genes (or ≈4% of the entire protein-coding genome) have been predicted from genome sequence analysis. Although numerous classification schemes have been proposed, the superfamily is classically divided into three main classes (A, B, and C) with no detectable shared sequence homology between classes. The largest class by far is class A, which accounts for nearly 85% of the GPCR genes. Of class A GPCRs, over half of these are predicted to encode olfactory receptors while the remaining receptors are liganded by known endogenous compounds or are classified as orphan receptors. Despite the lack of sequence homology between classes, all GPCRs share a common structure and mechanism of signal transduction.

In all, GPCRs can be grouped into 6 classes based on sequence homology and functional similarity:[8][9][10][11]

The very large rhodopsin A group has been further subdivided into 19 subgroups (A1-A19).[12] More recently, an alternative classification system called GRAFS (Glutamate, Rhodopsin, Adhesion, Frizzled/Taste2, Secretin) has been proposed.[13]

The human genome encodes thousands of G protein-coupled receptors,[14] about 350 of which detect hormones, growth factors, and other endogenous ligands. Approximately 150 of the GPCRs found in the human genome have unknown functions.

Some web-servers[15] and bioinformatics prediction methods[16][17] have been used for predicting the classification of GPCRs according to their amino acid sequence alone, by means of the pseudo amino acid composition approach.

Physiological roles Edit

GPCRs are involved in a wide variety of physiological processes. Some examples of their physiological roles include:

  1. The visual sense: the opsins use a photoisomerization reaction to translate electromagnetic radiation into cellular signals. Rhodopsin, for example, uses the conversion of 11-cis-retinal to all-trans-retinal for this purpose
  2. The sense of smell: receptors of the olfactory epithelium bind odorants (olfactory receptors) and pheromones (vomeronasal receptors)
  3. Behavioral and mood regulation: receptors in the mammalian brain bind several different neurotransmitters, including serotonin, dopamine, GABA, and glutamate
  4. Regulation of immune system activity and inflammation: chemokine receptors bind ligands that mediate intercellular communication between cells of the immune system; receptors such as histamine receptors bind inflammatory mediators and engage target cell types in the inflammatory response
  5. Autonomic nervous system transmission: both the sympathetic and parasympathetic nervous systems are regulated by GPCR pathways, responsible for control of many automatic functions of the body such as blood pressure, heart rate, and digestive processes
  6. Cell density sensing: A novel GPCR role in regulating cell density sensing.
  7. Homeostasis modulation (e.g., water balance).[18]
  8. Involved in tumor growth and metastasis.[19]

Receptor structure Edit

GPCRs are integral membrane proteins that possess seven membrane-spanning domains or transmembrane helices. The extracellular parts of the receptor can be glycosylated. These extracellular loops also contain two highly-conserved cysteine residues that form disulfide bonds to stabilize the receptor structure. Some seven-transmembrane helix proteins (channelrhodopsin) that resemble GPCRs may contain ion channels, within their protein.

Similar to GPCRs, the adiponectin receptors 1 and 2 (ADIPOR1 and ADIPOR2) also possess 7 transmembrane domains. However ADIPOR1 and ADIPOR2 are orientated oppositely to GPCRs in the membrane (i.e., cytoplasmic N-terminus, extracellular C-terminus) and do not associate with G proteins.[20]

Early structural models for GPCRs were based on their weak analogy to bacteriorhodopsin, for which a structure had been determined by both electron diffraction (PDB 2BRD, Template:PDB2)[21][22] and X ray-based crystallography (Template:PDB2).[23] In 2000, the first crystal structure of a mammalian GPCR, that of bovine rhodopsin (Template:PDB2), was solved.[24] While the main feature, the seven transmembrane helices, is conserved, the relative orientation of the helices differ significantly from that of bacteriorhodopsin. In 2007, the first structure of a human GPCR was solved (Template:PDB2, Template:PDB2).[25] This was followed immediately by a higher resolution structure of the same receptor (Template:PDB2).[26][27] This human β2-adrenergic receptor GPCR structure, proved highly similar to the bovine rhodopsin in terms of the relative orientation of the seven-transmembrane helices. However the conformation of the second extracellular loop is entirely different between the two structures. Since this loop constitutes the "lid" that covers the top of the ligand binding site, this conformational difference highlights the difficulties in constructing homology models of other GPCRs based only on the rhodopsin structure.

The structures of activated and/or agonist-bound GPCRs have also been determined.[28][29][30][31] These structures indicate how ligand binding at the extracellular side of a receptor leads to conformational changes in the cytoplasmic side of the receptor. The biggest change is an outward movement of the cytoplasmic part of the 5th and 6th transmembrane helix (TM5 and TM6). The structure of activated beta-2 adrenergic receptor in complex with Gs confirmed that the Gα binds to a cavity created by this movement.[32]

Structure-function relationships Edit

File:GPCR in membrane.jpg

Structurally, GPCRs are characterized by an extracellular N-terminus, followed by seven transmembrane (7-TM) α-helices (TM-1 to TM-7) connected by three intracellular (IL-1 to IL-3) and three extracellular loops (EL-1 to EL-3), and finally an intracellular C-terminus. The GPCR arranges itself into a tertiary structure resembling a barrel, with the seven transmembrane helices forming a cavity within the plasma membrane that serves a ligand-binding domain that is often covered by EL-2. Ligands may also bind elsewhere, however, as is the case for bulkier ligands (e.g., proteins or large peptides), which instead interact with the extracellular loops, or, as illustrated by the class C metabotropic glutamate receptors (mGluRs), the N-terminal tail. The class C GPCRs are distinguished by their large N-terminal tail, which also contains a ligand-binding domain. Upon glutamate-binding to an mGluR, the N-terminal tail undergoes a conformational change that leads to its interaction with the residues of the extracellular loops and TM domains. The eventual effect of all three types of agonist-induced activation is a change in the relative orientations of the TM helices (likened to a twisting motion) leading to a wider intracellular surface and "revelation" of residues of the intracellular helices and TM domains crucial to signal transduction function (i.e., G-protein coupling). Inverse agonists and antagonists may also bind to a number of different sites, but the eventual effect must be prevention of this TM helix reorientation.

The structure of the N- and C-terminal tails of GPCRs may also serve important functions beyond ligand-binding. In particular, the C-terminus often contains serine (Ser) or threonine (Thr) residues that, when phosphorylated, increase the affinity of the intracellular surface for the binding of scaffolding proteins called β-arrestins (β-arr).[33] Once bound, β-arrestins both sterically prevent G-protein coupling and may recruit other proteins leading to the creation of signaling complexes involved in extracellular-signal regulated kinase (ERK) pathway activation or receptor endocytosis (internalization). As the phosphorylation of these Ser and Thr residues often occurs as a result of GPCR activation, the β-arr-mediated G-protein-decoupling and internalization of GPCRs are important mechanisms of desensitization.[34]

A final common structural theme among GPCRs is palmitoylation of one or more sites of the C-terminal tail or the intracellular loops. Palmitoylation is the covalent modification of cysteine (Cys) residues via addition of hydrophobic acyl groups, and has the effect of targeting the receptor to cholesterol- and sphingolipid-rich microdomains of the plasma membrane called lipid rafts. As many of the downstream transducer and effector molecules of GPCRs (including those involved in negative feedback pathways) are also targeted to lipid rafts, this has the effect of facilitating rapid receptor signaling.

GPCRs respond to extracellular signals mediated by a huge diversity of agonists, ranging from proteins to biogenic amines to protons, but all transduce this signal via a mechanism of G-protein coupling. This is made possible by virtue of a guanine-nucleotide exchange factor (GEF) domain primarily formed by a combination of IL-2 and IL-3 along with adjacent residues of the associated TM helices.

Mechanism Edit

File:GPCR activation.jpg

The G protein-coupled receptor is activated by an external signal in the form of a ligand or other signal mediator. This creates a conformational change in the receptor, causing activation of a G protein. Further effect depends on the type of G protein.

Ligand binding Edit

GPCRs include receptors for sensory signal mediators (e.g., light and olfactory stimulatory molecules); adenosine, bombesin, bradykinin, endothelin, γ-aminobutyric acid (GABA), hepatocyte growth factor (HGF), melanocortins, neuropeptide Y, opioid peptides, opsins, somatostatin, GH, tachykinins, members of the vasoactive intestinal peptide family, and vasopressin; biogenic amines (e.g., dopamine, epinephrine, norepinephrine, histamine, glutamate (metabotropic effect), glucagon, acetylcholine (muscarinic effect), and serotonin); chemokines; lipid mediators of inflammation (e.g., prostaglandins, prostanoids, platelet-activating factor, and leukotrienes); and peptide hormones (e.g., calcitonin, C5a anaphylatoxin, follicle-stimulating hormone (FSH), gonadotropin-releasing hormone (GnRH), neurokinin, thyrotropin-releasing hormone (TRH), cannabinoids, and oxytocin). GPCRs that act as receptors for stimuli that have not yet been identified are known as orphan receptors.

Whereas, in other types of receptors that have been studied, wherein ligands bind externally to the membrane, the ligands of GPCRs typically bind within the transmembrane domain. However, protease-activated receptors are activated by cleavage of part of their extracellular domain.[36]

Conformational change Edit

File:Beta2Receptor-with-Gs.png

The transduction of the signal through the membrane by the receptor is not completely understood. It is known that the inactive G protein is bound to the receptor in its inactive state. Once the ligand is recognized, the receptor shifts conformation and, thus, mechanically activates the G protein, which detaches from the receptor. The receptor can now either activate another G protein or switch back to its inactive state. This is an overly simplistic explanation, but suffices to convey the overall set of events.

It is believed that a receptor molecule exists in a conformational equilibrium between active and inactive biophysical states.[37] The binding of ligands to the receptor may shift the equilibrium toward the active receptor states.[38] Three types of ligands exist: agonists are ligands that shift the equilibrium in favour of active states; inverse agonists are ligands that shift the equilibrium in favour of inactive states; and neutral antagonists are ligands that do not affect the equilibrium. It is not yet known how exactly the active and inactive states differ from each other.

G-protein activation/deactivation cycle Edit

File:GPCR cycle.jpg
See also: G protein

When the receptor is inactive, the GEF domain may be bound to an also inactive α-subunit of a heterotrimeric G-protein. These "G-proteins" are a trimer of α, β, and γ subunits (known as Gα, Gβ, and Gγ, respectively) that is rendered inactive when reversibly bound to Guanosine diphosphate (GDP) (or alternatively, no guanine nucleotide) but active when bound to Guanosine triphosphate (GTP). Upon receptor activation, the GEF domain, in turn, allosterically activates the G-protein by facilitating the exchange of a molecule of GDP for GTP at the G-protein's α-subunit. The cell maintains a 10:1 ratio of cytosolic GTP:GDP so exchange for GTP is ensured. At this point, the subunits of the G-protein dissociate from the receptor, as well as each other, to yield a Gα-GTP monomer and a tightly interacting Gβγ dimer, which are now free to modulate the activity of other intracellular proteins. The extent to which they may diffuse, however, is limited due to the palmitoylation of Gα and the presence of a molecule of Glycosylphosphatidylinositol (GPI) that has been covalently added to the C-termini of Gγ. The phosphatidylinositol moiety of the GPI-linkage contains two hydrophobic acyl groups that anchor any GPI-linked proteins (e.g. Gβγ) to the plasma membrane, and also, to some extent, to the local lipid raft. (Compare this to the effect of palmitoylation on GPCR localization discussed above)

Because Gα also has slow GTP→GDP hydrolysis capability, the inactive form of the α-subunit (Gα-GDP) is eventually regenerated, thus allowing reassociation with a Gβγ dimer to form the "resting" G-protein, which can again bind to a GPCR and await activation. The rate of GTP hydrolysis is often accelerated due to the actions of another family of allosteric modulating proteins called Regulators of G-protein Signaling, or RGS proteins, which are a type of GTPase-Activating Protein, or GAP. In fact, many of the primary effector proteins (e.g. adenylate cyclases) that become activated/inactivated upon interaction with Gα-GTP also have GAP activity. Thus, even at this early stage in the process, GPCR-initiated signaling has the capacity for self-termination.

GPCR signaling Edit

File:GPCR mechanism.png

If a receptor in an active state encounters a G protein, it may activate it. Some evidence suggests that receptors and G proteins are actually pre-coupled. For example, binding of G proteins to receptors affects the receptor's affinity for ligands. Activated G proteins are bound to GTP.

Further signal transduction depends on the type of G protein. The enzyme adenylate cyclase is an example of a cellular protein that can be regulated by a G protein, in this case the G protein Gs. Adenylate cyclase activity is activated when it binds to a subunit of the activated G protein. Activation of adenylate cyclase ends when the G protein returns to the GDP-bound state.

Adenylate cyclases (of which 9 membrane-bound and one cytosolic forms are known in humans) may also be activated or inhibited in other ways (e.g., Ca2+/Calmodulin binding), which can modify the activity of these enzymes in an additive or synergistic fashion along with the G proteins.

The signaling pathways activated through a GPCR are limited by the primary sequence and tertiary structure of the GPCR itself but ultimately determined by the particular conformation stabilized by a particular ligand, as well as the availability of transducer molecules. Currently, GPCRs are considered to utilize two primary types of transducers: G-proteins and β-arrestins. Because β-arr’s only have high affinity to the phosphorylated form of most GPCRs (see above or below), the majority of signaling is ultimately dependent upon G-protein activation. However, the possibility for interaction does allow for G-protein independent signaling to occur.

G-protein-dependent signaling Edit

There are three main G-protein-mediated signaling pathways, mediated by four sub-classes of G-proteins distinguished from each other by sequence homology (Gαs, Gαi/o, Gαq/11, and Gα12/13). Each sub-class of G-protein consists of multiple proteins, each the product of multiple genes and/or splice variations that may imbue them with differences ranging from subtle to distinct with regard to signaling properties, but in general they appear reasonably grouped into four classes. Because the signal transducing properties of the various possible βγ combinations do not appear to radically differ from one another, these classes are defined according to the isoform of their α-subunit.[6]:1163

While most GPCRs are capable of activating more than one Gα-subtype, they also show a preference for one subtype over another. When the subtype activated depends on the ligand that is bound to the GPCR, this is called functional selectivity (also known as agonist-directed trafficking, or conformation specific agonism). However, the binding of any single particular agonist may also initiate activation of multiple different G-proteins, as it may be capable of stabilizing more than one conformation of the GPCR’s GEF domain, even over the course of a single interaction. Additionally, a conformation that preferably activates one isoform of Gα may activate another if the preferred is less available. Furthermore, feedback pathways may result in receptor modifications (e.g. phosphorylation) that alter the G-protein preference. Regardless of these various nuances, the GPCR’s preferred coupling partner is usually defined according to the G-protein most obviously activated by the endogenous ligand under most physiological and/or experimental conditions.

Gα signaling Edit

  1. The effector of both the Gαs and Gαi/o pathways is the Cyclic-adenosine monophosphate (cAMP) generating enzyme Adenylate Cyclase, or AC. While there are ten different AC gene products in mammals, each with subtle differences in tissue distribution and/or function, all catalyze the conversion of cytosolic Adenosine Triphosphate (ATP) to cAMP, and all are directly stimulated by G-proteins of the Gαs class. Conversely, interaction with Gα subunits of the Gαi/o type inhibits AC from generating cAMP. Thus, a GPCR coupled to Gαs counteracts the actions of a GPCR coupled to Gαi/o, and vice versa. The level of cytosolic cAMP may then determine the activity of various ion channels as well as members of the ser/thr specific Protein Kinase A (PKA) family. Thus cAMP is considered a second messenger and PKA a secondary effector.
  2. The effector of the Gαq/11 pathway is Phospholipase C-β (PLCβ), which catalyzes the cleavage of membrane-bound phosphatidylinositol 4,5-biphosphate (PIP2) into the second messengers inositol (1,4,5) trisphosphate (IP3) and diacylglycerol (DAG). IP3 acts on IP3 receptors found in the membrane of the endoplasmic reticulum (ER) to elicit Ca2+ release from the ER, while DAG diffuses along the plasma membrane where it may activate any membrane localized forms of a second ser/thr kinase called Protein Kinase C (PKC). Since many isoforms of PKC are also activated by increases in intracellular Ca2+, both these pathways can also converge on each other to signal through the same secondary effector. Elevated intracellular Ca2+ also binds and allosterically activates proteins called Calmodulins, which in turn go on to bind and allosterically activate enzymes such as Ca2+/Calmodulin-dependant Kinases (CAMKs).
  3. The effectors of the Gα12/13 pathway are three RhoGEFs (p115-RhoGEF, PDZ-RhoGEF, and LARG), which, when bound to Gα12/13 allosterically activate the cytosolic small GTPase, Rho. Once bound to GTP, Rho can then go on to activate various proteins responsible for cytoskeleton regulation such as Rho-kinase (ROCK). Most GPCRs that couple to Gα12/13 also couple to other sub-classes, often Gαq/11.

Gβγ signaling Edit

The above descriptions ignore the effects of Gβγ–signalling, which can also be important, in particular in the case of activated Gαi/o-coupled GPCRs. The primary effectors of Gβγ are various ion channels, such as G-protein-regulated Inwardly Rectifying K+ channels (GIRKs), P/Q- and N-type voltage-gated Ca2+ Channels, as well as some isoforms of AC and PLC, along with some Phosphoinositide-3-Kinase (PI3K) isoforms.

G-Protein-independent signaling Edit

Although they are classically thought of working only together, GPCRs may signal through G-protein-independent mechanisms, and heterotrimeric G-proteins may play functional roles independent of GPCRs. GPCRs may signal independently through many proteins already mentioned for their roles in G-protein-dependent signaling such as β-arrs, GRKs, and Srcs. Additionally, further scaffolding proteins involved in subcellular localization of GPCRs (e.g., PDZ-domain-containing proteins) may also act as signal transducers. Most often the effector is a member of the MAPK family.

Examples Edit

In the late 1990s, evidence began accumulating to suggest that some GPCRs are able to signal without G proteins. The ERK2 mitogen-activated protein kinase, a key signal transduction mediator downstream of receptor activation in many pathways, has been shown to be activated in response to cAMP-mediated receptor activation in the slime mold D. discoideum despite the absence of the associated G protein α- and β-subunits.[39]

In mammalian cells, the much-studied β2-adrenoceptor has been demonstrated to activate the ERK2 pathway after arrestin-mediated uncoupling of G-protein-mediated signaling. Therefore it seems likely that some mechanisms previously believed purely related to receptor desensitisation are actually examples of receptors switching their signaling pathway, rather than simply being switched off.

In kidney cells, the bradykinin receptor B2 has been shown to interact directly with a protein tyrosine phosphatase. The presence of a tyrosine-phosphorylated ITIM (immunoreceptor tyrosine-based inhibitory motif) sequence in the B2 receptor is necessary to mediate this interaction and subsequently the antiproliferative effect of bradykinin.[40]

GPCR-independent signaling by heterotrimeric G-proteins Edit

Although it is a relatively immature area of research, it appears that heterotrimeric G-proteins may also take part in non-GPCR signaling. There is evidence for roles as signal transducers in nearly all other types of receptor-mediated signaling, including integrins, receptor tyrosine kinases (RTKs), cytokine receptors (JAK/STATs), as well as modulation of various other "accessory" proteins such as GEFs, Guanine-nucleotide Dissociation Inhibitors (GDIs) and protein phosphatases. There may even be specific proteins of these classes whose primary function is as part of GPCR-independent pathways, termed Activators of G-protein Signalling (AGS). Both the ubiquity of these interactions and the importance of Gα vs. Gβγ subunits to these processes are still unclear.

Details of cAMP and PIP2 pathways Edit

File:Proteinkinase 1.svg
File:The effect of Rs and Gs in cAMP signal pathway.jpg
File:The effect of Ri and Gi in cAMP signal pathway.jpg

There are two principal signal transduction pathways involving the G protein-linked receptors: cAMP signal pathway and Phosphatidylinositol signal pathway.[5]

cAMP signal pathway Edit

Main article: cAMP-dependent pathway

The cAMP signal transduction contains 5 main characters: stimulative hormone receptor (Rs) or inhibitory hormone receptor (Ri);Stimulative regulative G-protein (Gs) or inhibitory regulative G-protein (Gi);Adenylyl cyclase; Protein Kinase A (PKA); and cAMP phosphodiesterase.

Stimulative hormone receptor (Rs) is a receptor that can bind with stimulative signal molecules, while inhibitory hormone (Ri) is a receptor that can bind with inhibitory signal molecules.

Stimulative regulative G-protein is a G protein-linked to stimulative hormone receptor (Rs) and its α subunit upon activation could stimulate the activity of an enzyme or other intracellular metabolism. On the contrary, inhibitory regulative G-protein is linked to an inhibitory hormone receptor and its α subunit upon activation could inhibit the activity of an enzyme or other intracellular metabolism.

The Adenylyl cyclase is a 12-transmembrane glucoprotein that catalyzes ATP to form cAMP with the help of cofactor Mg2+ or Mn2+. The cAMP produced is a second messenger in cellular metabolism and is an allosteric activator to Protein kinase A.

Protein kinase A is an important enzyme in cell metabolism due to its ability to regulate cell metabolism by phosphorylating specific committed enzymes in the metabolic pathway. It can also regulate specific gene expression, cellular secretion, and membrane permeability. The protein enzyme contains two catalytic subunits and two regulatory subunits. When there is no cAMP,the complex is inactive. When cAMP binds to the regulatory subunits, their conformation is altered, causing the dissociation of the regulatory subunits, which activates protein kinase A and allows further biological effects.

cAMP phosphodiesterase is an enzyme that can degrade cAMP to 5'-AMP, which terminates the signal.

Phosphatidylinositol signal pathway Edit

In the phosphatidylinositol signal pathway, the extracellular signal molecule binds with the G-protein receptor (Gq) on the cell surface and activates phospholipase C, which is located on the plasma membrane. The lipase hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2) into two second messengers: Inositol 1,4,5-trisphosphate (IP3) and Diacylglycerol (DAG). IP3 binds with the receptor in the membrane of the smooth endoplasmic reticulum and mitochondria, help open the Ca2+ channel. DAG helps activate Protein Kinase C (PKC), which phosphorylates many other proteins, changing their catalytic activities, leading to cellular responses. The effects of Ca2+ is also remarkable: it cooperates with DAG in activating PKC and can activate CaM kinase pathway, in which calcium modulated protein calmodulin (CaM) binds Ca2+, undergoes a change in conformation, and activates CaM kinase II, which has unique ability to increase its binding affinity to CaM by autophosphorylation, making CaM unavailable for the activation of other enzymes. The kinase then phosphorylates target enzymes, regulating their activities. The two signal pathways are connected together by Ca2+-CaM, which is also a regulatory subunit of adenylyl cyclase and phosphodiesterase in cAMP signal pathway.

Receptor regulation Edit

GPCRs become desensitized when exposed to their ligand for a prolonged period of time. There are two recognized forms of desensitization: 1) homologous desensitization, in which the activated GPCR is downregulated; and 2) heterologous desensitization, wherein the activated GPCR causes downregulation of a different GPCR. The key reaction of this downregulation is the phosphorylation of the intracellular (or cytoplasmic) receptor domain by protein kinases.

Phosphorylation by cAMP-dependent protein kinases Edit

Cyclic AMP-dependent protein kinases (protein kinase A) are activated by the signal chain coming from the G protein (that was activated by the receptor) via adenylate cyclase and cyclic AMP (cAMP). In a feedback mechanism, these activated kinases phosphorylate the receptor. The longer the receptor remains active, the more kinases are activated, the more receptors are phosphorylated. In β2-adrenoceptors, this phosphorylation results in the switching of the coupling from the Gs class of G-protein to the Gi class.[41] cAMP-dependent PKA mediated phosphorylation can cause heterologous desensitisation in receptors other than those activated.[42]

Phosphorylation by GRKs Edit

The G protein-coupled receptor kinases (GRKs) are protein kinases that phosphorylate only[citation needed] active GPCRs.

Phosphorylation of the receptor can have two consequences:

  1. Translocation: The receptor is, along with the part of the membrane it is embedded in, brought to the inside of the cell, where it is dephosphorylated within the acidic vesicular environment[43] and then brought back. This mechanism is used to regulate long-term exposure, for example, to a hormone, by allowing resensitisation to follow desensitisation. Alternatively, the receptor may undergo lysozomal degradation, or remain internalised, where it is thought to participate in the initiation of signalling events, the nature of which depend on the internalised vesicle's subcellular localisation.[42]
  2. Arrestin linking: The phosphorylated receptor can be linked to arrestin molecules that prevent it from binding (and activating) G proteins, effectively switching it off for a short period of time. This mechanism is used, for example, with rhodopsin in retina cells to compensate for exposure to bright light. In many cases, arrestin binding to the receptor is a prerequisite for translocation. For example, beta-arrestin bound to β2-adrenoreceptors acts as an adaptor for binding with clathrin, and with the beta-subunit of AP2 (clathrin adaptor molecules); thus the arrestin here acts as a scaffold assembling the components needed for clathrin-mediated endocytosis of β2-adrenoreceptors.[44][45]

Mechanisms of GPCR signal termination Edit

As mentioned above, G-proteins may terminate their own activation due to their intrinsic GTP→GDP hydrolysis capability. However, this reaction proceeds at a slow rate (≈.02 times/sec) and thus it would take around 50 seconds for any single G-protein to deactivate if other factors did not come into play. Indeed, there are around 30 isoforms of RGS proteins that, when bound to Gα through their GAP domain, accelerate the hydrolysis rate to ≈30 times/sec. This 1500-fold increase in rate allows for the cell to respond to external signals with high speed, as well as spatial resolution due to limited amount of second messenger that can be generated and limited distance a G-protein can diffuse in .03 seconds. For the most part, the RGS proteins are promiscuous in their ability to activate G-proteins, while which RGS is involved in a given signaling pathway seems more determined by the tissue and GPCR involved than anything else. Additionally, RGS proteins have the additional function of increasing the rate of GTP-GDP exchange at GPCRs, (i.e. as a sort of co-GEF) further contributing to the time resolution of GPCR signaling.

In addition, the GPCR may be desensitized itself. This can occur as:

  1. a direct result of ligand occupation, wherein the change in conformation allows recruitment of GPCR-Regulating Kinases (GRKs), which go on to phosphorylate various serine/threonine residues of IL-3 and the C-terminal tail. Upon GRK phosphorylation, the GPCR's affinity for β-arrestin (β-arrestin-1/2 in most tissues) is increased, at which point β-arrestin may bind and act to both sterically hinder G-protein coupling as well as initiate the process of receptor internalization through clathrin-mediated endocytosis. Because only the liganded receptor is desensitized by this mechanism, it is called homologous desensitization
  2. the affinity for β-arr may increased in a ligand occupation and GRK-independent manner through phosphorylation of different ser/thr sites (but also of IL-3 and the C-terminal tail) by PKC and PKA. These phosphorylations are often sufficient to impair G-protein coupling on their own as well.[citation needed]
  3. PKC/PKA may, instead, phosphorylate GRKs, which can also lead to GPCR phosphorylation and β-arrestin binding in an occupation-independent manner. These latter two mechanisms allow for desensitization of one GPCR due to the activities of others, or heterologous desensitization. GRKs may also have GAP domains and so may contribute to inactivation through non-kinase mechanisms as well. A combination of these mechanisms may also occur.

Once β-arrestin is bound to a GPCR, it undergoes a conformational change allowing it to serve as a scaffolding protein for an adaptor complex termed AP-2, which in turn recruits another protein called clathrin. If enough receptors in the local area recruit clathrin in this manner, they aggregate and the membrane buds inwardly as a result of interactions between the molecules of clathrin, in a process called opsonization. Once the pit has been pinched off, the plasma membrane due to the actions of two other proteins called amphiphysin and dynamin, it is now an endocytic vesicle. At this point, the adapter molecules and clathrin have dissociated, and the receptor is either trafficked back to the plasma membrane or targeted to lysosomes for degradation.

At any point in this process, the β-arrestins may also recruit other proteins—such as the non-receptor tyrosine kinase (nRTK), c-SRC—which may activate ERK1/2, or other mitogen-activated protein kinase (MAPK) signaling through, for example, phosphorylation of the small GTP-ase, Ras, or recruit the proteins of the ERK cascade directly (i.e., Raf-1, MEK, ERK-1/2) at which point signaling is initiated due to their close proximity to one another. Another target of c-SRC are the dynamin molecules involved in endocytosis. Dynamins polymerize around the neck of an incoming vesicle, and their phosphorylation by c-SRC provides the energy necessary for the conformational change allowing the final "pinching off" from the membrane.

GPCR cellular regulation Edit

Receptor desensitization is mediated through a combination phosphorylation, β-arr binding, and endocytosis as described above. Downregulation occurs when endocytosed receptor is embedded in an endosome that is trafficked to merge with an organelle called a lysosome. Because lysosomal membranes are rich in proton pumps, their interiors have low pH (≈4.8 vs. the pH≈7.2 cytosol), which acts to denature the GPCRs. Additionally, lysosomes contain many degradative enzymes, including proteases, which can function only at such low pH, and so the peptide bonds joining the residues of the GPCR together may be cleaved. Whether or not a given receptor is trafficked to a lysosome, detained in endosomes, or trafficked back to the plasma membrane depends on a variety of factors, including receptor type and magnitude of the signal. GPCR regulation is additionally mediated by gene transcription factors. These factors can increase or decrease gene transcription and thus increase or decrease the generation of new receptors (up- or down-regulation) that travel to the cell membrane.

Receptor oligomerization Edit

Main article: GPCR oligomer

G-protein-coupled receptor oligomerisation is a widespread phenomenon. One of the best-studied example is the metabotropic GABAB receptor. This so-called constitutive receptor is formed by heterodimerization of GABABR1 and GABABR2 subunits. Expression of the GABABR1 without the GABABR2 in heterologous systems leads to retention of the subunit in the endoplasmic reticulum. Expression of the GABABR2 subunit alone, meanwhile, leads to surface expression of the subunit, although with no functional activity (i.e., the receptor does not bind agonist and cannot initiate a response following exposure to agonist). Expression of the two subunits together leads to plasma membrane expression of functional receptor. It has been shown that GABABR2 binding to GABABR1 causes masking of a retention signal[46] of functional receptors.[47]

Origin and diversification of the superfamily Edit

Signal transduction mediated by the superfamily of GPCRs dates back to the origin of multicellularity. Mammalian-like GPCRs are found in fungi, and have been classified according to the GRAFS classification system based on GPCR fingerprints.[48] Identification of the superfamily members across the eukaryotic domain, and comparison of the family-specific motifs, have shown that the superfamily of GPCRs have a common origin.[49] Characteristic motifs indicate that three of the five GRAFS families, Rhodopsin, Adhesion, and Frizzled, evolved from the Dictyostelium discoideum cAMP receptors before the split of Opisthokonts. Later the Secretin family evolved from the Adhesion GPCR receptor family before the split of nematodes.

Dictyostelium discoideum Edit

A novel GPCR containing a lipid kinase domain has recently been identified in Dictyostelium discoideum that regulates cell density sensing.[50]

See also Edit

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