G protein gamma subunit, a hidden master regulator of GPCR signaling
2022; Elsevier BV; Volume: 298; Issue: 12 Linguagem: Inglês
10.1016/j.jbc.2022.102618
ISSN1083-351X
AutoresDinesh Kankanamge, Mithila Tennakoon, Ajith Karunarathne, N. Gautam,
Tópico(s)Neuropeptides and Animal Physiology
ResumoHeterotrimeric G proteins (αβγ subunits) that are activated by G protein-coupled receptors (GPCRs) mediate the biological responses of eukaryotic cells to extracellular signals. The α subunits and the tightly bound βγ subunit complex of G proteins have been extensively studied and shown to control the activity of effector molecules. In contrast, the potential roles of the large family of γ subunits have been less studied. In this review, we focus on present knowledge about these proteins. Induced loss of individual γ subunit types in animal and plant models result in strikingly distinct phenotypes indicating that γ subtypes play important and specific roles. Consistent with these findings, downregulation or upregulation of particular γ subunit types result in various types of cancers. Clues about the mechanistic basis of γ subunit function have emerged from imaging the dynamic behavior of G protein subunits in living cells. This shows that in the basal state, G proteins are not constrained to the plasma membrane but shuttle between membranes and on receptor activation βγ complexes translocate reversibly to internal membranes. The translocation kinetics of βγ complexes varies widely and is determined by the membrane affinity of the associated γ subtype. On translocating, some βγ complexes act on effectors in internal membranes. The variation in translocation kinetics determines differential sensitivity and adaptation of cells to external signals. Membrane affinity of γ subunits is thus a parsimonious and elegant mechanism that controls information flow to internal cell membranes while modulating signaling responses. Heterotrimeric G proteins (αβγ subunits) that are activated by G protein-coupled receptors (GPCRs) mediate the biological responses of eukaryotic cells to extracellular signals. The α subunits and the tightly bound βγ subunit complex of G proteins have been extensively studied and shown to control the activity of effector molecules. In contrast, the potential roles of the large family of γ subunits have been less studied. In this review, we focus on present knowledge about these proteins. Induced loss of individual γ subunit types in animal and plant models result in strikingly distinct phenotypes indicating that γ subtypes play important and specific roles. Consistent with these findings, downregulation or upregulation of particular γ subunit types result in various types of cancers. Clues about the mechanistic basis of γ subunit function have emerged from imaging the dynamic behavior of G protein subunits in living cells. This shows that in the basal state, G proteins are not constrained to the plasma membrane but shuttle between membranes and on receptor activation βγ complexes translocate reversibly to internal membranes. The translocation kinetics of βγ complexes varies widely and is determined by the membrane affinity of the associated γ subtype. On translocating, some βγ complexes act on effectors in internal membranes. The variation in translocation kinetics determines differential sensitivity and adaptation of cells to external signals. Membrane affinity of γ subunits is thus a parsimonious and elegant mechanism that controls information flow to internal cell membranes while modulating signaling responses. G protein-coupled receptors (GPCRs) on the plasma membrane sense external signals and activate heterotrimeric (αβγ) G proteins. Activation of the G proteins results in the α subunit exchanging GDP for GTP and the dissociation of a tightly associated βγ complex. Both the α-GTP and the βγ complex are independently capable of modulating the activity of effectors. The α subunits of G proteins are GTPase switches that are active in the GTP-bound form and deactivated in the GDP bound form (1Gilman A.G. G proteins: transducers of receptor-generated signals.Annu. Rev. Biochem. 1987; 56: 615-649Crossref PubMed Google Scholar). α and βγ subunits are large families of diverse members that act on a number of effectors such as adenylyl cyclase and phospholipase C (2Simon M.I. Strathmann M.P. Gautam N. Diversity of G proteins in signal transduction.Science. 1991; 252: 802-808Crossref PubMed Google Scholar, 3Gautam N. Downes G.B. Yan K. Kisselev O. The G-protein betagamma complex.Cell Signal. 1998; 10: 447-455Crossref PubMed Scopus (158) Google Scholar). The βγ complex acts on various effectors including G protein-gated inwardly rectifying K+ channels (4Logothetis D.E. Kurachi Y. Galper J. Neer E.J. Clapham D.E. The beta gamma subunits of GTP-binding proteins activate the muscarinic K+ channel in heart.Nature. 1987; 325: 321-326Crossref PubMed Scopus (876) Google Scholar), adenylyl cyclase (5Tang W.J. Gilman A.G. Type-specific regulation of adenylyl cyclase by G protein beta gamma subunits.Science. 1991; 254: 1500-1503Crossref PubMed Google Scholar), phospholipase C (PLC) (6Katz A. Wu D. Simon M.I. Subunits beta gamma of heterotrimeric G protein activate beta 2 isoform of phospholipase C.Nature. 1992; 360: 686-689Crossref PubMed Scopus (0) Google Scholar), GPCR kinases (7Daaka Y. Pitcher J.A. Richardson M. Stoffel R.H. Robishaw J.D. Lefkowitz R.J. Receptor and G betagamma isoform-specific interactions with G protein-coupled receptor kinases.Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 2180-2185Crossref PubMed Scopus (155) Google Scholar), and phosphoinositide 3-kinase γ (PI3Kγ) (8Brock C. Schaefer M. Reusch H.P. Czupalla C. Michalke M. Spicher K. et al.Roles of G beta gamma in membrane recruitment and activation of p110 gamma/p101 phosphoinositide 3-kinase gamma.J. Cell Biol. 2003; 160: 89-99Crossref PubMed Scopus (0) Google Scholar). Recent reviews have focused on various aspects of the α and βγ subunits (9Hewavitharana T. Wedegaertner P.B. Non-canonical signaling and localizations of heterotrimeric G proteins.Cell Signal. 2012; 24: 25-34Crossref PubMed Scopus (71) Google Scholar, 10Campbell A.P. Smrcka A.V. Targeting G protein-coupled receptor signalling by blocking G proteins.Nat. Rev. Drug Discov. 2018; 17: 789-803Crossref PubMed Scopus (76) Google Scholar, 11Tennakoon M. Senarath K. Kankanamge D. Ratnayake K. Wijayaratna D. Olupothage K. et al.Subtype-dependent regulation of Gbetagamma signalling.Cell Signal. 2021; 82: 109947Crossref PubMed Scopus (0) Google Scholar). In contrast, studies of the γ subunits have been limited and their structure and potential functions have not been reviewed. This review focuses on present knowledge about the γ subunits, their potential roles in signaling based on this information, gaps that remain in our knowledge, and potential future experimental directions that can address these lacunae. The γ subunit of transducin, the G protein found in rod outer segments of the retina was the first γ to be characterized at the protein and cDNA level (12Hurley J.B. Fong H.K. Teplow D.B. Dreyer W.J. Simon M.I. Isolation and characterization of a cDNA clone for the gamma subunit of bovine retinal transducin.Proc. Natl. Acad. Sci. U. S. A. 1984; 81: 6948-6952Crossref PubMed Scopus (0) Google Scholar). The identification of the cDNA for a γ subunit associated with the Gi/o proteins using peptide analysis and PCR showed that the primary structures of the two γ subunits diverged considerably, and it was evolutionarily related to the small GTP binding Ras family of proteins (13Gautam N. Baetscher M. Aebersold R. Simon M.I. A G protein gamma subunit shares homology with ras proteins.Science. 1989; 244: 971-974Crossref PubMed Google Scholar). Identification of additional subunits suggested that the γ subunits were potentially a large family of structurally diverse proteins (14Gautam N. Northup J. Tamir H. Simon M.I. G protein diversity is increased by associations with a variety of gamma subunits.Proc. Natl. Acad. Sci. U. S. A. 1990; 87: 7973-7977Crossref PubMed Google Scholar). Over the years, 12 γ subunit types were identified based on cDNA sequences (14Gautam N. Northup J. Tamir H. Simon M.I. G protein diversity is increased by associations with a variety of gamma subunits.Proc. Natl. Acad. Sci. U. S. A. 1990; 87: 7973-7977Crossref PubMed Google Scholar, 15Fisher K.J. Aronson Jr., N.N. Characterization of the cDNA and genomic sequence of a G protein gamma subunit (gamma 5).Mol. Cell Biol. 1992; 12: 1585-1591Crossref PubMed Scopus (0) Google Scholar, 16Kalyanaraman S. Kalyanaraman V. Gautam N. A brain-specific G protein gamma subunit.Biochem. Biophys. Res. Commun. 1995; 216: 126-132Crossref PubMed Scopus (0) Google Scholar, 17Ray K. Kunsch C. Bonner L.M. Robishaw J.D. Isolation of cDNA clones encoding eight different human G protein gamma subunits, including three novel forms designated the gamma 4, gamma 10, and gamma 11 subunits.J. Biol. Chem. 1995; 270: 21765-21771Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar, 18Morishita R. Nakayama H. Isobe T. Matsuda T. Hashimoto Y. Okano T. et al.Primary structure of a gamma subunit of G protein, gamma 12, and its phosphorylation by protein kinase C.J. Biol. Chem. 1995; 270: 29469-29475Abstract Full Text Full Text PDF PubMed Scopus (79) Google Scholar, 19Ryba N.J. Tirindelli R. A novel GTP-binding protein gamma-subunit, G gamma 8, is expressed during neurogenesis in the olfactory and vomeronasal neuroepithelia.J. Biol. Chem. 1995; 270: 6757-6767Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar, 20Downes G.B. Gautam N. The G protein subunit gene families.Genomics. 1999; 62: 544-552Crossref PubMed Scopus (228) Google Scholar, 21Huang L. Shanker Y.G. Dubauskaite J. Zheng J.Z. Yan W. Rosenzweig S. et al.Ggamma13 colocalizes with gustducin in taste receptor cells and mediates IP3 responses to bitter denatonium.Nat. Neurosci. 1999; 2: 1055-1062Crossref PubMed Scopus (285) Google Scholar). The primary structures of the γ subunits were conserved in different mammalian species indicating that the differences in amino acid residues among these subunits were of functional importance (3Gautam N. Downes G.B. Yan K. Kisselev O. The G-protein betagamma complex.Cell Signal. 1998; 10: 447-455Crossref PubMed Scopus (158) Google Scholar). The presence of a γ subunit in yeast (22Whiteway M. Hougan L. Dignard D. Thomas D.Y. Bell L. Saari G.C. et al.The STE4 and STE18 genes of yeast encode potential beta and gamma subunits of the mating factor receptor-coupled G protein.Cell. 1989; 56: 467-477Abstract Full Text PDF PubMed Scopus (0) Google Scholar) and γ subunits in plants (23Mason M.G. Botella J.R. Completing the heterotrimer: isolation and characterization of an Arabidopsis thaliana G protein gamma-subunit cDNA.Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 14784-14788Crossref PubMed Scopus (0) Google Scholar, 24Mason M.G. Botella J.R. Isolation of a novel G-protein gamma-subunit from Arabidopsis thaliana and its interaction with Gbeta.Biochim. Biophys. Acta. 2001; 1520: 147-153Crossref PubMed Scopus (0) Google Scholar) also showed that the G protein γ subunit has been retained over a long period of evolution in all eukaryotes and further emphasized the potential for an independent role in signaling. In the plant Arabidopsis thaliana, an atypical γ subunit has been identified with a primary structure that is distinctly different from all other γ subunits (25Chakravorty D. Trusov Y. Zhang W. Acharya B.R. Sheahan M.B. McCurdy D.W. et al.An atypical heterotrimeric G-protein gamma-subunit is involved in guard cell K(+)-channel regulation and morphological development in Arabidopsis thaliana.Plant J. 2011; 67: 840-851Crossref PubMed Scopus (167) Google Scholar). This suggests that the γ subunits have evolutionarily diverged considerably in plants to play specialized roles. Though the lipidation of α subunits with a covalent 16-carbon palmitate group and/or 14 carbon myristate group at their N terminus was discovered in the late 1980s (26Wedegaertner P.B. Wilson P.T. Bourne H.R. Lipid modifications of trimeric G proteins.J. Biol. Chem. 1995; 270: 503-506Abstract Full Text Full Text PDF PubMed Scopus (394) Google Scholar), it was only in 1990 that the anchoring of βγ to the membrane via a prenyl moiety by covalent posttranslational modifications at the C terminus of γ subunit was identified (27Fukada Y. Takao T. Ohguro H. Yoshizawa T. Akino T. Shimonishi Y. Farnesylated gamma-subunit of photoreceptor G protein indispensable for GTP-binding.Nature. 1990; 346: 658-660Crossref PubMed Scopus (0) Google Scholar, 28Xie H. Yamane H. Stephenson R. Ong O. Fung B. Clarke S. Analysis of prenylated carboxyl-terminal cysteine methyl esters in proteins.Methods. 1990; 1: 276-282Crossref Scopus (17) Google Scholar, 29Chen C.A. Manning D.R. Regulation of G proteins by covalent modification.Oncogene. 2001; 20: 1643-1652Crossref PubMed Scopus (169) Google Scholar). γ subunits are lipidated with a prenyl group, either farnesyl (15 Carbon) (27Fukada Y. Takao T. Ohguro H. Yoshizawa T. Akino T. Shimonishi Y. Farnesylated gamma-subunit of photoreceptor G protein indispensable for GTP-binding.Nature. 1990; 346: 658-660Crossref PubMed Scopus (0) Google Scholar, 30Lai R.K. Perez-Sala D. Canada F.J. Rando R.R. The gamma subunit of transducin is farnesylated.Proc. Natl. Acad. Sci. U. S. A. 1990; 87: 7673-7677Crossref PubMed Google Scholar) or geranylgeranyl (20 Carbon) (31Mumby S.M. Casey P.J. Gilman A.G. Gutowski S. Sternweis P.C. G protein gamma subunits contain a 20-carbon isoprenoid.Proc. Natl. Acad. Sci. U. S. A. 1990; 87: 5873-5877Crossref PubMed Scopus (0) Google Scholar, 32Yamane H.K. Farnsworth C.C. Xie H.Y. Howald W. Fung B.K. Clarke S. et al.Brain G protein gamma subunits contain an all-trans-geranylgeranylcysteine methyl ester at their carboxyl termini.Proc. Natl. Acad. Sci. U. S. A. 1990; 87: 5868-5872Crossref PubMed Google Scholar), through a stable thioether linkage to the C-terminal Cys. A four-residue conserved amino acid sequence called; "the CaaX motif" on the C terminus of the γ subunit determines the type of prenylation on a specific γ subtype. CaaX is composed of a Cys, two aliphatic amino acids-aa, and a prenyl transferase determining residue, X. The Cys is farnesylated when X is Met, Ser, Glu, or Ala (as in γ1, γ9, and γ11), and geranylgeranylated when X is a Leu (the rest of the nine γ subunits) (33Vogler O. Barcelo J.M. Ribas C. Escriba P.V. Membrane interactions of G proteins and other related proteins.Biochim. Biophys. Acta. 2008; 1778: 1640-1652Crossref PubMed Scopus (0) Google Scholar). The last three residues (aaX) of prenylated γ are proteolytically cleaved off by an endoprotease; Ras converting CaaX endopeptidase, and subsequently the prenyl Cys is carboxy methylated by a methyltransferase, isoprenyl-cysteine carboxyl methyl transferase (26Wedegaertner P.B. Wilson P.T. Bourne H.R. Lipid modifications of trimeric G proteins.J. Biol. Chem. 1995; 270: 503-506Abstract Full Text Full Text PDF PubMed Scopus (394) Google Scholar, 34Gao J. Liao J. Yang G.Y. CAAX-box protein, prenylation process and carcinogenesis.Am. J. Transl Res. 2009; 1: 312-325PubMed Google Scholar). In contrast to prenylation which is restricted to a small set of proteins and is retained through the life of the modified proteins, phosphorylation is ubiquitous and transient. Phosphorylation of γ subunits was shown to occur in the case of γ12, and the results suggested a role for the phosphorylation in effector regulation in specific G protein pathways (18Morishita R. Nakayama H. Isobe T. Matsuda T. Hashimoto Y. Okano T. et al.Primary structure of a gamma subunit of G protein, gamma 12, and its phosphorylation by protein kinase C.J. Biol. Chem. 1995; 270: 29469-29475Abstract Full Text Full Text PDF PubMed Scopus (79) Google Scholar, 35Yasuda H. Lindorfer M.A. Myung C.S. Garrison J.C. Phosphorylation of the G protein gamma12 subunit regulates effector specificity.J. Biol. Chem. 1998; 273: 21958-21965Abstract Full Text Full Text PDF PubMed Scopus (45) Google Scholar, 36Ueda H. Yamauchi J. Itoh H. Morishita R. Kaziro Y. Kato K. et al.Phosphorylation of F-actin-associating G protein gamma12 subunit enhances fibroblast motility.J. Biol. Chem. 1999; 274: 12124-12128Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar). More recently, phosphorylation of the yeast γ subunit has been shown to be essential for downstream signaling activity (37Nassiri Toosi Z. Su X. Austin R. Choudhury S. Li W. Pang Y.T. et al.Combinatorial phosphorylation modulates the structure and function of the G protein gamma subunit in yeast.Sci. Signal. 2021; 14eabd2464Crossref PubMed Scopus (3) Google Scholar). An examination of the sequences of γ subunits has shown that eight of the subunits contain putative phosphorylation sites in the N-terminal 14 residues (38Chakravorty D. Assmann S.M. G protein subunit phosphorylation as a regulatory mechanism in heterotrimeric G protein signaling in mammals, yeast, and plants.Biochem. J. 2018; 475: 3331-3357Crossref PubMed Scopus (0) Google Scholar). In the future it will become clearer whether these sites are phosphorylated, and it is a general theme in regulating the activity of these subunits. Since there is a possibility that βγ complexes made up of different combinations of β and γ subunit types could have distinct functions, it was important to determine the rules for the association of various β and γ subunit types. Do all β subunits associate with all γ subunits or is there a selective association? Such selectivity would suggest that even if a cell expresses many subunit types, only certain βγ complexes are possible. A variety of experimental methods showed that associations between β and γ subunit types were selective (39Pronin A.N. Gautam N. Interaction between G-protein beta and gamma subunit types is selective.Proc. Natl. Acad. Sci. U. S. A. 1992; 89: 6220-6224Crossref PubMed Scopus (0) Google Scholar, 40Schmidt C.J. Thomas T.C. Levine M.A. Neer E.J. Specificity of G protein beta and gamma subunit interactions.J. Biol. Chem. 1992; 267: 13807-13810Abstract Full Text PDF PubMed Google Scholar, 41Yan K. Kalyanaraman V. Gautam N. Differential ability to form the G protein betagamma complex among members of the beta and gamma subunit families.J. Biol. Chem. 1996; 271: 7141-7146Abstract Full Text Full Text PDF PubMed Scopus (84) Google Scholar, 42Dingus J. Wells C.A. Campbell L. Cleator J.H. Robinson K. Hildebrandt J.D. G Protein betagamma dimer formation: gbeta and Ggamma differentially determine efficiency of in vitro dimer formation.Biochemistry. 2005; 44: 11882-11890Crossref PubMed Scopus (0) Google Scholar). Importantly, purifying native βγ complexes from tissues has confirmed selective association between β and γ subtypes (43Asano T. Morishita R. Ueda H. Kato K. Selective association of G protein beta(4) with gamma(5) and gamma(12) subunits in bovine tissues.J. Biol. Chem. 1999; 274: 21425-21429Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar). When individual G protein heterotrimers based on α subunit identity from different tissues were examined, they were found to contain different γ subunits suggesting again that the γ subunit types play different roles (14Gautam N. Northup J. Tamir H. Simon M.I. G protein diversity is increased by associations with a variety of gamma subunits.Proc. Natl. Acad. Sci. U. S. A. 1990; 87: 7973-7977Crossref PubMed Google Scholar, 44Hildebrandt J.D. Codina J. Rosenthal W. Birnbaumer L. Neer E.J. Yamazaki A. et al.Characterization by two-dimensional peptide mapping of the gamma subunits of Ns and Ni, the regulatory proteins of adenylyl cyclase, and of transducin, the guanine nucleotide-binding protein of rod outer segments of the eye.J. Biol. Chem. 1985; 260: 14867-14872Abstract Full Text PDF PubMed Google Scholar, 45Tamir H. Fawzi A.B. Tamir A. Evans T. Northup J.K. G-Protein beta gamma forms: identity of beta and diversity of gamma subunits.Biochemistry. 1991; 30: 3929-3936Crossref PubMed Google Scholar, 46Wilcox M.D. Dingus J. Balcueva E.A. McIntire W.E. Mehta N.D. Schey K.L. et al.Bovine brain GO isoforms have distinct gamma subunit compositions.J. Biol. Chem. 1995; 270: 4189-4192Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar, 47Richardson M. Robishaw J.D. The alpha2A-adrenergic receptor discriminates between Gi heterotrimers of different betagamma subunit composition in Sf9 insect cell membranes.J. Biol. Chem. 1999; 274: 13525-13533Abstract Full Text Full Text PDF PubMed Scopus (75) Google Scholar). Knocking down individual γ subtypes in a cell line with antisense oligonucleotides provided support for such specific roles by selectively affecting distinct signaling pathways (48Kleuss C. Scherubl H. Hescheler J. Schultz G. Wittig B. Selectivity in signal transduction determined by gamma subunits of heterotrimeric G proteins.Science. 1993; 259: 832-834Crossref PubMed Google Scholar). After the early mapping of the mouse genes and the elucidation of the structure of a γ subunit gene (49Downes G.B. Copeland N.G. Jenkins N.A. Gautam N. Structure and mapping of the G protein gamma3 subunit gene and a divergently transcribed novel gene, gng3lg.Genomics. 1998; 53: 220-230Crossref PubMed Scopus (0) Google Scholar), the genomics of γ subunits is now comprehensive in both mouse and human (Tables 1 and 2). The earlier studies showed that genes for two subunits γ1 and γ11 which are closely related by homology are arranged together in a head to tail orientation suggesting that they may have arisen as a result of gene duplication, and the γ3 gene is also in a head to tail orientation with a gene Gng3lg (20Downes G.B. Gautam N. The G protein subunit gene families.Genomics. 1999; 62: 544-552Crossref PubMed Scopus (228) Google Scholar). This gene was later named in humans as BSCL2, and mutations in this gene are associated with congenital lipodystrophy, Berardinelli–Seip syndrome (50Magre J. Delepine M. Khallouf E. Gedde-Dahl Jr., T. Van Maldergem L. Sobel E. et al.Identification of the gene altered in Berardinelli-Seip congenital lipodystrophy on chromosome 11q13.Nat. Genet. 2001; 28: 365-370Crossref PubMed Scopus (567) Google Scholar).Table 1Mouse γ subunit genesaData adapted from database resources of the National Center for Biotechnology Information.Gene symbolGene idChromosome noNumber of exonsGNG114,69966GNG214,702148GNG314,704193GNG414,706135GNG514,70733GNG714,708107GNG814,70976GNG914,710117GNG1014,70043GNG1166,06662GNG1214,70166GNG1364,337174a Data adapted from database resources of the National Center for Biotechnology Information. Open table in a new tab Table 2Human Gγ subunit genesaData adapted from database resources of the National Center for Biotechnology Information.Gene symbolGene idChromosome noNumber of exonsGNG1279273GNG254,3311414GNG32785115GNG4278618GNG5278714GNG72788196GNG894,235195GNG92793175GNG10279093GNG11279172GNG1255,97017GNG1351,764163a Data adapted from database resources of the National Center for Biotechnology Information. Open table in a new tab There were suggestions that the specific role in signaling that γ subunit types play is through selective and direct interaction with receptors. Studies with purified proteins showed that the βγ complex was an obligatory requirement for receptor activation of the α subunit (1Gilman A.G. G proteins: transducers of receptor-generated signals.Annu. Rev. Biochem. 1987; 56: 615-649Crossref PubMed Google Scholar, 51Birnbaumer L. G proteins in signal transduction.Annu. Rev. Pharmacol. Toxicol. 1990; 30: 675-705Crossref PubMed Google Scholar). A set of results suggested that the γ subunit interaction with a receptor is a requirement for G protein activation. Peptides from the C-terminal domain of the γ1 subunit stabilized the photoactivated form of rhodopsin, and mutations in this region prevented heterotrimer activation by rhodopsin (52Kisselev O.G. Ermolaeva M.V. Gautam N. A farnesylated domain in the G-protein gamma-subunit is a specific determinant of receptor coupling.J. Biol. Chem. 1994; 269: 21399-21402Abstract Full Text PDF PubMed Google Scholar, 53Kisselev O. Pronin A. Ermolaeva M. Gautam N. Receptor-G protein coupling is established by a potential conformational switch in the beta-gamma complex.Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 9102-9106Crossref PubMed Scopus (0) Google Scholar). Consistent with these results, a conformational change in the C-terminal domain peptide of γ subunit when bound to light-activated rhodopsin was detected while the same peptide remained disordered in the presence of inactive dark-adapted rhodopsin (54Kisselev O.G. Downs M.A. Rhodopsin controls a conformational switch on the transducin gamma subunit.Structure. 2003; 11: 367-373Abstract Full Text Full Text PDF PubMed Scopus (62) Google Scholar). A geranylgeranylated peptide corresponding to the C terminus of γ5 subunit, but not γ7 or γ12 subunits were shown to inhibit M2 muscarinic receptor signaling, also indicating Gγ-receptor interactions (55Azpiazu I. Cruzblanca H. Li P. Linder M. Zhuo M. Gautam N. A G protein gamma subunit-specific peptide inhibits muscarinic receptor signaling.J. Biol. Chem. 1999; 274: 35305-35308Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar). This role for the γ subunit is also supported by findings that particular γ subunit types are more potent in supporting G protein activation by a receptor (47Richardson M. Robishaw J.D. The alpha2A-adrenergic receptor discriminates between Gi heterotrimers of different betagamma subunit composition in Sf9 insect cell membranes.J. Biol. Chem. 1999; 274: 13525-13533Abstract Full Text Full Text PDF PubMed Scopus (75) Google Scholar, 55Azpiazu I. Cruzblanca H. Li P. Linder M. Zhuo M. Gautam N. A G protein gamma subunit-specific peptide inhibits muscarinic receptor signaling.J. Biol. Chem. 1999; 274: 35305-35308Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar, 56Yasuda H. Lindorfer M.A. Woodfork K.A. Fletcher J.E. Garrison J.C. Role of the prenyl group on the G protein gamma subunit in coupling trimeric G proteins to A1 adenosine receptors.J. Biol. Chem. 1996; 271: 18588-18595Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar, 57Hou Y. Azpiazu I. Smrcka A. Gautam N. Selective role of G protein gamma subunits in receptor interaction.J. Biol. Chem. 2000; 275: 38961-38964Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar, 58Lim W.K. Myung C.S. Garrison J.C. Neubig R.R. Receptor-G protein gamma specificity: gamma11 shows unique potency for A(1) adenosine and 5-HT(1A) receptors.Biochemistry. 2001; 40: 10532-10541Crossref PubMed Scopus (0) Google Scholar). There are 20 available structures of the receptor–G protein heterotrimer, all of them containing γ2 with or without the prenylation site. Their PDB IDs and the particular receptor–G protein complex (Table 3). The structure of the complete C-terminal domain of the γ subunit is not clear in any of these structures likely due to the hypervariable nature of the C-terminal domain. Since structures of the GPCR-G protein complex capture frozen states of this dynamic interaction in a narrow time window, it is possible that they have not captured the states when direct interaction between the receptor and the γ subunit occurs. Consistent with this notion, recent modeling shows how the existent findings fit into a model of receptor–G protein interaction where the γ subunit tail interaction occurs transiently with an intracellular hydrophobic site in the receptor facilitating subsequent interaction with the α subunit (59McIntire W.E. A model for how Gbetagamma couples Galpha to GPCR.J. Gen. Physiol. 2022; 154e202112982Crossref PubMed Scopus (1) Google Scholar). Structures in the future that capture transient states of the receptor–G protein complex after activation can more directly address questions about the interaction of the γ subunit with the receptor.Table 3Cryo-EM and X-ray crystallographic structure information of different GPCR-G protein complexesaStructural data of different GPCR-G proteins adapted from Protein Data Bank.a Structural data of different GPCR-G proteins adapted from Protein Data Bank. Open table in a new tab Once it was determined that γ subunits are a family, their expression in mammalian tissues was examined (Table 4). There were early suggestions that γ subtypes are expressed selectively in mammalian tissue. When antisera specific to γ2 and γ3 subunits were used, they were detected in G protein heterotrimers purified from brain but not some other tissues (14Gautam N. Northup J. Tamir H. Simon M.I. G protein diversity is increased by associations with a variety of gamma subunits.Proc. Natl. Acad. Sci. U. S. A. 1990; 87: 7973-7977Crossref PubMed Google Scholar). The presence of γ2 and γ3 was further established when brain extracts were examined for G protein γ subunits. These studies showed that γ5, γ10, and γ11 were present in several different tissues, although γ5 and γ10 were barely detectable in brain (60Cali J.J. Balcueva E.A. Rybalkin I. Robishaw J.D. Selective tissue distribution of G protein gamma subunits, including a new form of the gamma subunits identified by cDNA cloning.J. Biol. Chem. 1992; 267: 24023-24027Abstract Full Text PDF PubMed Google Scholar, 61Asano T. Morishita R. Ohashi K. Nagahama M. Miyake T. Kato K. Localization of various forms of the gamma subunit of G protein in neural and nonneural tissues.J. Neurochem. 1995; 64: 1267-1273Crossref PubMed Google Scholar, 62Morishita R. Ueda H. Kato K. Asano T. Identification of two forms of the gamma subunit of G protein, gamma10 and gamma11, in bovine lung and their tissue distribution in the rat.FEBS Lett. 1998; 428: 85-88Crossref PubMed Scopus (0) Google Scholar). This selectivity in mammalian tissues was an indication that they have distinct roles.Table 4Genomic location and tissue-specific expression of human Gγ subunitsaGenomic data adapt
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