Differential Contribution of GTPase Activation and Effector Antagonism to the Inhibitory Effect of RGS Proteins on Gq-mediated Signaling in Vivo

Abstract

RGS proteins act as negative regulators of G protein signaling by serving as GTPase-activating proteins (GAP) for α subunits of heterotrimeric G proteins (Gα), thereby accelerating G protein inactivation. RGS proteins can also block Gα-mediated signal production by competing with downstream effectors for Gα binding. Little is known about the relative contribution of GAP and effector antagonism to the inhibitory effect of RGS proteins on G protein-mediated signaling. By comparing the inhibitory effect of RGS2, RGS3, RGS5, and RGS16 on Gαq-mediated phospholipase Cβ (PLCβ) activation under conditions where GTPase activation is possible versus nonexistent, we demonstrate that members of the R4 RGS subfamily differ significantly in their dependence on GTPase acceleration. COS-7 cells were transiently transfected with either muscarinic M3 receptors, which couple to endogenous Gq protein and mediate a stimulatory effect of carbachol on PLCβ, or constitutively active Gαq*, which is inert to GTP hydrolysis and activates PLCβ independent of receptor activation. In M3-expressing cells, all of the RGS proteins significantly blunted the efficacy and potency of carbachol. In contrast, Gαq* -induced PLCβ activation was inhibited by RGS2 and RGS3 but not RGS5 and RGS16. The observed differential effects were not due to changes in M3, Gαq/Gαq*, PLCβ, or RGS expression, as shown by receptor binding assays and Western blots. We conclude that closely related R4 RGS family members differ in their mechanism of action. RGS5 and RGS16 appear to depend on G protein inactivation, whereas GAP-independent mechanisms (such as effector antagonism) are sufficient to mediate the inhibitory effect of RGS2 and RGS3. RGS proteins act as negative regulators of G protein signaling by serving as GTPase-activating proteins (GAP) for α subunits of heterotrimeric G proteins (Gα), thereby accelerating G protein inactivation. RGS proteins can also block Gα-mediated signal production by competing with downstream effectors for Gα binding. Little is known about the relative contribution of GAP and effector antagonism to the inhibitory effect of RGS proteins on G protein-mediated signaling. By comparing the inhibitory effect of RGS2, RGS3, RGS5, and RGS16 on Gαq-mediated phospholipase Cβ (PLCβ) activation under conditions where GTPase activation is possible versus nonexistent, we demonstrate that members of the R4 RGS subfamily differ significantly in their dependence on GTPase acceleration. COS-7 cells were transiently transfected with either muscarinic M3 receptors, which couple to endogenous Gq protein and mediate a stimulatory effect of carbachol on PLCβ, or constitutively active Gαq*, which is inert to GTP hydrolysis and activates PLCβ independent of receptor activation. In M3-expressing cells, all of the RGS proteins significantly blunted the efficacy and potency of carbachol. In contrast, Gαq* -induced PLCβ activation was inhibited by RGS2 and RGS3 but not RGS5 and RGS16. The observed differential effects were not due to changes in M3, Gαq/Gαq*, PLCβ, or RGS expression, as shown by receptor binding assays and Western blots. We conclude that closely related R4 RGS family members differ in their mechanism of action. RGS5 and RGS16 appear to depend on G protein inactivation, whereas GAP-independent mechanisms (such as effector antagonism) are sufficient to mediate the inhibitory effect of RGS2 and RGS3. Many extracellular stimuli elicit intracellular responses by activating seven-transmembrane receptors that are coupled to heterotrimeric G proteins comprising α and βγ subunits (1Neer E.J. Cell. 1995; 80: 249-257Abstract Full Text PDF PubMed Scopus (1287) Google Scholar, 2Gilman A.G. Biosci. Rep. 1995; 15: 65-97Crossref PubMed Scopus (175) Google Scholar). The duration of the response of a cell to external signals is largely determined by the activation/inactivation cycle of G proteins. Activated receptors trigger GTP-for-GDP exchange on Gα subunits, thus dissociating Gα from Gβ γ and subsequent activation of downstream effectors (such as enzymes and ion channels). The duration of G protein activation is limited by GTPase activity intrinsic to Gα subunits that catalyzes the conversion of active GTP-bound Gα into inactive GDP-bound Gα, which in turn can reassociate with Gβγ and receptors. RGS proteins belong to a family of more than 20 proteins with a conserved RGS core domain of ∼120 amino acids that is necessary and sufficient for binding to Gα subunits (3Hollinger S. Hepler J.R. Pharmacol. Rev. 2002; 54: 527-559Crossref PubMed Scopus (599) Google Scholar). They are divided into several subfamilies based on their structural similarities, gene organization, and function. RGS proteins act as regulators of G protein signaling by limiting the signals generated by G protein-coupled receptors. They markedly increase the rate at which Gα subunits hydrolyze GTP to GDP, a property that defines them as GTPase-activating proteins (or GAPs) 1The abbreviations used are: GAPGTPase-activating proteinPLCphospholipase CHAhemagglutininPBSphosphate-buffered salineGTPγSguanosine 5′-O-(3-thiotriphosphate).1The abbreviations used are: GAPGTPase-activating proteinPLCphospholipase CHAhemagglutininPBSphosphate-buffered salineGTPγSguanosine 5′-O-(3-thiotriphosphate). (4Ross E.M. Wilkie T.M. Annu. Rev. Biochem. 2000; 69: 795-827Crossref PubMed Scopus (926) Google Scholar). Hastening of Gα inactivation facilitates the reassociation of Gα with Gβγ subunits. As a result, RGS proteins inhibit both Gα- and Gβγ-mediated downstream effects. GAP effects have been described for virtually all proteins featuring an RGS core domain (5Wieland T. Mittmann C. Pharmacol. Ther. 2003; 97: 95-115Crossref PubMed Scopus (114) Google Scholar). Some RGS proteins can also diminish Gα-mediated signal generation by functionally inhibiting Gα-effector coupling (so-called effector antagonism) (6Hepler J.R. Berman D.M. Gilman A.G. Kozasa T. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 428-432Crossref PubMed Scopus (335) Google Scholar, 7Yan Y. Chi P.P. Bourne H.R. J. Biol. Chem. 1997; 272: 11924-11927Abstract Full Text Full Text PDF PubMed Scopus (126) Google Scholar). The absolute requirement and relative significance of the GAP effect and GAP-independent mechanisms for negative regulation of G protein-mediated signaling pathways by RGS proteins in mammalian cells are still poorly understood. GTPase-activating protein phospholipase C hemagglutinin phosphate-buffered saline guanosine 5′-O-(3-thiotriphosphate). GTPase-activating protein phospholipase C hemagglutinin phosphate-buffered saline guanosine 5′-O-(3-thiotriphosphate). In the present study, we compared the effect of RGS proteins in a setting where they can exert GAP function with one where GAP function cannot be elicited to test the following hypotheses: (i) that RGS proteins do not necessarily depend on their GAP function to modulate mammalian cell signaling and (ii) that the relative contribution of their GAP and effector antagonistic effects differs among different RGS proteins. We examined several members of the R4 RGS protein subfamily (RGS2, RGS3, RGS5, and RGS16), because they are primarily composed of the RGS core domain and have the shortest N- and C-terminal flanking regions among the entire RGS family. Other RGS subfamilies contain longer N- and C-terminal sequences with various additional structural and functional domains that serve as binding sites for other proteins and impart subfamily specific activities, subcellular localization, or regulation (4Ross E.M. Wilkie T.M. Annu. Rev. Biochem. 2000; 69: 795-827Crossref PubMed Scopus (926) Google Scholar). Because RGS3 has a much longer N terminus than all of the other R4 subfamily members (8Kehrl J.H. Srikumar D. Harrison K. Wilson 2002; PubMed Scopus Google we an RGS3 with a flanking in to other R4 RGS in K. J. 2000; PubMed Scopus Google Scholar). R4 RGS family members with Gq and proteins in and their inactivation (4Ross E.M. Wilkie T.M. Annu. Rev. Biochem. 2000; 69: 795-827Crossref PubMed Scopus (926) Google for of and signaling has been in mammalian cells T. Annu. Rev. Pharmacol. 2000; PubMed Scopus Google Scholar). We PLCβ activation as for the present study, because effector antagonism has been described as a mechanism of for of Gq signaling by RGS proteins (6Hepler J.R. Berman D.M. Gilman A.G. Kozasa T. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 428-432Crossref PubMed Scopus (335) Google Scholar, 7Yan Y. Chi P.P. Bourne H.R. J. Biol. Chem. 1997; 272: 11924-11927Abstract Full Text Full Text PDF PubMed Scopus (126) Google Scholar, Hepler J.R. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: PubMed Scopus Google Scholar). COS-7 cells were transiently transfected with either muscarinic M3 receptors or constitutively active Gαq* M3 receptors were to couple to which is to the activation/inactivation cycle for heterotrimeric G proteins. In contrast, Gαq* is inert to GTP hydrolysis and not depend on receptor activation to PLCβ S. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). the inhibitory effect of RGS protein in both about the functional of GTPase activation for the inhibitory effect of RGS proteins on signaling in We demonstrate that R4 RGS proteins differ in their dependence on GAP activation as a mechanism of action. RGS5 and RGS16 not exert inhibitory effect they were to act as a whereas RGS2 and RGS3 markedly blunted signaling in the of that other as effector are sufficient to mediate their inhibitory of for constitutively active G protein α U. A. A. J. E.J. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google was into RGS proteins were at their N or C terminus with a by RGS2 Hepler J.R. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: PubMed Scopus Google K. J. 2000; PubMed Scopus Google RGS3 and J.H. PubMed Scopus Google RGS5 and RGS16 T. E.J. PubMed Scopus Google were used as The were into with of the sequences were by The muscarinic M3 receptor and as as were from the and J. and cells were in with and They were transiently transfected for with and at an The of transfected in for was used to the of transfected and cells were and the proteins were as described E.J. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). the cells on were for in and by in the protein for The cells were in and for at in a The were with protein for and for The were with a at by the of of protein in for at The were with and with The proteins were with and by protein were examined by Western cells were in and in and for on a at cell were on and to was used to and The were in and with and in 20 and with the proteins of were with of activity was by in U. A. A. J. E.J. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). the cells were in with The was the of carbachol at the were in 20 by exchange and in a and protein were for of the different The are as to the protein in not and the COS-7 cells were with or for of was as described the of the in in was and cells were and their and were by for at of protein of were to a of and of that been in 20 for at at was and the were at for and by by of the cells were with and for at with in the or of to binding. The binding was by the and the with by cell in and the of was determined by and to the of protein present protein binding assays were by binding to the of binding sites and binding the for at independent the were by was used to by and to for effect and in the or of was was to the inhibitory effect of R4 RGS proteins on Gαq-mediated PLCβ activation under conditions where GTPase activation is possible versus was by transiently COS-7 cells with either muscarinic M3 receptors or constitutively active Gαq* in the or of RGS2, RGS3, RGS5, or RGS16. the different and for them in the present (i) In COS-7 cells PLCβ activation in response to receptor activation with carbachol was to be by endogenous proteins. COS-7 cells were to the contribution of Gβγ protein activation. COS-7 cells and which are to but to Gβγ Rev. 2000; 80: PubMed Scopus Google Scholar, Annu. Rev. Biochem. PubMed Scopus Google Scholar). an that is to is not D. A. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). endogenous Gq proteins exchange for GTP whereas inactivation GTP is to RGS proteins exert an inhibitory effect on Gq signaling by as effector or (ii) In contrast, in Gαq* COS-7 cells PLCβ was to be by constitutively active We used Gαq* to from endogenous The of amino acids not with Gαq* PLCβ activation Bourne H.R. J. Biol. Chem. 1995; Full Text Full Text PDF PubMed Scopus Google Scholar). Gαq* is inert to GTP hydrolysis S. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google and not to GTPase by RGS proteins D.M. Wilkie T.M. Gilman A.G. Cell. Full Text Full Text PDF PubMed Scopus Google Scholar). were in setting to the and RGS proteins a of were transfected for Gαq* and RGS and the of endogenous for PLCβ activation was by response for the muscarinic receptor carbachol. their relative all of the RGS proteins were at their N The in function of several R4 RGS proteins to be by the of an D.M. Wilkie T.M. Gilman A.G. Cell. Full Text Full Text PDF PubMed Scopus Google Scholar, C. J. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google because the N terminus of R4 RGS proteins has structural for subcellular (5Wieland T. Mittmann C. Pharmacol. Ther. 2003; 97: 95-115Crossref PubMed Scopus (114) Google we RGS proteins and as of from COS-7 cells and Western of the and a increase in RGS protein with of RGS The among the different RGS proteins. of RGS protein was cells that were transfected with the M3 receptor and transfected with Gαq* that functional and Gαq* cells cannot be to a in their of RGS the inhibitory effects of RGS protein in into the functional of GTPase activation and effector antagonism for the inhibitory effect of R4 RGS proteins on signaling in mammalian the effect of RGS proteins in a mammalian where they can act as both and effector COS-7 cells were transiently transfected with muscarinic M3 receptors, and production was in response to carbachol that M3 receptor was for activation. The of endogenous muscarinic receptors (such as M3, and in COS-7 cells was by sites in cells not The of endogenous Gq and PLCβ was sufficient to couple transfected M3 receptors to downstream signaling because the stimulatory effect of carbachol on PLCβ in M3-expressing cells was not by of coupling of transfected M3 receptors with Gq was by of PLCβ activation to of of COS-7 cells with members of the G protein family (such as but not by at the C-terminal in demonstrate that under the conditions G proteins in in was Gα subunits were in COS-7 cells that been with of proteins to PLCβ which be their βγ subunits Annu. Rev. Biochem. PubMed Scopus Google was by a of effect of of a C-terminal of receptor not a protein used as a Gβγ J. J. Biol. Chem. Full Text PDF PubMed Google Scholar). of the M3 receptor was sufficient to PLCβ activation in COS-7 that was by endogenous Gq protein α and as to the G protein activation/inactivation of RGS the activity of in the of RGS proteins The effect was observed at carbachol and to a The of carbachol was We compared PLCβ activation in the or of different RGS proteins of RGS2, RGS3 RGS5, and RGS16 a of the effect of carbachol as as a of the for carbachol The activation of PLCβ by carbachol was to of cells and with the for carbachol was by a of and all versus transfected COS-7 The inhibitory effect of RGS protein was a of stimulatory the M3 as shown by with of RGS that were at different carbachol for RGS protein that RGS protein of RGS RGS proteins the at the N or C terminus not that the not RGS protein effect of RGS proteins on Gαq-mediated PLCβ COS-7 cells were transiently transfected with M3 receptors of in a in the or of of RGS and that RGS5 was transfected at different production was at different of carbachol and The are in The the of or not changes in the of of the in the signaling to the observed receptor binding assays and Western were binding was the and the were by a binding as shown for COS-7 cells The of binding sites to protein with a of was not significantly of RGS2 of RGS3 RGS5 and RGS16 and in cells of was in of of the RGS proteins not Western in that the of the endogenous and was also largely by the or of M3 receptors or RGS proteins. is known to inhibit signaling in a of cell and T. Annu. Rev. Pharmacol. 2000; PubMed Scopus Google and we to RGS protein in in the of was and markedly of the M3 receptor and inhibitory effect of on activation was observed not In RGS2, RGS3 RGS5, and RGS16 inhibited PLCβ which not be to changes in the of of signaling the effect of the RGS proteins under conditions where they cannot inactivation, COS-7 cells were transiently transfected with constitutively active in the or of the different RGS proteins. we determined of Gαq* and RGS or in the of endogenous PLCβ their from COS-7 that were transfected with of Gαq* in the or of a of RGS and were not of Gαq* of the RGS proteins not the of Gαq* and RGS proteins was The of Gαq* was with RGS2, and RGS3, for which the effect was RGS2 and not were markedly in the of The of their to with the of Gαq* in the RGS2 In to RGS2 and RGS3, RGS5 and RGS16 were largely in the or of Gαq*, and Gαq* was not significantly by the of RGS5 or RGS16. of Gαq* with RGS2 or RGS3 not RGS5 and a increase in protein expression, which to be into for the of functional of RGS for a constitutively active Gαq* PLCβ in the of receptor the effector antagonistic effects of RGS2, RGS3, RGS5, and of were (i) of Gαq* in the of a of RGS protein and (ii) of RGS protein in the of a of Gαq* different RGS2 and RGS3 the Gαq* to the with an increase in compared with RGS2 and both of RGS3 blunted the stimulatory effect of Gαq* on PLCβ inhibitory effect was much more in cells Gαq* versus in which their effect to and of Gαq* and versus In to RGS2 and RGS3, RGS5 and RGS16 not significantly Gαq* -induced PLCβ activation under either that were sufficient to an inhibitory effect on Gαq-mediated PLCβ activation RGS5 and RGS16 were to exert an inhibitory effect in a setting where they not act as The of is that members of the R4 RGS family differ in their dependence on GTPase as a mechanism of for their negative regulation of Gαq-mediated signaling in RGS2 and RGS3 markedly inhibited PLCβ activation in the of GTPase that GAP-independent (such as effector antagonism) are sufficient to mediate their inhibitory In contrast, RGS5 and RGS16 were to exert an inhibitory effect on Gαq* -induced PLCβ in a setting where they not exert GAP several members of the R4 RGS we the that RGS proteins signaling do not necessarily depend on their GAP function to modulate cell signaling in and that the relative contribution of their GAP effects and effector antagonism differs among RGS proteins. RGS of GAP activity not be to all RGS described have significantly Gα binding J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar, J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar, J. Biol. Chem. 2003; Full Text Full Text PDF PubMed Scopus Google which in turn not their GAP activity but also effector antagonistic effects. we to of a in Gαq* that GTPase activity and thereby constitutively active S. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). PLCβ and RGS proteins to as as Gα subunits. constitutively active Gα subunits do not GTPase function in the of a GAP D.M. Wilkie T.M. Gilman A.G. Cell. Full Text Full Text PDF PubMed Scopus Google Scholar). The sites of PLCβ on have been to and in the domain of S. C. K. 1995; PubMed Scopus Google that the in the of Gαq* is to markedly binding to the sites of RGS on Gα are in the regions Berman D.M. Gilman A.G. Cell. 1997; Full Text Full Text PDF PubMed Scopus Google and the in binding of RGS proteins for the of is than that for constitutively active (4Ross E.M. Wilkie T.M. Annu. Rev. Biochem. 2000; 69: 795-827Crossref PubMed Scopus (926) Google Scholar). in the binding of RGS proteins for and Gαq* in can not be their contribution to the and RGS proteins were a of of of of the PLCβ of all PLCβ signaling endogenous and were for from both to their to the observed changes in PLCβ activity in the of a RGS In the COS-7 endogenous and M3 receptor were largely The observed in M3 receptor in cells was and is to be a to the of PLCβ activity in the of The of and was not in of the conditions which is of for the of the because PLCβ is known to act as a GAP on J.H. E.M. Cell. Full Text PDF PubMed Scopus Google Scholar, E.M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar, 2002; 54: PubMed Scopus Google Scholar). the of transfected RGS proteins was not significantly of the M3 receptor was a increase in RGS2 and in cells with PLCβ activation effect was also observed in Gαq* cells not and is not limited to COS-7 Little is known about the regulation of expression, but RGS2 has been to be in different cell in response to a of stimuli J.H. S. J. Biochem. Biol. 2002; PubMed Scopus Google signaling PubMed Scopus Google Scholar). do not on regulation of endogenous and the of endogenous not be in because cannot be from Gαq* by the that RGS2 and but of the other RGS proteins by the were in their in the of Gαq* possible and protein which was for other RGS proteins T. S. PubMed Scopus Google Scholar, Y. D. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). Because RGS2 and are of Gαq* PLCβ their as a negative mechanism the cell to PLCβ activation. and with Gαq* was in the of RGS proteins of effect RGS2, and RGS3, for which the effect was a and RGS proteins that modulate is to the mechanism the increase in and Gαq* as as the differential regulation of the RGS3 RGS2 and RGS3 in the of GTPase and RGS3 were of markedly inhibiting PLCβ activity in response to with carbachol or constitutively active Gαq* independent of their to act as We as an that mechanisms other than G protein inactivation effector antagonism) are sufficient to mediate their inhibitory effects. RGS2 was to be more as a GAP than effector in S. Hepler J.R. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). The of signaling cannot be in because protein can be as in the present We the of Gαq*, and PLCβ by and that RGS2 and both of RGS3 markedly inhibited Gαq* -induced PLCβ activation a of that they are of Gq signaling in mammalian cells in in the of GTPase The the RGS2 or RGS3 and Gαq*, the was their inhibitory with effector inhibitory effect of RGS2 and RGS3 on PLCβ, to the RGS2 and S. S. J.H. J. Biol. Chem. 2003; Full Text Full Text PDF PubMed Scopus Google is because be to a in PLCβ activity that was not GAP-independent mechanisms (such as effector antagonism) to the inhibitory effect of RGS2 and RGS3 GTPase is possible (such as in the M3 cannot be from As as they are to they be to have the to functionally inhibit the for effector antagonistic (or other effects are much in M3-expressing cells because of the of GTP hydrolysis by RGS proteins. was not the of to the of among different RGS proteins on the relative of RGS protein be a expression, RGS2 a inhibitory with RGS2 proteins for Gq signaling Hepler J.R. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). also inhibited PLCβ activation in M3-expressing cells and In was much than that of RGS3, whereas the inhibitory effect was with the that the C-terminal of RGS3 structural domains for signal J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus (114) Google Scholar, A. S. J.H. Pharmacol. 2000; PubMed Scopus Google Scholar). C-terminal RGS3 can with Gβγ and inhibit Gβγ-mediated PLCβ activation S. J.H. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). is to a in study, because PLCβ activation is not by the α of Gq in the used RGS5 and RGS16 GTPase to with that RGS5 and RGS16 can to and GTPase activity in and modulate signaling in S. Y. J. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: PubMed Scopus Google Scholar, J. K. J. A. S. Sci. PubMed Scopus Google Scholar, Kozasa T. C. C. J.H. J. 2003; PubMed Scopus Google both RGS5 and RGS16 an inhibitory effect on PLCβ activation in M3-expressing COS-7 cells RGS5 was shown not to be in the negative regulation of M3 receptor signaling in cells J. Biol. Chem. 2002; Full Text Full Text PDF PubMed Scopus Google Scholar). among cell and of RGS5 versus of endogenous the of of RGS5 in signaling. The of the present is that the inhibitory effects of both RGS5 and RGS16 were in cells Gαq* sufficient to inhibit PLCβ activation that to RGS2 and RGS5 and RGS16 depend on their GAP function to exert an inhibitory effect on signaling. of the RGS domains of RGS2 and RGS3 with of RGS5 and RGS16 not that their differential Berman D.M. Gilman A.G. Cell. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar, A. S. J.H. Pharmacol. 2000; PubMed Scopus Google Scholar). the to the RGS core domain in RGS3 is different in and from all other R4 RGS RGS2 is in to RGS5 and an domain J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar, C. K. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). the N of RGS2 and differ in their C. J. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google and the of sites J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). 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As RGS proteins to Gα, they have the to act as GTPase-activating proteins as effector that RGS2 and RGS3, but not RGS5 and are of inhibiting Gαq-mediated PLCβ activation in the of GTPase acceleration. on the relative contribution of and mechanisms of to the inhibitory effect of RGS proteins cannot be from study, functional among closely related R4 RGS family members with structural is to the which in the RGS G and effectors and in their receptor or function. As more are of RGS with mechanisms by are (i) G protein inactivation is also a for the inhibitory effect of RGS5 and RGS16 on signaling C. J. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar, S. Y. J. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: PubMed Scopus Google Scholar, Kozasa T. C. C. J.H. J. 2003; PubMed Scopus Google and (ii) a differential in RGS function can be members of other RGS We for of the