Mammalian RGS Proteins: Barbarians at the Gate

Hundreds or thousands of chemical and physical stimuli regulate the functions of eukaryotic cells by controlling the activities of a surprisingly small number of core signaling units that have been duplicated and adapted to achieve the necessary diversity. The most prevalent of these units, at least in animal cells, are three-protein modules consisting of signal recognition elements (receptors) and signal generators (effectors) whose activities are linked and coordinated by heterotrimeric guanine nucleotide-binding proteins or G proteins. 1The abbreviations used are: G protein, heterotrimeric guanine nucleotide-binding regulatory protein; GAP, GTPase-activating protein; RGS, regulator of G protein signaling; GTPγS, guanosine 5′-3-O-(thio)triphosphate; GAIP, Gα-interacting protein, an RGS protein. 1The abbreviations used are: G protein, heterotrimeric guanine nucleotide-binding regulatory protein; GAP, GTPase-activating protein; RGS, regulator of G protein signaling; GTPγS, guanosine 5′-3-O-(thio)triphosphate; GAIP, Gα-interacting protein, an RGS protein.Collectively, mammalian cells contain hundreds of G protein-coupled receptors and dozens of effectors. It is difficult to count functionally distinct G proteins because we do not understand the significance of the heterogeneity offered by the possible combination of 16 α, 5 β, and at least 12 γ subunits (for reviews, see Refs.1Gilman A.G. Annu. Rev. Biochem. 1987; 56: 615-649Crossref PubMed Scopus (4682) Google Scholar, 2Bourne H.R. Sanders D.A. McCormick F. Nature. 1990; 348: 125-132Crossref PubMed Scopus (1825) Google Scholar, 3Simon M.I. Strathmann M.P. Gautam N. Science. 1991; 252: 802-808Crossref PubMed Scopus (1576) Google Scholar, 4Hepler J.R. Gilman A.G. Trends Biochem. Sci. 1992; 17: 383-387Abstract Full Text PDF PubMed Scopus (919) Google Scholar, 5Neer E.J. Cell. 1995; 80: 249-257Abstract Full Text PDF PubMed Scopus (1283) Google Scholar).GDP-bound G protein α subunits have high affinity for a tight complex of β and γ subunits. This interaction of α with βγ occludes the sites of interaction of both of these signaling molecules with downstream effectors, and the inactive state is maintained by an extremely slow rate of dissociation of GDP from the oligomer (k ∼ 0.01/min). An agonist-bound receptor (typically a 35–60-kDa protein with seven plasma membrane-spanning helices) activates an appropriate G protein by poorly understood interactions that promote dissociation of GDP. High intracellular concentrations of GTP ensure a transient existence of the nucleotide-free G protein, and binding of GTP causes conformational changes in α that result in dissociation of GTP-α from βγ. Both of these complexes can then activate or inhibit signaling pathways by engaging in interactions with effectors such as adenylyl cyclases, phospholipases, cyclic nucleotide phosphodiesterases, and ion channels. Termination of signaling is dependent on the GTPase activity of α. Typically slow (k cat ∼ 4/min) hydrolysis of GTP to GDP (which remains protein bound) promotes dissociation of α from effectors and reassociation with βγ.The slow intrinsic rate of GTP hydrolysis by Gα proteins is regulated by interactions with so-called GTPase-activating proteins or GAPs. GAPs were first recognized as regulators of protein synthesis factors and low molecular weight GTPases such as Ras. It is now appreciated that certain effectors in G protein-regulated pathways act as GAPs on cognate Gα proteins (6Berstein G. Blank J.L. Jhon D.Y. Exton J.H. Rhee S.G. Ross E.M. Cell. 1992; 70: 411-418Abstract Full Text PDF PubMed Scopus (346) Google Scholar, 7Arshavsky V.Y. Bownds M.D. Nature. 1992; 357: 416-417Crossref PubMed Scopus (218) Google Scholar) and that there exists a large, newly discovered family of GAPs for Gα proteins known asregulators of G protein signaling or RGS proteins. Although one critical biochemical property of this novel RGS protein family has been defined, knowledge of the requisite regulation of these regulators is negligible. There are hints, however, that these proteins may be poised at centers of signaling to intercept activated G proteins, acting, from a G protein's point of view, as "barbarians at the gate" of cellular signaling.The RGS Protein FamilyRGS proteins were discovered functionally as negative regulators of G protein signaling in Saccharomyces cerevisiae (Sst2p) (reviewed in Ref. 8Dohlman H.G. Thorner J. J. Biol. Chem. 1997; 272: 3871-3874Abstract Full Text Full Text PDF PubMed Scopus (447) Google Scholar) and Caenorhabditis elegans (EGL10) (9Koelle M.R. Horvitz H.R. Cell. 1996; 84: 115-125Abstract Full Text Full Text PDF PubMed Scopus (475) Google Scholar). This information converged quickly with demonstrations of interaction of a mammalian RGS protein with Gαi3 in a two-hybrid screen (10De Vries L. Mousli M. Wurmser A. Farquhar M.G. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 11916-11920Crossref PubMed Scopus (264) Google Scholar); induction of related messages by mitogenic stimuli in human B (11Newton J.S. Deed R.W. Mitchell E.L.D. Murphy J.J. Norton J.D. Biochim. Biophys. Acta. 1993; 1216: 314-316Crossref PubMed Scopus (29) Google Scholar, 12Hong J.X. Wilson G.L. Fox C.H. Kehrl J.H. J. Immunol. 1993; 150: 3895-3904PubMed Google Scholar) and T (13Siderovski D.P. Heximer S.P. Forsdyke D.R. Mol. Cell. Biol. 1994; 13: 125-147Google Scholar) cells; and identification of related sequences by data base searches, application of polymerase chain reaction technology, rescue of the Sst2p-deficient phenotype, and homology-based screening (9Koelle M.R. Horvitz H.R. Cell. 1996; 84: 115-125Abstract Full Text Full Text PDF PubMed Scopus (475) Google Scholar, 14Druey K.M. Blumer K.J. Kang V.H. Kehrl J.H. Nature. 1996; 379: 742-746Crossref PubMed Scopus (404) Google Scholar, 15Chen C.K. Wieland T. Simon M.I. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 12885-12889Crossref PubMed Scopus (124) Google Scholar, 16Chen C. Zheng B. Han J. Lin S.-C. J. Biol. Chem. 1997; 272: 8679-8685Abstract Full Text Full Text PDF PubMed Scopus (142) Google Scholar, 17Faurobert E. Hurley J.B. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 2945-2950Crossref PubMed Scopus (93) Google Scholar, 18Snow B.E. Antonio L. Suggs S. Gutstein H.B. Siderovski D.P. Biochem. Biophys. Res. Commun. 1997; 233: 770-777Crossref PubMed Scopus (98) Google Scholar). These developments have been reviewed previously by others (8Dohlman H.G. Thorner J. J. Biol. Chem. 1997; 272: 3871-3874Abstract Full Text Full Text PDF PubMed Scopus (447) Google Scholar, 19Siderovski D.P. Hessel A. Chung S. Mak T.W. Tyers M. Curr. Biol. 1996; 6: 211-212Abstract Full Text Full Text PDF PubMed Scopus (175) Google Scholar, 20Neer E.J. Curr. Biol. 1997; 7: R31-R33Abstract Full Text Full Text PDF PubMed Google Scholar, 21Koelle M.R. Curr. Opin. Cell Biol. 1997; 9: 143-147Crossref PubMed Scopus (172) Google Scholar). To date, 19 mammalian genes are known to encode proteins that contain the diagnostic RGS core domain (Fig. 1). Typically, this 120-amino acid core is flanked on both sides by highly variable arms to constitute a 25-kDa protein. However, the RGS core may be split (observed only in lower organisms), and larger family members have been identified (e.g. RGS3, RGS7).RGS Proteins Are Gα GAPsGiven the genetic evidence that RGS proteins are negative regulators of G protein signaling that act at the level of the dissociated Gα subunit or above (but not on βγ or below), the most obvious hypotheses were that RGS proteins might act as inhibitors of GDP dissociation, blocking G protein activation, or stimulators of GTPase activity, facilitating deactivation. There is no evidence for the former mechanism. However, of the eight mammalian RGS proteins that have been examined biochemically, all act as GAPs (15Chen C.K. Wieland T. Simon M.I. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 12885-12889Crossref PubMed Scopus (124) Google Scholar, 17Faurobert E. Hurley J.B. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 2945-2950Crossref PubMed Scopus (93) Google Scholar, 22Berman D.M. Wilkie T.M. Gilman A.G. Cell. 1996; 86: 445-452Abstract Full Text Full Text PDF PubMed Scopus (648) Google Scholar, 23Watson N. Linder M.E. Druey K.M. Kehrl J.H. Blumer K.J. Nature. 1996; 383: 172-175Crossref PubMed Scopus (472) Google Scholar, 24Hunt T.W. Fields T.A. Casey P.J. Peralta E.G. Nature. 1996; 383: 175-177Crossref PubMed Scopus (306) Google Scholar).In the absence of a receptor, the rate-limiting step insteady-state hydrolysis of GTP by a Gα protein is release of product (GDP dissociation). To examine the effect of an RGS protein on actual hydrolysis of GTP, it is necessary to bypass the rate-limiting step or accelerate it substantially. The first approach requires preparation of substrate, GTP-Gα, by incubation of the Gα protein with GTP in the absence of Mg2+. Catalysis is then initiated by addition of Mg2+ in the presence or absence of an RGS protein, and a single round of GTP hydrolysis is monitored over a typical time course of seconds to minutes. Alternatively, reconstitution of a heterotrimeric G protein with an appropriate receptor in phospholipid vesicles permits addition of a receptor agonist to speed product dissociation; the accelerating effect of a GAP on steady-state GTP hydrolysis can then be measured.RGS4 is readily expressed in bacteria and purified, and its GAP activity has been studied most extensively. Whatever the actual mechanism for acceleration of GTP hydrolysis, GAPs such as RGS4 can be conceptualized to act as enzymes, binding substrate (e.g.Gα-GTP) and facilitating its conversion to product (Gα-GDP) (25Ross E.M. Recent Prog. Horm. Res. 1995; 50: 207-219PubMed Google Scholar, 26Berman D.M. Kozasa T. Gilman A.G. J. Biol. Chem. 1996; 271: 27209-27212Abstract Full Text Full Text PDF PubMed Scopus (298) Google Scholar, 27Nekrasova E.R. Berman D.M. Rustandi R.R. Hamm H.E. Gilman A.G. Arshavsky V.Y. Biochemistry. 1997; 36: 7638-7643Crossref PubMed Scopus (52) Google Scholar). Parameters that characterize the reaction include affinity of the GAP for a pseudosubstrate complex (e.g. Gα-GTPγS),K m for "substrate" andV max (estimated by measurement of initial rates of GTP hydrolysis at varying concentrations of Gα-GTP), and the pre-steady state rate of GTP hydrolysis (k cat) at saturating concentrations of the GAP. This intrinsic rate of hydrolysis may exceed the V max for steady-state turnover of Gα-GTP by the GAP if, for example, dissociation of the GAP from Gα-GDP is slow. The interactions of RGS4 with Gαo and Gαt are catalytic; a single molecule of RGS4 can accelerate the GTPase activity of multiple molecules of Gα. The maximal rate enhancement is estimated to be roughly 100-fold with Gαt (to 0.5/s at 4 °C).V max and K m (at 22 °C) are roughly 3/s and 2 μm, respectively; the affinity of RGS4 for GTPγS-bound Gαt or Gαo is approximately equal to the K m.The capacities of Gα proteins to serve as substrates for RGS protein GAPs are influenced by palmitoylation of Gα, at least in vitro. Thus, palmitoylation of Gαz and Gαi decreased their affinities for certain RGS proteins by at least 90%, as well as the maximal rate of GTP hydrolysis (28Tu Y. Wang J. Ross E.M. Science. 1997; 278: 1132-1135Crossref PubMed Scopus (130) Google Scholar). These observations are particularly intriguing because palmitoylation of Gα is reversible and, at least in some cases, is regulated in response to activation of cognate receptors.Berman et al. (26Berman D.M. Kozasa T. Gilman A.G. J. Biol. Chem. 1996; 271: 27209-27212Abstract Full Text Full Text PDF PubMed Scopus (298) Google Scholar) compared affinities of GTPγS-, GDP-, and GDP-AlF4−-bound forms of Gαo for RGS4 by testing their capacity to compete with GTP-Gαo for the GAP. The GDP-AlF4−-bound forms of Gα proteins approximate transition-state complexes (29Coleman D.E. Berghuis A.M. Lee E. Linder M.E. Gilman A.G. Sprang S.R. Science. 1994; 265: 1405-1412Crossref PubMed Scopus (749) Google Scholar, 30Sondek J. Lambright D.G. Noel J.P. Hamm H.E. Sigler P.B. Nature. 1994; 372: 276-279Crossref PubMed Scopus (532) Google Scholar), and RGS4 has markedly higher affinity for this complex than for the substrate or product complexes. Others have made similar observations with different RGS proteins (15Chen C.K. Wieland T. Simon M.I. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 12885-12889Crossref PubMed Scopus (124) Google Scholar, 23Watson N. Linder M.E. Druey K.M. Kehrl J.H. Blumer K.J. Nature. 1996; 383: 172-175Crossref PubMed Scopus (472) Google Scholar, 31Natochin M. Granovsky A.E. Artemyev N.O. J. Biol. Chem. 1997; 272: 17444-17449Abstract Full Text Full Text PDF PubMed Scopus (49) Google Scholar, 32Wieland T. Chen C.-K. Simon M.I. J. Biol. Chem. 1997; 272: 8853-8856Abstract Full Text Full Text PDF PubMed Scopus (41) Google Scholar). However, preferential affinity for the transition-state complex is not always manifest. A protein designated Gz GAP, now known to be a member of the RGS family (28Tu Y. Wang J. Ross E.M. Science. 1997; 278: 1132-1135Crossref PubMed Scopus (130) Google Scholar), has equally high affinities for both GTPγS-Gαz and GDP-AlF4−-Gαz (33Wang J. Tu Y.P. Woodson J. Song X.L. Ross E.M. J. Biol. Chem. 1997; 272: 5732-5740Abstract Full Text Full Text PDF PubMed Scopus (65) Google Scholar).The high affinity interaction between RGS4 and Gαi1-GDP-AlF4−permitted crystallization and solution of the structure of the complex at 2.8-Å resolution (34Tesmer J.J.G. Berman D.M. Gilman A.G. Sprang S.R. Cell. 1997; 89: 251-261Abstract Full Text Full Text PDF PubMed Scopus (680) Google Scholar) (Fig. 1). Only the core domain of RGS4 was visible in the crystal. Importantly, it has been demonstrated that this domain contains all of the crucial elements for GAP activity (10De Vries L. Mousli M. Wurmser A. Farquhar M.G. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 11916-11920Crossref PubMed Scopus (264) Google Scholar, 17Faurobert E. Hurley J.B. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 2945-2950Crossref PubMed Scopus (93) Google Scholar,35Popov S. Yu K. Kozasa T. Wilkie T.M. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 7216-7220Crossref PubMed Scopus (148) Google Scholar, 36Dohlman H.G. Apaniesk D. Chen Y. Song J.P. Nusskern D. Mol. Cell. Biol. 1995; 15: 3635-3643Crossref PubMed Scopus (166) Google Scholar). The core of RGS4 contains a classic right-handed, antiparallel four-helix bundle that interacts (via loops along its base) with switches I, II, and III of Gαi1. The switches of Gα proteins are those regions whose conformations are sensitive to the identity of the bound nucleotide (GTP or GDP), and the residues of switches I and II are intimately involved with binding and hydrolysis of GTP. RGS4 does not contribute any residues that interact directly with either GDP or AlF4−. However, a conserved Asn residue in RGS4 (Asn128) may interact in the ground state with the hydrolytic water molecule or with the side chain of Gαi1 residue Gln204, a critical residue that orients and polarizes the catalytic water in the transition state. It is thus suggested that RGS4 acts as a GAP by stabilizing the flexible switch regions of Gα proteins in conformations resembling those found in the transition state, thus lowering the activation energy barrier; Asn128 may further contribute to the chemistry of hydrolysis by interactions with water or Gln204.Comparison with RasGAPComparisons of the activities and structures of low molecular weight GTPases with those of heterotrimeric G proteins continue to be useful. The basal rate of GTP hydrolysis catalyzed by Ras is 2 orders of magnitude below that catalyzed by a typical Gα protein; the rates of GAP-stimulated GTP hydrolysis by both proteins are similar. Much of the difference in the basal activities and, thus, the correspondingly greater efficiency of RasGAP is ascribable to a single Arg residue at the active site of G protein α subunits. Arg178 in Gαi1 participates directly in catalysis by stabilization of the negative charge on γ-phosphoryl oxygen atoms in the transition state. Ras lacks such a residue and is essentially inactive as a GTPase when compared with Gαi1. RasGAP participates directly in GTP hydrolysis by insertion of an "arginine finger" into the active site (37Scheffzek K. Ahmadian M.R. Kabsch W. Wiesmuller L. Lautwein A. Schmitz F. Wittinghofer A. Science. 1997; 277: 333-338Crossref PubMed Scopus (1181) Google Scholar). RasGAP also binds to the mobile switches of Ras, orienting Gln61 appropriately (Gln61 corresponds to Gln204 in Gαi1). The active sites of the transition-state structures of Ras and Gαi1(associated with their respective GAPs) are amazingly similar. In particular, the critical Arg and Gln residues are in nearly identical positions, even though the Arg residues point into the active site from different directions, and one (Gαi1) is contributed in cis, the other in trans (Fig.2).Figure 2Superposition of the active sites of the Gα i 1 -GDP-AlF 4 − (blue) and Ras-GDP-AlF3-GAP334 (orange/red) complexes. Reproduced from Ref. 37Scheffzek K. Ahmadian M.R. Kabsch W. Wiesmuller L. Lautwein A. Schmitz F. Wittinghofer A. Science. 1997; 277: 333-338Crossref PubMed Scopus (1181) Google Scholar, with permission.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Specificity of RGS-Gα InteractionsEarly recognition of the large number of RGS proteins, a number roughly comparable with that of Gα subunits, prompted speculation of high specificity in their interactions; this has not been realized to the extent anticipated. G protein α subunits are usually categorized as members of one of four subfamilies, designated Gs (two genes), Gi (eight genes), Gq (four genes), and G12 (two genes). Most RGS proteins tested to date act as GAPs toward members of the Gi subfamily and appear to discriminate minimally among them; Gi proteins inhibit adenylyl cyclases, activate K+ channels, inhibit Ca2+ channels, and activate cyclic nucleotide phosphodiesterases. Some of these RGS proteins also act as GAPs toward members of the Gq subfamily (phospholipase Cβ activators) (38Hepler 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). Because GAP assays with Gαq are difficult technically, this interaction has been tested in only a few cases. GAPs for Gαs and Gα12 subfamily members have not been detected, although inhibitory effects of RGS proteins on Gs-mediated signaling pathways have been observed (see Although we that greater specificity of RGS-Gα protein interactions than is be it that an for the large number of RGS proteins does not with any resembling their with G protein α Proteins RGS4 binds to the switch regions of Gαi1. these switches are also involved in because effectors interact with α subunits. This or of binding the that RGS proteins can compete for binding to Gα. has been observed between RGS4 and compete for binding to (38Hepler 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). In both RGS4 and activation of Cβ by Because RGS4 does not hydrolysis of by Gα proteins, a GAP mechanism be of RGS4 in cells activation of synthesis by also with an mechanism Y. H.R. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar). observations have been made with RGS proteins, and its the γ subunit of a cyclic E.R. Berman D.M. Rustandi R.R. Hamm H.E. Gilman A.G. Arshavsky V.Y. Biochemistry. 1997; 36: 7638-7643Crossref PubMed Scopus (52) Google Scholar, 32Wieland T. Chen C.-K. Simon M.I. J. Biol. Chem. 1997; 272: 8853-8856Abstract Full Text Full Text PDF PubMed Scopus (41) Google for binding not be an mechanism for negative regulation of G signaling by a protein is an active GAP. The interaction of RGS4 with Gα proteins is transient and to signaling the GAP mechanism. The affinity of RGS4 for Gα proteins is low and to of Gα. However, it is possible that other members of the RGS protein family might have preferential affinity for or forms of α, than the transition-state These proteins in act as or of α such that it not interact with effectors or βγ. of these have different from those of a GAP mechanism. of the GTPase activity of Gα the protein and with blocking downstream interactions by both α and βγ. The other signaling by α signaling et al. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar) demonstrated that of a of in cells of cyclic by the or adenylyl These are and the of mechanism is thus of have tested in and GAP activity toward Gαi and Gαo not M. Berman and A. G. interactions between and Gαs have not been examined to the that this RGS protein might act as an blocking the binding of Gαs to adenylyl The first structure of a Gα protein with an has now been that of Gαs with the catalytic domain of adenylyl J. G. K. A. G. and S. in of the structure of Gαi1 with RGS4 on that of Gαs with adenylyl that the interaction of an RGS protein with a G protein α subunit not binding of a Gα protein to adenylyl Although both RGS4 and adenylyl to switch II of Gα, the structures that the interactions be the GAP activity of certain RGS proteins might be only in the presence of such as an of RGS proteins have catalytic in cells that are in the activities of pathways in from their in as GAPs. example, RGS4 and of adenylyl C. J.R. Gilman A.G. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: PubMed Scopus Google Scholar), and to a extent and or activation of K.M. Blumer K.J. Kang V.H. Kehrl J.H. Nature. 1996; 379: 742-746Crossref PubMed Scopus (404) Google Scholar, Y. H.R. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar, J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar). RGS3, and synthesis of Y. H.R. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar, J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar, C. J.R. Gilman A.G. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: PubMed Scopus Google Scholar, J.D. A. Druey K.M. Kehrl J.H. 1997; Scopus Google Scholar). To date, the result from such is of activation of adenylyl by existence of such activities their regulation in Although there are hints, we do not RGS proteins are regulated in or to The most obvious of G protein-regulated of well known in these signaling and regulation is at The first RGS protein to be in S. this and is regulated by of H.G. Apaniesk D. Chen Y. Song J.P. Nusskern D. Mol. Cell. Biol. 1995; 15: 3635-3643Crossref PubMed Scopus (166) Google Scholar, C. J. Mol. Cell. Biol. 1987; 7: PubMed Scopus Google Scholar). It is that this be at least a of the RGS protein in mammalian cells, of this is There has been only one of of a mammalian RGS protein in response to activation of a G protein-coupled receptor K.M. Blumer K.J. Kang V.H. Kehrl J.H. Nature. 1996; 379: 742-746Crossref PubMed Scopus (404) Google Scholar), regulation of of other RGS proteins has been in response to of L. S. Chen Y. N. L. J. J.S. N. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: PubMed Scopus Google Scholar) or activation of T cells and B cells (11Newton J.S. Deed R.W. Mitchell E.L.D. Murphy J.J. Norton J.D. Biochim. Biophys. Acta. 1993; 1216: 314-316Crossref PubMed Scopus (29) Google Scholar, 12Hong J.X. Wilson G.L. Fox C.H. Kehrl J.H. J. Immunol. 1993; 150: 3895-3904PubMed Google Scholar, D.P. Heximer S.P. Forsdyke D.R. Mol. Cell. Biol. 1994; 13: 125-147Google Scholar). The specificity of the interactions of RGS proteins with Gα proteins also in of the of RGS protein RGS proteins act as inhibitors of activated of between signaling or well understood are of and in regulation of RGS protein There are that these be for RGS proteins have no obvious to at sites of some do and there are of contains a domain the of insertion E. Hurley J.B. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 2945-2950Crossref PubMed Scopus (93) Google Scholar). and contain a E. Hurley J.B. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 2945-2950Crossref PubMed Scopus (93) Google Scholar, Vries L. E. L. Farquhar M.G. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: PubMed Scopus Google Scholar), and is and Vries L. E. L. Farquhar M.G. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: PubMed Scopus Google Scholar). An RGS protein was discovered as a that interacts with the of a large protein of in the of RGS4 and other members of the family appear to interact directly with adenylyl K. and A. G. RGS proteins to effectors for G protein poised to intercept and α subunits their dissociation from βγ. and Wilkie and T. have demonstrated that RGS protein and when tested in are markedly than their when tested in the observations in the are in some and of are we to an for in this Only of RGS protein can be now and we be The a of information regulation of RGS protein activity, and The of their interactions with G protein α subunits be defined, as interactions between RGS proteins and other cellular that the members of the