Sequence of interactions in receptor-G protein coupling

Guanine nucleotide exchange in heterotrimeric G proteins catalyzed by G protein-coupled receptors (GPCRs) is a key event in many physiological processes. The crystal structures of the GPCR rhodopsin and two G proteins as well as binding sites on both catalytically interacting proteins are known, but the temporal sequence of events leading to nucleotide exchange remains to be elucidated. We employed time-resolved near infrared light scattering to study the order in which the Galpha and Ggamma C-terminal binding sites on the holo-G protein interact with the active state of the GPCR rhodopsin (R*) in native membranes. We investigated these key binding sites within mass-tagged peptides and G proteins and found that their binding to R* is mutually exclusive. The interaction of the holo-G protein with R* requires at least one of the lipid modifications of the G protein (i.e. myristoylation of the Galpha N terminus and/or farnesylation of the Ggamma C terminus). A holo-G protein with a high affinity Galpha C terminus shows a specific change of the reaction rate in the GDP release and GTP uptake steps of catalysis. We interpret the data by a sequential fit model where (i) the initial encounter between R* and the G protein occurs with the Gbetagamma subunit, and (ii) the Galpha C-terminal tail then interacts with R* to release bound GDP, thereby decreasing the affinity of R* for the Gbetagamma subunit. The mechanism limits the time in which both C-terminal binding sites of the G protein interact simultaneously with R* to a short lived transitory state.     

Inhibition of GDP/GTP exchange on G alpha subunits by proteins containing G-protein regulatory motifs

A novel Galpha binding consensus sequence, termed G-protein regulatory (GPR) or GoLoco motif, has been identified in a growing number of proteins, which are thought to modulate G-protein signaling. Alternative roles of GPR proteins as nucleotide exchange factors or as GDP dissociation inhibitors for Galpha have been proposed. We investigated the modulation of the GDP/GTP exchange of Gialpha(1), Goalpha, and Gsalpha by three proteins containing GPR motifs (GPR proteins), LGN-585-642, Pcp2, and RapIGAPII-23-131, to elucidate the mechanisms of GPR protein function. The GPR proteins displayed similar patterns of interaction with Gialpha(1) with the following order of affinities: Gialpha(1)GDP > Gialpha(1)GDPAlF(4)(-) > or = Gialpha(1)GTPgammaS. No detectable binding of the GPR proteins to Gsalpha was observed. LGN-585-642, Pcp2, and RapIGAPII-23-131 inhibited the rates of spontaneous GTPgammaS binding and blocked GDP release from Gialpha(1) and Goalpha. The inhibitory effects of the GPR proteins on Gialpha(1) were significantly more potent, indicating that Gi might be a preferred target for these modulators. Our results suggest that GPR proteins are potent GDP dissociation inhibitors for Gialpha-like Galpha subunits in vitro, and in this capacity they may inhibit GPCR/Gi protein signaling in vivo.     

The [35S]GTPgammaS binding assay: approaches and applications in pharmacology

Receptors of the of seven transmembrane spanning, heterotrimeric G protein coupled family (GPCR) play crucial roles in regulating physiological functions and consequently are targets for the action of many classes of drugs. Activation of receptor by agonist leads to the dissociation of GDP from Galpha of the Galphabetagamma heterotrimer, followed by the binding of GTP to Galpha and subsequent modulation of downstream effectors. The G protein heterotrimer is reformed by GTPase activity of the Galpha subunit, forming Galpha-GDP and so allowing Galpha and Gbetagamma to recombine. The [35S]GTPgammaS assay measures the level of G protein activation following agonist occupation of a GPCR, by determining the binding of the non-hydrolyzable analog [35S]GTPgammaS to Galpha subunits. Thus, the assay measures a functional consequence of receptor occupancy at one of the earliest receptor-mediated events. The assay allows for traditional pharmacological parameters of potency, efficacy and antagonist affinity, with the advantage that agonist measures are not subjected to amplification or other modulation that may occur when analyzing parameters further downstream of the receptor. In general the assay is experimentally more feasible for receptors coupled to the abundant G(i/o) proteins. Nevertheless, [35S]GTPgammaS binding assays are used with GPCRs that couple to the G(s) and G(q) families of G proteins, especially in artificial expression systems, or using receptor-Galpha constructs or immunoprecipitation of [35S]GTPgammaS-labeled Galpha. The relative simplicity of the assay has made it very popular and its use is providing insights into contemporary pharmacological topics including the roles of accessory proteins in signaling, constitutive activity of receptors and agonist specific signaling.     

Structure of Galpha(i1) bound to a GDP-selective peptide provides insight into guanine nucleotide exchange

Heterotrimeric G proteins are molecular switches that regulate numerous signaling pathways involved in cellular physiology. This characteristic is achieved by the adoption of two principal states: an inactive, GDP bound state and an active, GTP bound state. Under basal conditions, G proteins exist in the inactive, GDP bound state; thus, nucleotide exchange is crucial to the onset of signaling. Despite our understanding of G protein signaling pathways, the mechanism of nucleotide exchange remains elusive. We employed phage display technology to identify nucleotide state-dependent Galpha binding peptides. Herein, we report a GDP-selective Galpha binding peptide, KB-752, that enhances spontaneous nucleotide exchange of Galpha(i) subunits. Structural determination of the Galpha(i1)/peptide complex reveals unique changes in the Galpha switch regions predicted to enhance nucleotide exchange by creating a GDP dissociation route. Our results cast light onto a potential mechanism by which Galpha subunits adopt a conformation suitable for nucleotide exchange.     

Mammalian Ric-8A (synembryn) is a heterotrimeric Galpha protein guanine nucleotide exchange factor

The activation of heterotrimeric G proteins is accomplished primarily by the guanine nucleotide exchange activity of ligand-bound G protein-coupled receptors. The existence of nonreceptor guanine nucleotide exchange factors for G proteins has also been postulated. Yeast two-hybrid screens with Galpha(o) and Galpha(s) as baits were performed to identify binding partners of these proteins. Two mammalian homologs of the Caenorhabditis elegans protein Ric-8 were identified in these screens: Ric-8A (Ric-8/synembryn) and Ric-8B. Purification and biochemical characterization of recombinant Ric-8A revealed that it is a potent guanine nucleotide exchange factor for a subset of Galpha proteins including Galpha(q), Galpha(i1), and Galpha(o), but not Galpha(s). The mechanism of Ric-8A-mediated guanine nucleotide exchange was elucidated. Ric-8A interacts with GDP-bound Galpha proteins, stimulates release of GDP, and forms a stable nucleotide-free transition state complex with the Galpha protein; this complex dissociates upon binding of GTP to Galpha.     

The receptor-bound "empty pocket" state of the heterotrimeric G-protein alpha-subunit is conformationally dynamic

Heterotrimeric G-protein activation by a G-protein-coupled receptor (GPCR) requires the propagation of structural signals from the receptor-interacting surfaces to the guanine nucleotide-binding pocket. To probe conformational changes in the G-protein alpha-subunit (G(alpha)) associated with activated GPCR (R*) interactions and guanine nucleotide exchange, high-resolution solution NMR methods are being applied in studying signaling of the G-protein, transducin, by light-activated rhodopsin. Using these methods, we recently demonstrated that an isotope-labeled G(alpha) reconstituted heterotrimer forms functional complexes under NMR experimental conditions with light-activated, detergent-solubilized rhodopsin and a soluble mimic of R*, both of which trigger guanine nucleotide exchange [Ridge, K. D., et al. (2006) J. Biol. Chem. 281, 7635-7648]. Here, it is shown that both light-activated rhodopsin and the soluble mimic of R form trapped intermediate complexes with a GDP-released "empty pocket" state of the heterotrimer in the absence of GTP (or GTPgammaS). In contrast to guanine nucleotide-bound forms of G(alpha), the NMR spectra of the GDP-released, R-bound empty pocket state of G(alpha) display severe line broadening suggestive of a dynamic intermediate state. Interestingly, the conformation of a GDP-depleted, Mg(2+)-bound state of G(alpha) generated in a manner independent of R* does not exhibit a similar degree of line broadening but rather appears structurally similar to the GDP/Mg(2+)-bound form of the protein. Taken together, these results suggest that R*-mediated changes in the receptor-interacting regions of G(alpha), and not the absence of bound guanine nucleotide, are the predominant factors which dictate G(alpha) conformation and dynamics in this R*-bound state of the heterotrimer.     

Molecular cloning of a cDNA for a small GTP binding protein, BRho, from the embryo of Bombyx mori and its characterization after expression and purification

A cDNA clone encoding a small GTP binding protein (Brho) was isolated from an embryonic cDNA library of Bombyx mori that encoded a polypeptide with 202 amino acids sharing 60-80% similarity with the Rho1 family of GTP binding proteins. The effector site and one of the guanine nucleotide binding sites differed from other members of the Rho family. To characterize the biochemical properties of Brho, the clone was expressed in Escherichia coli as a glutathione S-transferase (GST) fusion protein. The recombinant protein was purified to homogeneity with glutathione S-Sepharose. The fusion protein bound [(35)S] GTPgammaS and [(3)H] GDP with association constants of 11x10(6) M(-1) and 6.2x10(6) M(-1), respectively. The binding of [(35)S] GTPgammaS was inhibited by GTP and GDP, but by no other nucleotides. The calculated GTP-hydrolysis activity was 89.6 m mol/min/mol of Brho. Bound [(35)S] GTPgammaS and [(3)H] GDP were exchanged with GTPgammaS most efficiently in the presence of 6 mM MgCl(2). These results suggest that Brho has a higher affinity for GTP than GDP, converts from the GTP-bound state into the GDP-bound state by intrinsic GTP hydrolytic activity, and returns to the GTP-bound state with the exchange of GDP with GTP. Arch.     

Heterotrimeric G-protein alpha-subunit adopts a "preactivated" conformation when associated with betagamma-subunits

Activation of a heterotrimeric G-protein by an agonist-stimulated G-protein-coupled receptor requires the propagation of structural signals from the receptor binding interface to the guanine nucleotide binding pocket of the G-protein. To probe the molecular basis of this signaling process, we are applying high resolution NMR to track structural changes in an isotope-labeled, full-length G-protein alpha-subunit (G(alpha)) chimera (ChiT) associated with G-protein betagamma-subunit (G(betagamma)) and activated receptor (R(*)) interactions. Here, we show that ChiT can be functionally reconstituted with G(betagamma) as assessed by aluminum fluoride-dependent changes in intrinsic tryptophan fluorescence and light-activated rhodopsin-catalyzed guanine nucleotide exchange. We further show that (15)N-ChiT can be titrated with G(betagamma) to form stable heterotrimers at NMR concentrations. To assess structural changes in ChiT upon heterotrimer formation, HSQC spectra of the (15)N-ChiT-reconstituted heterotrimer have been acquired and compared with spectra obtained for GDP/Mg(2+)-bound (15)N-ChiT in the presence and absence of aluminum fluoride and guanosine 5'-3-O-(thio)triphosphate (GTPgammaS)/Mg(2+)-bound (15)N-ChiT. As anticipated, G(betagamma) association with (15)N-ChiT results in (1)HN, (15)N chemical shift changes relative to the GDP/Mg(2+)-bound state. Strikingly, however, most (1)HN, (15)N chemical shift changes associated with heterotrimer formation are the same as those observed upon formation of the GDP.AlF(4)(-)/Mg(2+)- and GTPgammaS/Mg(2+)-bound states. Based on these comparative analyses, assembly of the heterotrimer appears to induce structural changes in the switch II and carboxyl-terminal regions of G(alpha) ("preactivation") that may facilitate the interaction with R(*) and subsequent GDP/GTP exchange.     

Mutagenesis in the switch IV of the helical domain of the human Gsalpha reduces its GDP/GTP exchange rate

The Galpha subunits of heterotrimeric G proteins are constituted by a conserved GTPase "Ras-like" domain (RasD) and by a unique alpha-helical domain (HD). Upon GTP binding, four regions, called switch I, II, III, and IV, have been identified as undergoing structural changes. Switch I, II, and III are located in RasD and switch IV in HD. All Galpha known functions, such as GTPase activity and receptor, effector, and Gbetagamma interaction sites have been found to be localized in RasD, but little is known about the role of HD and its switch IV region. Through the construction of chimeras between human and Xenopus Gsalpha we have previously identified a HD region, encompassing helices alphaA, alphaB, and alphaC, that was responsible for the observed functional differences in their capacity to activate adenylyl cyclase (Antonelli et al. [1994]: FEBS Lett 340:249-254). Since switch IV is located within this region and contains most of the nonconservative amino acid differences between both Gsalpha proteins, in the present work we constructed two human Gsalpha mutant proteins in which we have changed four and five switch IV residues for the ones present in the Xenopus protein. Mutants M15 (hGsalphaalphaS133N, M135P, P138K, P143S) and M17 (hGsalphaalphaS133N, M135P, V137Y, P138K, P143S) were expressed in Escherichia coli, purified, and characterized by their ability to bind GTPgammaS, dissociate GDP, hydrolyze GTP, and activate adenylyl cyclase. A decreased rate of GDP release, GTPgammaS binding, and GTP hydrolysis was observed for both mutants, M17 having considerably slower kinetics than M15 for all functions tested. Reconstituted adenylyl cyclase activity with both mutants showed normal activation in the presence of AlF(4)(-), but a decreased activation with GTPgammaS, which is consistent with the lower GDP dissociating rate they displayed. These data provide new evidence on the role that HD is playing in modulating the GDP/GTP exchange of the Gsalpha subunit.     

GTP binding and hydrolysis kinetics of human septin 2

Septins are a family of conserved proteins that are essential for cytokinesis in a wide range of organisms including fungi, Drosophila and mammals. In budding yeast, where they were first discovered, they are thought to form a filamentous ring at the bridge between the mother and bud cells. What regulates the assembly and function of septins, however, has remained obscure. All septins share a highly conserved domain related to those found in small GTPases, and septins have been shown to bind and hydrolyze GTP, although the properties of this domain and the relationship between polymerization and GTP binding/hydrolysis is unclear. Here we show that human septin 2 is phosphorylated in vivo at Ser218 by casein kinase II. In addition, we show that recombinant septin 2 binds guanine nucleotides with a Kd of 0.28 microm for GTPgammaS and 1.75 microm for GDP. It has a slow exchange rate of 7 x 10(-5) s(-1) for GTPgammaS and 5 x 10(-4) s(-1) for GDP, and an apparent kcat value of 2.7 x 10(-4) s(-1), similar to those of the Ras superfamily of GTPases. Interestingly, the nucleotide binding affinity appears to be altered by phosphorylation at Ser218. Finally, we show that a single septin protein can form homotypic filaments in vitro, whether bound to GDP or GTP.     

Copper and zinc inhibit Galphas function: a nucleotide-free state of Galphas induced by Cu2+ and Zn2+

The stimulatory GTP-binding protein of adenylyl cyclase (AC) regulates hormone-stimulated production of cAMP. Here, we demonstrate that Cu(2+) and Zn(2+) inhibit the steady-state GTPase activity of the alpha subunit of GTP-binding protein (Galpha(s)) but do not alter its intrinsic GTPase activity. Cu(2+) and Zn(2+) decrease steady-state GTPase activity by inhibiting the binding of GTP to Galpha(s). Moreover, Cu(2+) and Zn(2+) increase GDP dissociation from Galpha(s) and render the G protein in a nucleotide-free state. However, these cations do not alter the dissociation of the guanosine 5'-3-O-(thio)triphosphate (GTPgammaS) that is already bound to the Galpha(s). Because of their ability to inhibit GTPgammaS binding, preincubation of Cu(2+) or Zn(2+) with Galpha(s) does not permit GTPgammaS to activate Galpha(s) and stimulate AC activity. However, preincubation of Galpha(s) with GTPgammaS followed by addition of Cu(2+) or Zn(2+) did not alter the ability of Galpha(s) to stimulate AC activity. Interestingly, AlF(4)(-) partially restored the ability of Galpha(s), which had been preincubated with Cu(2+) or Zn(2+), to stimulate AC; AlF(4)(-) does not permit the re-association of unbound GDP with Galpha(s). Thus, the interaction of AlF(4)(-) with the nucleotide-free Galpha(s) is sufficient to activate AC. Using antibodies to the N and C termini of Galpha(s), we show that the Cu(2+) interaction site on the G protein is in the C terminus. We conclude that Cu(2+) and Zn(2+) generate a nucleotide-free state of Galpha(s) and that, in the absence of any nucleotide, the gamma-phosphate mimic of GTP, AlF(4)(-), alters Galpha(s) structure sufficiently to permit stimulation of AC activity. Moreover, our finding that isoproterenol-stimulated AC activity was more sensitive to inhibition by Cu(2+) and Zn(2+) as compared with forskolin-stimulated activity is consistent with Galpha(s) being a primary target of these cations in regulating the signaling from receptor to AC.     

A bifunctional Galphai/Galphas modulatory peptide that attenuates adenylyl cyclase activity

Signaling via G-protein coupled receptors is initiated by receptor-catalyzed nucleotide exchange on Galpha subunits normally bound to GDP and Gbetagamma. Activated Galpha . GTP then regulates effectors such as adenylyl cyclase. Except for Gbetagamma, no known regulators bind the adenylyl cyclase-stimulatory subunit Galphas in its GDP-bound state. We recently described a peptide, KB-752, that binds and enhances the nucleotide exchange rate of the adenylyl cyclase-inhibitory subunit Galpha(i). Herein, we report that KB-752 binds Galpha(s) . GDP yet slows its rate of nucleotide exchange. KB-752 inhibits GTPgammaS-stimulated adenylyl cyclase activity in cell membranes, reflecting its opposing effects on nucleotide exchange by Galpha(i) and Galpha(s).     

Mechanisms for reversible regulation between G13 and Rho exchange factors.

The heterotrimeric G proteins, G(12) and G(13), mediate signaling between G protein-coupled receptors and the monomeric GTPase, RhoA. One pathway for this modulation is direct stimulation by Galpha(13) of p115 RhoGEF, an exchange factor for RhoA. The GTPase activity of both Galpha(12) and Galpha(13) is increased by the N terminus of p115 Rho guanine nucleotide exchange factor (GEF). This region has weak homology to the RGS box sequence of the classic regulators of G protein signaling (RGS), which act as GTPase-activating proteins (GAP) for G(i) and G(q). Here, the RGS region of p115 RhoGEF is shown to be distinctly different in that sequences flanking the predicted "RGS box" region are required for both stable expression and GAP activity. Deletions in the N terminus of the protein eliminate GAP activity but retain substantial binding to Galpha(13) and activation of RhoA exchange activity by Galpha(13). In contrast, GTRAP48, a homolog of p115 RhoGEF, bound to Galpha(13) but was not stimulated by the alpha subunit and had very poor GAP activity. Besides binding to the N-terminal RGS region, Galpha(13) also bound to a truncated protein consisting only of the Dbl homology (DH) and pleckstrin homology (PH) domains. However, Galpha(13) did not stimulate the exchange activity of this truncated protein. A chimeric protein, which contained the RGS region of GTRAP48 in place of the endogenous N terminus of p115 RhoGEF, was activated by Galpha(13). These results suggest a mechanism for activation of the nucleotide exchange activity of p115 RhoGEF that involves direct and coordinate interaction of Galpha(13) to both its RGS and DH domains.     

The receptor-Galpha fusion protein as a tool for ligand screening: a model study using a nociceptin receptor-Galphai2 fusion protein

As a model system to screen endogenous ligands for G(i)-coupled receptors, we have prepared and characterized a fusion protein of nociceptin receptor and alpha subunit of G(i2). We detected nociceptin binding to the fusion protein by measuring stimulation of [(35)S]GTPgammaS binding with an EC(50) of 2.0 nM and a gain of approximately five times. The stimulation by nociceptin of [(35)S]GTPgammaS binding to the fusion protein was clearly observed in the presence of an appropriate concentration of GDP, because the affinity for GDP was decreased in the presence of agonist. Full and partial agonists differed in their effects on apparent the affinity of the fusion protein for GDP: the IC(50) values for GDP to displace 100 pM [(35)S]GTPgammaS were estimated to be 2 micro M, 0.4 micro M, and 0.05 micro M in the presence of full agonist (nociceptin), partial agonist (F/G-NC), and antagonist (NBZH), respectively. We also detected the activity to stimulate [(35)S]GTPgammaS binding to the fusion protein in the brain extract derived from 2-3 g wet weight tissue without false-positive results. The active component was identified as endogenous nociceptin itself. These results indicate that the fusion protein of GPCR and Galpha(i) is useful for screening of endogenous ligands.     

Binding of guanine nucleotides and Mg2+ to tubulin with a nucleotide-depleted exchangeable site

Binding of GTP and GDP to tubulin in the presence or absence of Mg2+ was measured following depletion of the exchangeable site--(E-site) nucleotide. The E-site nucleotide was displaced with a large molar excess of the nonhydrolyzable GTP analogue, GMPPCP, followed by the removal of the analogue. Using a micropartition assay, the equilibrium constant measured in 0.1 M 1.4-piperazinediethanesulfonic acid (Pipes), pH 6.9, 1 mM ethylene glycol bis(beta-aminoethyl ether) N,N,N',N'-tetraacetic acid, 1 mM dithiothreitol, and 1 mM MgSO4 at 4 degrees C was 9.1 x 10(6) M-1 for GTP and 4.4 x 10(6) M-1 for GDP. Removal of Mg2+ reduced the binding affinity of GTP by 160-fold while the affinity of GDP remained essentially unchanged. Similar values were obtained if 0.1 M Tris, pH 7.0, was used instead of Pipes. Binding of Mg2+ to tubulin containing GTP, GDP, or no nucleotide at the E-site was also examined by the micropartition method. Tubulin-GTP contained one high affinity Mg2+ site (K alpha = 1.2 x 10(6) M-1) in addition to the one occupied by Mg2+ as tubulin is isolated, while only weak Mg2+ binding to tubulin-GDP and to tubulin with a vacant E-site (K alpha = 10(3) M-1) was observed. It is suggested that Mg2+ binds to the beta and gamma phosphates of GTP, and only to the beta phosphate of GDP, as shown for the H. ras p21 protein.     

New roles for Galpha and RGS proteins: communication continues despite pulling sisters apart

Large G protein alpha subunits and their attendant regulators of G-protein signaling (RGS) proteins control both intercellular signaling and asymmetric cell divisions by distinct pathways. The classical pathway, found throughout higher eukaryotic organisms, mediates intercellular communication via hormone binding to G-protein-coupled receptors (GPCRs). Recent studies have led to the discovery of GPCR-independent activation of Galpha subunits by the guanine nucleotide exchange factor RIC-8 in both asymmetric cell division and synaptic vesicle priming in metazoan organisms. Protein-protein interactions and protein function in each pathway are driven through the cycle of GTP binding and hydrolysis by the Galpha subunit. This review builds a conceptual framework for understanding RIC-8-mediated pathways by comparison with the mechanism of classical G-protein activation and inhibition in GPCR signaling.     

Selective regulation of Galpha(q/11) by an RGS domain in the G protein-coupled receptor kinase, GRK2

G protein-coupled receptor kinases (GRKs) are well characterized regulators of G protein-coupled receptors, whereas regulators of G protein signaling (RGS) proteins directly control the activity of G protein alpha subunits. Interestingly, a recent report (Siderovski, D. P., Hessel, A., Chung, S., Mak, T. W., and Tyers, M. (1996) Curr. Biol. 6, 211-212) identified a region within the N terminus of GRKs that contained homology to RGS domains. Given that RGS domains demonstrate AlF(4)(-)-dependent binding to G protein alpha subunits, we tested the ability of G proteins from a crude bovine brain extract to bind to GRK affinity columns in the absence or presence of AlF(4)(-). This revealed the specific ability of bovine brain Galpha(q/11) to bind to both GRK2 and GRK3 in an AlF(4)(-)-dependent manner. In contrast, Galpha(s), Galpha(i), and Galpha(12/13) did not bind to GRK2 or GRK3 despite their presence in the extract. Additional studies revealed that bovine brain Galpha(q/11) could also bind to an N-terminal construct of GRK2, while no binding of Galpha(q/11), Galpha(s), Galpha(i), or Galpha(12/13) to comparable constructs of GRK5 or GRK6 was observed. Experiments using purified Galpha(q) revealed significant binding of both Galpha(q) GDP/AlF(4)(-) and Galpha(q)(GTPgammaS), but not Galpha(q)(GDP), to GRK2. Activation-dependent binding was also observed in both COS-1 and HEK293 cells as GRK2 significantly co-immunoprecipitated constitutively active Galpha(q)(R183C) but not wild type Galpha(q). In vitro analysis revealed that GRK2 possesses weak GAP activity toward Galpha(q) that is dependent on the presence of a G protein-coupled receptor. However, GRK2 effectively inhibited Galpha(q)-mediated activation of phospholipase C-beta both in vitro and in cells, possibly through sequestration of activated Galpha(q). These data suggest that a subfamily of the GRKs may be bifunctional regulators of G protein-coupled receptor signaling operating directly on both receptors and G proteins.     

AGS3 inhibits GDP dissociation from galpha subunits of the Gi family and rhodopsin-dependent activation of transducin

A number of recently discovered proteins that interact with the alpha subunits of G(i)-like G proteins contain homologous repeated sequences named G protein regulatory (GPR) motifs. Activator of G protein signaling 3 (AGS3), identified as an activator of the yeast pheromone pathway in the absence of the pheromone receptor, has a domain with four such repeats. To elucidate the potential mechanisms of regulation of G protein signaling by proteins containing GPR motifs, we examined the effects of the AGS3 GPR domain on the kinetics of guanine nucleotide exchange and GTP hydrolysis by G(i)alpha(1) and transducin-alpha (G(t)alpha). The AGS3 GPR domain markedly inhibited the rates of spontaneous guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) binding to G(i)alpha and rhodopsin-stimulated GTPgammaS binding to G(t)alpha. The full-length AGS3 GPR domain, AGS3-(463-650), was approximately 30-fold more potent than AGS3-(572-629), containing two AGS3 GPR motifs. The IC(50) values for the AGS3-(463-650) inhibitory effects on G(i)alpha and transducin were 0.12 and 0.15 microm, respectively. Furthermore, AGS3-(463-650) and AGS3-(572-629) effectively blocked the GDP release from G(i)alpha and rhodopsin-induced dissociation of GDP from G(t)alpha. The potencies of AGS3-(572-629) and AGS3-(463-650) to suppress the GDP dissociation rates correlated with their ability to inhibit the rates of GTPgammaS binding. Consistent with the inhibition of nucleotide exchange, the AGS3 GPR domain slowed the rate of steady-state GTP hydrolysis by G(i)alpha. The catalytic rate of G(t)alpha GTP hydrolysis, measured under single turnover conditions, remained unchanged with the addition of AGS3-(463-650). Altogether, our results suggest that proteins containing GPR motifs, in addition to their potential role as G protein-coupled receptor-independent activators of Gbetagamma signaling pathways, act as GDP dissociation inhibitors and negatively regulate the activation of a G protein by a G protein-coupled receptor.     

Tubulin exchanges divalent cations at both guanine nucleotide-binding sites

The tubulin heterodimer binds a molecule of GTP at the nonexchangeable nucleotide-binding site (N-site) and either GDP or GTP at the exchangeable nucleotide-binding site (E-site). Mg2+ is known to be tightly linked to the binding of GTP at the E-site (Correia, J. J., Baty, L. T., and Williams, R. C., Jr. (1987) J. Biol. Chem. 262, 17278-17284). Measurements of the exchange of Mn2+ for bound Mg2+ (as monitored by atomic absorption and EPR) demonstrate that tubulin which has GDP at the E-site possesses one high affinity metal-binding site and that tubulin which has GTP at the E-site possesses two such sites. The apparent association constants are 0.7-1.1 x 10(6) M-1 for Mg2+ and approximately 4.1-4.9 x 10(7) M-1 for Mn2+. Divalent cations do bind to GDP at the E-site, but with much lower affinity (2.0-2.3 x 10(3) M-1 for Mg2+ and 3.9-6.6 x 10(3) M-1 for Mn2+). These data suggest that divalent cations are involved in GTP binding to both the N- and E-sites of tubulin. The N-site metal exchanges slowly (kapp = 0.020 min-1), suggesting a mechanism involving protein "breathing" or heterodimer dissociation. The N-site metal exchange rate is independent of the concentration of protein and metal, an observation consistent with the possibility that a dynamic breathing process is the rate-limiting step. The exchange of Mn2+ for Mg2+ has no effect on the secondary structure of tubulin at 4 degrees C or on the ability of tubulin to form microtubules. These results have important consequences for the interpretation of distance measurements within the tubulin dimer using paramagnetic ions. They are also relevant to the detailed mechanism of divalent cation release from microtubules after GTP hydrolysis.