Gz, a guanine nucleotide-binding protein with unique biochemical properties

Cloning of a complementary DNA (cDNA) for Gz alpha, a newly appreciated member of the family of guanine nucleotide-binding regulatory proteins (G proteins), has allowed preparation of specific antisera to identify the protein in tissues and to assay it during purification from bovine brain. Additionally, expression of the cDNA in Escherichia coli has resulted in the production and purification of the recombinant protein. Purification of Gz from bovine brain is tedious, and only small quantities of protein have been obtained. The protein copurifies with the beta gamma subunit complex common to other G proteins; another 26- kDa GTP-binding protein is also present in these preparations. The purified protein could not serve as a substrate for NAD-dependent ADP- ribosylation catalyzed by either pertussis toxin or cholera toxin. Purification of recombinant Gz alpha (rGz alpha) from E. coli is simple, and quantities of homogeneous protein sufficient for biochemical analysis are obtained. Purified rGz alpha has several properties that distinguish it from other G protein alpha subunit polypeptides. These include a very slow rate of guanine nucleotide exchange (k = 0.02 min^-1), which is reduced greater than 20-fold in the presence of mM concentrations of Mg2+. In addition, the rate of the intrinsic GTPase activity of Gz alpha is extremely slow. The hydrolysis rate (kcat) for rGz alpha at 30 degrees C is 0.05 min^-1, or 200-fold slower than that determined for other G protein alpha subunits. rGz alpha can interact with bovine brain beta gamma but does not serve as a substrate for ADP-ribosylation catalyzed by either pertussis toxin or cholera toxin. These studies suggest that Gz may play a role in signal transduction pathways that are mechanistically distinct from those controlled by the other members of the G protein family

G protein beta gamma subunits synthesized in Sf9 cells. Functional characterization and the significance of prenylation of gamma

Heterotrimeric guanine nucleotide-binding regulatory proteins (G proteins) consist of a nucleotide-binding alpha subunit and a high- affinity complex of beta and gamma subunits. There is molecular heterogeneity of beta and gamma, but the significance of this diversity is poorly understood. Different G protein beta and gamma subunits have been expressed both singly and in combinations in Sf9 cells. Although expression of individual subunits is achieved in all cases, beta gamma subunit activity (support of pertussis toxin-catalyzed ADP-ribosylation of rGi alpha 1) is detected only when beta and gamma are expressed concurrently. Of the six combinations of beta gamma tested (beta 1 or beta 2 with gamma 1, gamma 2, or gamma 3), only one, beta 2 gamma 1, failed to generate a functional complex. Each of the other five complexes has been purified by subunit exchange chromatography using Go alpha-agarose as the chromatographic matrix. We have detected differences in the abilities of the purified proteins to support ADP- ribosylation of Gi alpha 1; these differences are attributable to the gamma component of the complex. When assayed for their ability to inhibit calmodulin-stimulated type-I adenylylcyclase activity or to potentiate Gs alpha-stimulated type-II adenylylcyclase, recombinant beta 1 gamma 1 and transducin beta gamma are approximately 10 and 20 times less potent, respectively, than the other complexes examined. Prenylation and/or further carboxyl-terminal processing of gamma are not required for assembly of the beta gamma subunit complex but are indispensable for high affinity interactions of beta gamma with either G protein alpha subunits or adenylylcyclases

Structure of RGS4 bound to AlF\u3csub\u3e4\u3c/sub\u3e\u3csup\u3e-\u3c/sup\u3e-activated G(iα1): Stabilization of the Transition State for GTP Hydrolysis

RGS proteins are GTPase activators for heterotrimeric G proteins. We report here the 2.8 Å resolution crystal structure of the RGS protein RGS4 complexed with G(iα1)-Mg2+-GDP-AlF4. Only the core domain of RGS4 is visible in the crystal. The core domain binds to the three switch regions of G(iα1), but does not contribute catalytic residues that directly interact with either GDP or AlF4. Therefore, RGS4 appears to catalyze rapid hydrolysis of GTP primarily by stabilizing the switch regions of G(iα1), although the conserved Asn-128 from RGS4 could also play a catalytic role by interacting with the hydrolytic water molecule or the side chain of Gln-204. The binding site for RGS4 on G(iα1) is also consistent with the activity of RGS proteins as antagonists of G(α) effectors

Crystal structure of the adenylyl cyclase activator G(sα)

The crystal structure of G(sα), the heterotrimeric G protein α subunit that stimulates adenylyl cyclase, was determined at 2.5 Å in a complex with guanosine 5\u27-O-(3-thio-triphosphate) (GTPγS). G(sα) is the prototypic member of a of GTP-binding proteins that regulate the activities of effectors in a hormone-dependent manner. Comparison of the structure of G(sα)·GTPγS with that of G(iα)·GTPγS suggest that their effector specificity is primarily dictated by the shape of the binding surface formed by the switch II helix and the α3-β5 loop, despite the high sequence homology of these elements. In contrast, sequence divergence explains the inability of regulators of G protein signaling to stimutate the GTPase activity of G(sα). The βγ binding surface ofG(sα) is largely conserved in sequence and structure to that of G(iα), whereas differences in the surface formed by the carboxyl-terminal helix and the α4-β6 loop may mediate ceptor specificity

Identification of a G(iα) binding site on type V adenylyl cyclase

The stimulatory G protein α subunit G(sα) binds within a cleft in adenylyl cyclase formed by the α1-α2 and α3-β4 loops of the C2 domain. The pseudosymmetry of the C1 and C2 domains of adenylyl cyclase suggests that the homologous inhibitory α subunit G(iα) could bind to the analogous cleft within C1. We demonstrate that myristoylated guanosine 5\u27-3-O- (thio)triphosphate-G(iα1) forms a stable complex with the C1 (but not the C2) domain of type V adenylyl cyclase. Mutagenesis of the membrane-bound enzyme identified residues whose alteration either increased or substantially decreased the IC50 for inhibition by G(iα1). These mutations suggest binding of G(iα) within the cleft formed by the α2 and α3 helices of C1, analogous to the G(sα) binding site in C2. Adenylyl cyclase activity reconstituted by mixture of the C1 and C2 domains of type V adenylyl cyclase was also inhibited by G(iα). The C(1b) domain of the type V enzyme contributed to affinity for G(iα), but the source of C2 had little effect. Mutations in this soluble system faithfully reflected the phenotypes observed with the membrane-bound enzyme. The pseudosymmetrical structure of adenylyl cyclase permits bidirectional regulation of activity by homologous G protein α subunits

Crystal structure of the catalytic domains of adenylyl cyclase in a complex with G(sα)·GTPγΣ

The crystal structure of a soluble, catalytically active form of adenylyl cyclase in a complex with its stimulatory heterotrimeric G protein α subunit (G(sα)) and forskolin was determined to a resolution of 2.3 angstroms. When P-site inhibitors were soaked into native crystals of the complex, the active site of adenylyl cyclase was located and structural elements important for substrate recognition and catalysis were identified. On the basis of these and other structures, a molecular mechanism is proposed for the activation of adenylyl cyclase by G(sα)

Mechanism of GTP hydrolysis by G-protein α subunits

Hydrolysis of GTP by a variety of guanine nucleotide-binding proteins is a crucial step for regulation of these biological switches. Mutations that impair the GTPase activity of certain heterotrimeric signal-transducing G proteins or of p21(ras) cause tumors in man. A conserved glutamic residue in the α subunit of G proteins has been hypothesized to serve as a general base, thereby activating a water molecule for nucleophilic attack on GTP. The results of mutagenesis of this residue (Glu-207) in G(iα1) refute this hypothesis. Based on the structure of the complex of G(iα1) with GDP, Mg2+, and AlF4/-, which appears to resemble the transition state for GTP hydrolysis, we believe that Glu-204 of G(iα1), rather than Glu-207, supports catalysis of GTP hydrolysis by stabilization of the transition state

Structure of the GDP-Pi complex of Gly203→Ala G(iα1): a mimic of the ternary product complex of Galpha-catalyzed GTP hydrolysis

Background: G proteins play a vital role in transmembrane signalling events. In their inactive form G proteins exist as heterotrimers consisting of an α subunit, complexed with GDP and a dimer of βγ subunits. Upon stimulation by receptors, G protein α subunits exchange GDP for GTP and dissociate from βγ. Thus activated, α subunits stimulate or inhibit downstream effectors. The duration of the activated state corresponds to the single turnover rate of GTP hydrolysis, which is typically in the range of seconds. In G(iα1), the Gly203→Ala mutation reduces the affinity of the substrate for Mg2+, inhibits a key conformational step that occurs upon GTP binding and consequently inhibits the release of βγ subunits from the GTP complex. The structure of the Gly203→Ala mutant of G(iα1) (G203AG(iα1)) bound to the slowly hydrolyzing analog of GTP (GTPγS) has been determined in order to elucidate the structural changes that take place during hydrolysis. Results: We have determined the three dimensional structure of a Gly203→Ala mutant of G(iα1) at 2.6 Å resolution. Although crystals were grown in the presence of GTPγS and Mg2+, the catalytic site contains a molecule of GDP and a phosphate ion, but no Mg2+. The phosphate ion is bound to a site near that occupied by the γ-phosphate of GTPγS in the activated wild-type α subunit. A region of the protein, termed the Switch II helix, twists and bends to adopt a conformation that is radically different from that observed in other G(iα1) subunit complexes. Conclusions: Under the conditions of crystallization, the Gly203→Ala mutation appears to stabilize a conformation that may be similar, although perhaps not identical, to the transient ternary product complex of G(iα1)-catalyzed GTP hydrolysis. The rearrangement of the Switch II helix avoids a potential static conflict caused by the mutation. However, it appears that dissociation of the γ-phosphate from the pentacoordinate intermediate also requires a conformational change in Switch II. Thus, a conformational rearrangement of the Switch II helix may be required in Gα-catalyzed GTP hydrolysis

The A326S mutant of G(iα1) as an approximation of the receptor-bound state

Agonist-bound heptahelical receptors activate heterotrimeric G proteins by catalyzing exchange of GDP for GTP on their α subunits. In search of an approximation of the receptor-α subunit complex, we have considered the properties of A326S G(iα1), a mutation discovered originally in G(sα) (Iiri, T., Herzmark, P., Nakamoto, J. M., Van Dop, C., and Bourne, H. R. (1994) Nature 371, 164-168) that mimics the effect of receptor on nucleotide exchange. The mutation accelerates dissociation of GDP from the α(i1)β1γ2 heterotrimer by 250-fold. Nevertheless, affinity of mutant G(iα1) for GTPγS is high in the presence of Mg2+, and the mutation has no effect on the intrinsic GTPase activity of the α subunit. The mutation also uncouples two activities of βγ: stabilization of the GDP-bound α subunit (which is retained) and retardation of GDP dissociation heterotrimer (which is lost). For wild-type and mutant G(iαl), βγ prevents irreversible inactivation of the α subunit at 30 °C. However, the mutation accelerates irreversible inactivation of α at 37 °C despite the presence of βγ. Structurally, the mutation weakens affinity for GTPγS by steric crowding: a 2-fold increase in the number of close contacts between the protein and the purine ring of the nucleotide. By contrast, we observe no differences in structure at the GDP binding site between wild-type heterotrimers and those containing A326S G(i 1/2 ). However, the GDP binding site is only partially occupied in crystals of G protein heterotrimers containing A326S G(iα1). In contrast to original speculations about the structural correlates of receptor-catalyzed nucleotide exchange, rapid dissociation of GDP can be observed in the absence of substantial structural alteration of a G(α) subunit in the GDP-bound state