Stimulation and inhibition of human platelet membrane high-affinity GTPase by neomycin

The effect of the inositol phospholipid-binding antibiotic neomycin was studied on high-affinity GTPase in human platelet membranes. At low concentrations (up to 1 mM), neomycin by itself stimulated a high-affinity GTPase. This GTPase stimulation was additive with that caused by the hormonal factors, prostaglandin E1 and epinephrine, but not with thrombin. At concentrations higher than 1 mM, neomycin reduced control GTPase activity and eliminated the stimulation caused by thrombin. The data suggest that neomycin by a presently unknown mechanism can regulate activity states of signal transducing GTP-binding proteins.     

Production of antibodies against rhodopsin after immunization with beta gamma-subunits of transducin: evidence for interaction of beta gamma-subunits of guanosine 5'-triphosphate binding proteins with receptor

The light-detecting system of retinal rod outer segments is regulated by a guanyl nucleotide binding (G) protein, transducin, which is composed of alpha-, beta-, and gamma-subunits. Transducin couples rhodopsin to the intracellular effector enzyme, a cGMP phosphodiesterase. The beta gamma complex (T beta gamma) is required for the alpha-subunit (T alpha) to interact effectively with the photon receptor rhodopsin. It is not clear, however, whether T beta gamma binds directly to rhodopsin or promotes T alpha binding to rhodopsin only by binding to T alpha. We have found that serum from rabbits immunized with T beta gamma contained a population of antibodies that were reactive against rhodopsin. These antibodies could be separated from T beta gamma antibodies by absorbing the latter on immobilized transducin. Binding of purified rhodopsin antibodies was inhibited by T beta gamma, suggesting that the rhodopsin antibodies and T beta gamma bound to the same site on rhodopsin. We propose that the rhodopsin antibodies act both as antiidiotypic antibodies against the idiotypic T beta gamma antibodies and as antibodies against rhodopsin. This hypothesis is consistent with the conclusion that T beta gamma interacts directly with the receptor. It is probable that in an analogous way, G beta gamma interacts directly with receptors of the adenylate cyclase system.     

Sites of interaction in the complex between beta- and gamma-subunits of transducin

Transducin, the guanine nucleotide-binding regulatory protein in rod outer segments, is a heterotrimer consisting of alpha-, beta-, and gamma-subunits. Activation of the photoreceptor, rhodopsin, by light, results in activation of transducin which cleaves to form transducin alpha. GTP and a complex of beta gamma-subunits. We have investigated the point(s) of contact between the subunits of transducin by analyzing for the formation of intersubunit disulfide bond(s) in the presence of copper phenanthroline. The formation of a new species with an apparent molecular mass of 43 kDa was observed which had resulted from the formation of a disulfide bond between the beta- and gamma-subunits. The amino acid residues participating in the disulfide bond were identified as Cys-25 in the beta-subunit and Cys-36 and/or Cys-37 in the gamma-subunit. Thus, these cysteine residues and, probably, some of the adjacent amino acid residues form a point of contact between the beta- and gamma-subunits of transducin in the stable complex.     

Stimulation of human platelet guanylate cyclase by fatty acids.

Guanylate cyclase from human platelets was over 90% soluble, even when assayed in the presence of Triton X-100. A time-dependent increase in activity occurred when the enzyme was incubated at 37 degrees and this spontaneous activation was prevented by dithiothreitol. Arachidonic acid stimulated the soluble enzyme activity approximately 2- to 3-fold. Linear double reciprocal plots of guanylate cyclase activation as a function of arachidonic acid concentration were obtained with a Ka value of 2.1 muM. A Hill coefficient of 0.98 was obtained indicating that one fatty acid binding site is present for each catalytic site. Concentrations of arachidonic acid in excess of 10 muM caused less than maximal stimulation. Dihomo-gamma-linolenic acid and two polyunsaturated 22 carbon fatty acids stimulated the activity of guanylate cyclase to the same degree as did arachidonic acid. The methyl ester of arachidonic acid was much less effective. Diene, monoene, and saturated fatty acids of various carbon chain lengths as well as prostaglandins E1, E2, and F2alpha, had little or no effect. These data indicate that the structural determined required for stimulation by fatty acids of soluble platelet guanylate cyclase is a 1,4,7-octatriene group with its first double bond in the omega6 position. This structural group is similar to the substrate specificity determinants of fatty acid cyclooxygenase, the first enzyme of the prostaglandin synthetase complex. However, conversion of arachidonic acid to a metabolite of the cyclooxygenase pathway did not appear to be required for activation of the cyclase since activation occurred in the 105,000 X g supernatant fraction and pretreatment of this fraction with aspirin did not alter the ability of arachidonic acid to activate guanylate cyclase. Kinetic studies showed that the stimulation of guanylate cyclase by arachidonic acid is primarily an effect on maximal velocity. Arachidonic acid did not alter the concentration of free Mn2+ required for optimal activity. It is concluded that the activity of the soluble form of guanylate cyclase in cell-free preparations of human platelets can be increased by a lipid-protein interaction involving specific polyunsaturated fatty acids.     

Stimulation of human polymorphonuclear leukocytes by recombinant human interleukin-1 beta.

Leukocyte activation is a property of systemic infection. Animal experiments indicate interleukin-1 (IL-1) as a possible modulator, while contradictory results have been reported from in-vitro stimulation of isolated leukocytes. The purpose of the present study was to investigate the activation of isolated polymorphonuclear (PMN) leukocytes in vitro by preparations of recombinant human IL-1 beta and IL-1 receptor antagonist, which in earlier studies could elicit and abrogate, respectively, a sepsis-like syndrome in rabbits. They have also been shown to influence acute phase protein synthesis in mice and rats, and release of leukocyte cathepsin G in vivo. It was found that recombinant human IL-1 beta elicited a dose-dependent luminol-enhanced chemiluminescence response in isolated human PMN leukocytes in the dose range 8.8 x 10(-11)-8.8 x 10(-8) M. The effect could be blocked by prior treatment with the IL-1 receptor antagonist, indicating a direct effect on the specific IL-1 receptor. Preincubation by IL-1 beta enhanced the effect of a secondary challenge with phorbol 12-myristate 13-acetate or formyl-Met-Leu-Phe by 30-40%. The priming effect of rhIL-1 beta could also be blocked by the specific receptor antagonist. In this study, incubation of PMN leukocytes with rhIL-1 beta failed to induce degranulation of both azurophil (neutrophil proteinase 4/proteinase 3) and specific (lactoferrin) granules. rhIL-1 beta has been shown to induce degranulation in vivo, which is thus indicated as an indirect effect. We conclude that IL-1 beta is a direct and specific, but probably weak stimulator of the PMN leukocyte.(ABSTRACT TRUNCATED AT 250 WORDS)     

Beta gamma-subunit of bovine transducin composed of two components with distinctive gamma-subunits

During the process of transduction of a photon signal in vertebrate rod outer segments, transducin, a guanine nucleotide binding protein, mediates between a photobleaching intermediate of rhodopsin and a cGMP-phosphodiesterase. We report here that the beta gamma-subunit of bovine transducin (T beta gamma) characterized so far consists of two components (T beta gamma-1 and T beta gamma-2), which can be separated by anion exchange chromatography under nondenaturing conditions. Both components consisted of two polypeptides of Mr 36,000 (T beta) and about 8,000 (T gamma) in sodium dodecyl sulfate polyacrylamide (13%) gel electrophoresis. On a further analysis by 8 M urea/sodium dodecyl sulfate-polyacrylamide gel electrophoresis, T gamma subunits of T beta gamma-1 and T beta gamma-2 showed Mr values of 8,000 (T gamma-1) and 6,000 (T gamma-2), respectively. Amino acid compositions of both T gamma-1 and T gamma-2 roughly corresponded with that of T gamma previously reported and were quite different from that of gamma-subunit of cGMP-phosphodiesterase. Western blot analysis of freshly isolated rod outer segments by an antiserum raised against a mixture of T beta gamma-1 and T beta gamma-2 revealed the presence of both components in the membranes of a starting material. This observation excludes the possibility that one of the components might be produced artificially in the course of the purification. In the presence of a photobleaching intermediate of either unphosphorylated or phosphorylated rhodopsin, the binding of guanosine 5'-(beta, gamma-imido)triphosphate (GppNHp) to the alpha-subunit of transducin (T alpha) was remarkably enhanced with increasing concentrations of purified T beta gamma-2. On the contrary, T beta gamma-1 retained little ability, if any, to enhance the GppNHp binding to T alpha; the ability of T beta gamma-1 was at least 30 times lower than that of T beta gamma-2. Such a low activity of T beta gamma-1 was attributed to inability for coupling of T alpha with a photobleaching intermediate of rhodopsin. These results indicate that T gamma-2 is essential for the GTP binding of transducin. The role of T gamma-1 in vertebrate photoreceptor cells was discussed.     

Lack of association of Ca(2+)-calmodulin with the beta gamma-subunits of the photo-receptor G protein (transducin).

We previously reported that the beta gamma-subunit of transducin (T beta gamma) is composed of two components, T beta gamma-1 and T beta gamma-2 with distinctive gamma-subunits, T gamma-1 and T gamma-2, respectively. T beta gamma-2 enhances GTP binding to the alpha-subunit of transducin (T alpha) in the presence of a photobleaching intermediate of rhodopsin, while T beta gamma-1 is an inactive component with little enhancement ability (Fukada, Y., Ohguro, H., Saito, T., Yoshizawa, T., and Akino, T. (1989) J. Biol. Chem. 264: 5937-5943). To further elucidate the functional differences between T beta gamma-1 and T beta gamma-2, we examined the association of T beta gamma s with Ca(2+)-calmodulin, and the effect of Ca2+ on binding of GTP to T alpha in the presence of either T beta gamma-1 or T beta gamma-2. Ca2+ had no effect on the GTP binding activity of transducin and T beta gamma s could not associate with Ca(2+)-calmodulin, indicating that the relationship of T beta gamma with Ca(2+)-calmodulin of is different from that of the brain G protein.     

Regulation of exocytosis from rat peritoneal mast cells by G protein beta gamma-subunits.

We applied G protein-derived beta gamma-subunits to permeabilized mast cells to test their ability to regulate exocytotic secretion. Mast cells permeabilized with streptolysin-O leak soluble (cytosol) proteins over a period of 5 min and become refractory to stimulation by Ca2+ and GTPgammaS over approximately 20-30 min. beta gamma-Subunits applied to the permeabilized cells retard this loss of sensitivity to stimulation (run-down) and it can be inferred that they interact with the regulatory mechanism for secretion. While alpha-subunits are without effect, beta gamma-subunits at concentrations >10(-8 )M enhance the secretion due to Ca2+ and GTPgammaS. Unlike the small GTPases Rac and Cdc42, beta gamma-subunits cannot induce secretion in the absence of an activating guanine nucleotide, and thus further GTP-binding proteins (likely to be Rho-related GTPases) must be involved. The enhancement due to beta gamma-subunits is mediated largely through interaction with pleckstrin homology (PH) domains. It remains manifest in the face of maximum activation by PMA and inhibition of PKC with the pseudosubstrate inhibitory peptide. Soluble peptides mimicking PH domains inhibit the secretion due to GTPgammaS and block the enhancement due to beta gamma-subunits. Our data suggest that beta gamma-subunits are components of the pathway of activation of secretion due to receptor-mimetic ligands such as mastoparan and compound 48/80.     

Kinetics of inhibition of platelet calpain II by human kininogens.

Organophosphate-resistant and -susceptible strains of Culex quinquefasciatus (mosquito) have been compared on the basis of their esterase activities. The homozygous resistant strain (Dar) shows two highly active esterases after starch-gel electrophoresis, of Rm 0.2 and 0.4, which are absent from susceptible strains (Apo, Mon), and which previous selection studies have shown to be inseparable from organophosphate resistance. After SDS/polyacrylamide-gel electrophoresis and silver staining of total C. quinquefasciatus proteins, a 62 kDa band is observed in strain Dar at high concentrations, and in susceptible strains in trace amounts. After Western blotting, this 62 kDa protein is recognized by antisera raised against the two esterases eluted from starch gels. After chromatofocusing of Dar proteins, the 62 kDa protein is seen to be associated with esterase activity, and of a similar pI to that observed for esterases after isoelectric focusing. Post-translational modification is not required for recognition of the 62 kDa putative esterase, since the protein is immunoprecipitated by the anti-esterase serum from products of translation of Dar mRNA in vitro.     

A novel mechanism for the inhibition of adenylate cyclase via inhibitory GTP-binding proteins. Calmodulin-dependent inhibition of the cyclase catalyst by the beta gamma-subunits of GTP-binding proteins

The adenylate cyclase catalytic protein partially purified from rat brain membranes was activated by the stimulatory GTP-binding protein (Gs), forskolin, and Ca2+-calmodulin. The Ca2+-calmodulin-stimulated activity was markedly, but the Gs- or forskolin-stimulated activity was essentially not, inhibited by low concentrations of the beta gamma-subunits of the inhibitory GTP-binding protein (Gi). The inhibition appeared to be competitive with calmodulin. On the other hand, the association of increasing amounts of beta gamma with the alpha of Gi, which was measured based on the ADP-ribosylation by islet-activating protein, pertussis toxin, was apparently competed by Ca2+-calmodulin. Furthermore, beta gamma bound to calmodulin-Sepharose in the presence of Ca2+, but not in its absence. Thus, the direct interaction of beta gamma with calmodulin is a likely mechanism involved in beta gamma-induced inhibition of the calmodulin-stimulated adenylate cyclase.     

Stimulation of human granulocyte elastase by platelet factor 4 and heparin

Highly purified platelet factor 4 (PF₄) was found to be a potent stimulator of human granulocyte elastase activity against native elastin and solubilized α elastin. Heparin neutralized this stimulation of elastolysis by PF₄, but independently stimulated granulocyte elastase activity. Chondroitin sulfate, a constituent of the PF₄ carrier molecule, also stimulated granulocyte elastase activity. The stimulation of granulocyte elastase by PF₄ occurs at known serum concentrations of PF₄.     

Interactions of pure beta gamma-subunits of G-proteins with purified beta 1-adrenoceptor

The role of the beta gamma-subunits in the interaction of G-proteins was examined with beta 1-adrenoceptors purified from turkey erythrocytes and pure beta gamma-subunits prepared from turkey erythrocytes and bovine brain. On a non-denaturing polyacrylamide gel, the mobility of beta gamma-subunits was increased when incubated with beta 1-adrenoceptor and the beta 1-adrenergic agonist 1-(-)-isoproterenol, whereas on incubation with the antagonist 1-alprenolol the mobility was unchanged. Furthermore, the beta 1-adrenoceptor was retarded on a Sephadex G-50 column equilibrated with beta gamma-subunits and agonist. No retardation occurred in the presence of antagonist. These data suggest a direct interaction of activated beta 1-adrenoceptors with isolated beta gamma-subunits of G-proteins.     

Stimulation and inhibition of human T-lymphocyte colony cell proliferation by hemopoietic cell factors.

Human lymphocytes, isolated from peripheral blood and stimulated with phytohemagglutinin M (PHA) prior to being seeded on a two-layer medium of soft agar which contained the mitogen, developed into colonies 3–4 days after seeding in the culture system. The cloning potential of PHA-treated lymphocytes is significantly enhanced by adding, to the soft agar culture, culture fluid (CF) obtained from mitogen-treated lymphocytes or a feeder layer (FL) prepared either from lymphocytes isolated from peripheral blood or from T-cell enriched populations. PHA seems to stimulate the release of lymphocyte colony enhancing factor (LCEF) from the T-sensitized lymphocytes. The addition of CF or FL to the culture medium appears to increase the amount of LCEF, resulting in enhancement of the number and size of lymphocyte colonies. When CF derived from spleen cells or from the peripheral blood adherent-cell population was added to the lower layer of the soft agar culture, the growth and development of lymphocyte colonies was inhibited. This suggests that monocyte-macrophages release a lymphocyte colony inhibiting factor (LCIF) into the CF. The extent of inhibition or stimulation of colony formation is a function of the number and type of cells used to prepare the CF or FL and the concentration of CF in the culture medium. The presence of FL or CF derived from spleen non-adherent cells, white blood cells, bone marrow cells, or a B-cell enriched population had no effect on colonies growing in the culture. This may possibly be due to the paucity of T lymphocytes and monocyte-macrophages present in these materials. A control system in which LCIF, produced by monocyte-macrophages, and LCEF, produced by T lymphocytes, participate in the regulation of lymphocyte production is postulated.     

Stimulation of Ca2+-activated human platelet phospholipase A2 by diacylglycerol.

The adenosine analogues 5'-(N-ethyl)carboxamidoadenosine (NECA) and N6-(phenylisopropyl)adenosine (PIA) activate glycogen phosphorylase 5-fold and 4.2-fold respectively in rat hepatocytes incubated in the absence of endogenous adenosine. Half-maximally effective concentrations are 0.5 microM for NECA and 20 microM for PIA, demonstrating the presence of A2-adenosine receptors. Exogenous adenosine activates phosphorylase 4.6-fold, but high rates of adenosine disappearance from the medium render estimates of its half-maximally effective concentration unreliable. These effects of NECA and adenosine are inhibited by 0.1 mM-caffeine. Activation of phosphorylase by a physiological concentration of adenosine (3.3 microM) was 50% inhibited by a physiological concentration of caffeine (35 microM).     

Ni-mediated inhibition of human platelet adenylate cyclase by thrombin

Cefoxitin, a poor substrate of the RTEM beta-lactamase (penicillin amido-beta-lactam hydrolase, EC 3.5.2.6), induces a reversible change in the conformation of the enzyme. The change is manifested in gradual loss of catalytic activity and increased susceptibility to proteolytic inactivation. It is prevented by antibodies, which stabilize the native conformation. By contrast, divalent cations, which have no effect on the native enzyme, delay recovery from the cefoxitin-induced state, presumably by reacting with sites made accessible in the partly unfolded enzyme. Prolonged exposure to excess of cefoxitin causes a similar delay. The kinetic evidence, namely, the initial burst of consumption of cefoxitin and the subsequent gradual recovery of activity with better substrates, appears to be consistent with acylation of the active site by cefoxitin followed by a slower deacylation step [Fisher et al. (1980) Biochemistry 19, 2895-2901]. However, additional evidence leads us to conclude that the kinetics observed reflect deformation of the active site, rather than its blockage, by cefoxitin. Of most significance is the transient change in specificity, i. e. a preferential interaction of the recovering enzyme with substrates which are closest in structure to cefoxitin.     

Mechanism of inhibition of transducin GTPase activity by fluoride and aluminum

Transducin, the guanyl nucleotide-binding protein of the retinal light-activated cGMP phosphodiesterase system, is structurally and functionally similar to the inhibitory and stimulatory guanyl nucleotide-binding proteins, Gi and Gs, of the adenylate cyclase complex. All are heterotrimers composed of alpha, beta, and gamma subunits. Gs and Gi can be activated by NaF with AlCl3 as well as by agonists acting through specific receptors. The effects of NaF and AlCl3 on transducin were investigated in a reconstituted system consisting of the purified subunits of transducin (T alpha, T beta, gamma) and rhodopsin. NaF noncompetitively inhibited the GTPase activity of T alpha in a concentration- and time-dependent manner. Inhibition by NaF was enhanced synergistically by AlCl3 which alone only slightly inhibited GTPase activity. None of the other anions tested reproduced the effect of fluoride. Fluoride inhibited [3H]guanosine 5'-(beta, gamma-imido)triphosphate binding to T alpha and release of bound GDP. The ADP-ribosylation of T alpha by pertussis toxin and binding of T alpha to rhodopsin, both of which are enhanced in the presence of T beta gamma, were inhibited by NaF and AlCl3. These findings are consistent with the hypothesis that fluoride enhances the dissociation of T alpha from T beta gamma, resulting in the inhibition of GTP-GDP exchange, and therefore, GTP hydrolysis.     

Binding of alpha- and beta gamma-subunits of Go to beta 1-adrenoceptor in sealed unilamellar lipid vesicles.

First, we describe a preparation of sealed unilamellar lipid vesicles. When this preparation was subjected to sucrose density gradient centrifugation, two rather uniform fractions emerged, one consisting of lighter lipid-rich vesicles with average diameters ranging over 150-200 nm (fraction I), the other consisting of heavier vesicles with average diameters ranging over 30-70 nm (fraction II). When the lipid mixture containing dimyristoylglycerophosphocholine, cholesterol, dipalmitoylglycerophosphoserine and dipalmitoylglycerophosphoethanolamine at molar ratios of 54:35:10:1 was reconstituted with alpha- and beta gamma-subunits of Go-proteins purified to homogeneity from bovine brain, the lipid-rich lighter vesicle fraction I took up these subunits nearly exclusively. Whereas, when a beta 1-adrenoceptor preparation purified from turkey erythrocyte membranes was reconstituted, it was found nearly completely in the smaller heavier vesicle fraction II where it was incorporated inside-out. On co-reconstitution of either alpha o or beta gamma alone with beta 1-adrenoceptors, some of these subunits appear together with beta 1-adrenoceptors in the small vesicle fraction II, but much more alpha o was bound to the receptor in the presence of beta gamma-subunits. The observations reported are novel and surprising in several respects: firstly, they suggest that beta gamma-subunits can bind to the non-activated beta 1-receptor where they may serve as an anchor for alpha-subunits. Secondly, the binding of alpha o- and beta gamma-subunits to the beta 1-adrenoceptors enhances the basal GTPase activity of alpha o. Thirdly, since the binding domains of the beta 1-adrenoceptor for G-proteins were facing outwards in our sealed vesicle preparations, it follows that interactions of G-proteins with the beta-receptor can occur at the aqueous membrane interface as was postulated originally by M. Chabre [Trends Biochem. Sci. 12, 213-215 (1987)] for the transducin-rhodopsin interactions. Finally, the binding of Go-subunits from bovine brain to a beta 1-adrenoceptor from turkey erythrocytes was not expected, since these polypeptides are not likely to be physiological partners.     

Inhibition of hormonally regulated adenylate cyclase by the beta gamma subunit of transducin.

Transducin (T), the GTP-binding protein of the retina activates the cGMP phosphodiesterase system, and presents analogies with the proteins GS and Gi which respectively mediate adenylate cyclase activation and inhibition by hormone receptors. These proteins are all comprised of an alpha subunit carrying the GTP-binding site and a beta gamma subunit made of two peptides. The beta peptide (35 kd) appears similar in the three proteins. We demonstrate here that purified T beta gamma inhibits adenylate cyclase from human platelet membranes. This inhibition was observed when adenylate cyclase was stimulated by GTP, prostaglandin E1 (PGE1), NaF and forskolin, but not when stimulated by GTP(gamma)S. In the presence of GTP and forskolin, the T beta gamma-induced maximal inhibition was not additive with the alpha 2-receptor-induced adenylate cyclase inhibition mediated by Gi. Both inhibitions were suppressed at high Mg2+ concentrations, which as also known to dissociate T beta gamma from T alpha-GDP. This suggests that these adenylate cyclase inhibitions are due to the formation of inactive complexes of GS alpha-GDP with T beta gamma or Gi beta gamma. T beta gamma-induced inhibition did not require detergent and could be suppressed by simple washing. T beta gamma effects are dependent on its concentration rather than on its total amount. This suggests that T beta gamma can operate in solution with no integration into the membrane. Similar inhibitory effects of T beta gamma are observed on adenylate cyclase from anterior pituitary and lymphoma S49 cell lines.     

Mechanism of inhibition of transducin guanosine triphosphatase activity by vanadate

The visual excitation system of the retinal rod outer segments and the hormone-sensitive adenylate cyclase complex are regulated through guanine nucleotide-binding proteins, transducin in the former and inhibitory and stimulatory regulatory components, Gi and Gs, in the latter. These proteins are functionally and structurally similar; all are heterotrimers composed of alpha, beta, and gamma subunits and exhibit guanosine triphosphatase activity stimulated by light-activated rhodopsin or the agonist-receptor complex. Adenylate cyclase can be stimulated by vanadate, which, like NaF, probably acts through Gs. Effects of vanadate on the function of a guanine nucleotide-binding protein were investigated in a reconstituted model system consisting of purified transducin subunits (T alpha, T beta gamma) and rhodopsin in phosphatidylcholine vesicles. Vanadate (decameric) inhibited [3H]GTP binding to T alpha and noncompetitively inhibited GTP hydrolysis in a concentration-dependent manner with maximal inhibition of approximately 90% at 3-5 mM. Vanadate also inhibited release of bound GDP but did not affect the rate of hydrolysis of bound GTP (single turnover rate), indicating that vanadate did not interfere with the intrinsic GTPase activity of T alpha. Binding of T alpha to rhodopsin and the ADP-ribosylation of T alpha by pertussis toxin, both of which are enhanced in the presence of T beta gamma, were inhibited by vanadate. These findings are consistent with the conclusion that vanadate can cause the dissociation of T alpha from T beta gamma, resulting in the inhibition of GDP-GTP exchange and thereby GTP hydrolysis. Adenylate cyclase activation could result from a similar effect of vanadate on Gs.