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.