ADP-RIBOSYLATION OF G(S) BY CHOLERA-TOXIN IS POTENTIATED BY AGONIST ACTIVATION OF BETA-ADRENERGIC RECEPTORS IN THE ABSENCE OF GTP

Purified G(s) is a substrate for ADP-ribosylation catalyzed by cholera toxin (CTx). In S49 cyc- membranes complemented with in vitro transla ted G(s)alpha, the beta-adrenergic agonist isoproterenol enhanced the ADP-ribosylation rate. This effect was maximal if all guanyl nucleotid es were suppressed but was blocked by the beta-adrenergic antagonist a lprenolol. Enhancement was partially diminished if addition of GDP fol lowed that of isoproterenol. When added in the absence of agonist, the GTP analogues guanosine 5'-O-(gamma-thiotriphosphate) and guanosine 5 '-(beta,gamma-imido)triphosphate potentiated CTx-catalyzed ADP-ribosyl ation of G(s)alpha consistent with their activating ADP-ribosylation f actors. However, this effect was lessened when the same nucleotides we re tested in the presence of agonist. Taken altogether, these results indicate that like G(t) and G(i), G(s) is an optimal substrate for CTx when coupled to an agonist-activated receptor and depleted of nucleot ide. Therefore, coupling to the receptor and subsequent departure of t he GDP turn out to be the common features underlying the sensitivity o f all GTP-binding proteins to CTx-catalyzed ADP-ribosylation.