Mutations in the GTP-binding site of G(sα) alter stimulation of adenylyl cyclase

Mutational replacements of specific residues in the GTP-binding pocket of the 21-kDa ras proteins (p21(ras)) reduce their GTPase activity. To test the possibility that the cognate regions of G protein α chains participate in GTP binding and hydrolysis, we compared signaling functions of normal and mutated α chains (termed α(s)) of G(s), the stimulatory regulator of adenylyl cyclase. α(s) chains were expressed in an α(s)-deficient S49 mouse lymphoma cell line, cyc^- α(s) in which leucine replaces glutamine 227 (corresponding to glutamine 61 of p21(ras)) constitutively activates adenylyl cyclase and reduces the k(cat) for GTP hydrolysis more than 100-fold. There is a smaller reduction in GTPase activity in another mutant in which valine replaces glycine 49 (corresponding to glycine 12 of p21(ras)). This mutant α(s) is a poor activator of adenylyl cyclase. Moreover, the glycine 49 protein, unlike normal α(s), is not protected against tryptic cleavage by hydrolysis resistant GTP analogs; this finding suggests impairment of the mutant protein's ability to attain the active (GTP-bound) conformation. We conclude that α(s) residues near glutamine 227 and glycine 49 participate in binding and hydrolysis of GTP, although the GTP binding regions of α(s) and p21(ras) are not identical.