Inhibition of catalytic unit of adenylate cyclase and activation of GTPase of Ni protein by beta gamma-subunits of GTP-binding proteins.

A protein factor which inhibited adenylate cyclase was purified to apparent homogeneity from rat brain and identified as the beta gamma-subunits of the GTP-binding regulatory proteins of adenylate cyclase. (i) The beta gamma-subunits (protein factor) inhibited the partially purified catalytic unit of adenylate cyclase in the presence of an activator, forskolin or the stimulative regulatory protein (Ns), to 60 and 40% of the control, respectively; inhibition of the catalytic unit in the presence of forskolin required no guanine nucleotides. (ii) The subunits enhanced the GTPase activity of the purified alpha-subunit of the inhibitory regulatory protein (Ni alpha) 3.8-fold. (iii) The subunits stimulated ADP-ribosylation of Ni alpha catalyzed by islet-activating protein (pertussis toxin). ADP-ribosylation had no effect on the GTPase activity of Ni alpha in the presence of the beta gamma-subunits. The results suggest that direct inhibition of the catalytic unit by the beta gamma-subunits liberated from Ni is essential for the receptor-mediated inhibition of adenylate cyclase.