Inhibition of catalytic unit of adenylate cyclase and activation of GTPase of Ni protein by βγ‐subunits of GTP‐binding proteins

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Abstract
A protein factor which inhibited adenylate cyclase was purified to apparent homogeneity from rat brain and identified as the βγ-subunits of the GTP-binding regulatory proteins of adenylate cyclase. (i) The βγ-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 α-subunit of the inhibitory regulatory protein (Niα) 3.8-fold, (iii) The subunits stimulated ADP-ribosylation of niα catalyzed by islet-activating protein (pertussis toxin). ADP-ribosylation had no effect on the GTPase activity of Niα in the presence of the βγ-subunits. The results suggest that direct inhibition of the catalytic unit by the βγ-subunits liberated from Ni is essential for the receptor-mediated inhibition of adenylate cyclase