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.

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