G Protein βγ-Subunits Inhibit Purified Adenylate Cyclase Independent of the Activation by Ca²⁺ and Calmodulin

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Adenylate cyclase purified by affinity chromatography was activated about 2.5-fold in a Ca²⁺- and calmodulin-dependent fashion. G protein βγ-subunits, an inhibitor in the receptor-mediated inhibition of adenylate cyclase, inhibited the purified cyclase by more than 80%. The extent of βγ-induced inhibition was not affected by the activation with Ca²⁺ and calmodulin. Moreover, the prior addition of the βγ-subunits to the cyclase did not prevent the subsequent activation of the enzyme by Ca²⁺ and calmodulin. We conclude that the βγ-subunits inhibit adenylate cyclase activity in a calmodulin-independent mode.