ABSTRACT-Adenylate cyclase purified by affinity chromatography was activated about 2.5-fold in a Ca2+ and calmodulin-dependent fashion. G protein pr-subunits, an inhibitor in the receptor-mediated inhibition of adenylate cyclase, inhibited the purified cyclase by more than 80°Io. The extent of pr-induced inhibition was not affected by the activation with Ca 2+ and calmodulin. Moreover, the prior addition of the Pr subunits to the cyclase did not prevent the subsequent activation of the enzyme by Ca 2+ and calmodulin. We conclude that the pr-subunits inhibit adenylate cyclase activity in a calmodulin-independent mode.

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