Mechanism and Properties of Inhibition of Purified Rat Brain Adenylate Cyclase by G Protein βγ-Subunits

Abstract

The mode of the inhibition of purified rat brain adenylate cyclase by the βγ-subunits of G protein (βγ) was studied. These subunits inhibited the catalytic activity of the cyclase with the maximal inhibition of 85% and the half-maximal inhibition at about 0.7 nM βγ. The complex of βγ and adenylate cyclase isolated by density gradient centrifugation contained 1.8-2.0 mol βγ per mol of the cyclase when βγ was assayed by immunoblotting and by its inhibitory activity on adenylate cyclase. However, the βγ concentration-inhibition curves suggest that one of the two βγ molecules bound may be essential for the inhibition. The role for the second βγ molecule is unknown. As a tentative estimate, 70% of the adenylate cyclase activity remained inhibited by βγ when the complex was isolated. The inhibition was not dependent on Gαs or calmodulin. Although purified adenylate cyclase contained a protein (0.06-0.08 mol/mol of adenylate cyclase) that reacted with anti-Gαs antibody, this protein was not liberated from the cyclase when it formed a complex with βγ. In addition, guanine nucleotide analogs little affected the cyclase activity or the inhibition by βγ. The inhibition by βγ was reversed by the dilution of the complex, and the following re-addition of βγ suppressed the enzyme activity to about 15% of the initial activity again. These findings provide strong evidence that βγ inhibits adenylate cyclase directly and reversibly through the formation of the complex.