Calcium-dependent protein phosphorylation in bovine anterior pituitary membranes and intact cells

Abstract

A calmodulin-activated protein kinase has been identified in bovine anterior pituitary membranes. This enzyme phosphorylated one endogenous substrate of subunit molecular weight 53000 in the membranes. Phosphorylation of this protein was rapid, was half-maximal at 2.5 μM calcium in the presence of saturating concentrations of calmodulin (CaM), and was inhibited by trifluoperazine and thioridazine. A second protein was phosphorylated by an endogenous protein kinase in anterior pituitary membranes. Phosphorylation of this 42 000 M r protein was reduced by calcium, was independent of exogenously added CaM, and was increased by trifluoperazine or thioridazine. The 42 000 M r, protein may be the α-subunit of pyruvate dehydrogenase. Calcium-dependent protein phosphorylation was also observed in intact cells; the largest increases were seen in proteins of M r 42 000, 21 000 and 17 000.