Protein kinase and its endogenous substrates in coated vesicles.

Coated vesicles prepared from bovine brains contained a protein kinase activity which catalyzed the phosphorylation of endogenous structural proteins, Mr 150 000, 120 000, 48 000 and 32 000. An endogenous protein, Mr 48 000 was most strongly phosphorylated by this kinase. This protein kinase also phosphorylated exogenous proteins, phosvitin intensely and casein slightly but not histone or protamine. The enzyme activity was independent of cyclic nucleotides or Ca2+/calmodulin. Mg2+ stimulated the kinase activity. Some divalent cations were substituted for Mg2+; the potency decreased in the order Mn2+, Mg2+, Co2+, Ca2+, Zn2+. Two separate subfractions, the outer coat and the inner vesicle (core), were prepared from coated vesicles by a urea treatment followed by sucrose density gradient centrifugation and dialysis. The kinase activity was found predominantly in the coat subfraction.