The soluble, cyclic AMP-stimulated protein kinase catalyses the phosphorylation of different membrane proteins from those which are phosphorylated by the membrane bound enzyme.

Soluble, cyclic AMP stimulated protein kinase (type a1 protein kinase) stimulated the phosphorylation of membrane proteins in preparations from a wide variety of tissues. This kinase does not, however, appear to catalyse the phosphorylation of the same membrane proteins which are phosphorylated by the intrinsic protein kinase activity of the preparations. This was demonstrated in two ways: 1. If preparations of membrane fragments are incubated with [gamma 32P] ATP in the absence of type a1 protein kinase, until all membrane bound protein acceptor sites are saturated with phosphate addition of type a1 protein kinase causes the incorporation of more phosphate into the membrane proteins. 2. The nature of the membrane proteins phosphorylated by type a1 protein kinase and the intrinsic protein kinase of the preparation was compared by SDS gel electrophoresis. Membrane preparations were phosphorylated with [gamma 32P] ATP in the absence of added type a1 protein kinase and with [gamma 33P] ATP in the presence of the enzyme. Aliquots of the two samples were then mixed, separated by SDS gel electrophoresis, and the distribution of radioactivity measured. A comparison of the distribution of protein bound 33P and 32P made it quite clear that type a1 protein kinase catalysed the phosphorylation of different membrane proteins from those which are phosphorylated by the intrinsic protein kinase activity of the preparations.