Distribution and properties of protein kinase and protein phosphatase activities in synaptosomal plasma membranes and...

Some characteristics of the protein kinase activity associated with a synaptosomal plasma membrane (synaptic membrane) fraction and a synaptic junction fraction have been compared. Autoradiography of the phosphorylated fractions separated on sodium dodecyl sulfate polyacrylamide gels showed that cyclic AMP stimulates the phosphorylation of five polypeptides in synaptic membranes, whereas no cyclic AMP dependency could be detected in synaptic junctions. Kinetic studies demonstrated that synaptic junctions contain a high Km and a low Km protein kinase activity while only the high Km activity could be detected in synaptic membranes. The intrinsic ATPase activity of synaptic membranes was shown to strongly interfere with measurements of protein kinase activity. Cyclic AMP binding experiments revealed a 2.6-fold enrichment of cyclic AMP binding capacity in synaptic junctions as compared to synaptic membranes. Protein phosphatase activity was not detected in synaptic junctions but was associated with synaptic membranes, where cyclic AMP was shown to either stimulate or inhibit the dephosphorylation of different polypeptides.