Synaptic vesicles isolated from super(32)P-prelabeled synaptosomes contain a phosphorprotein of apparent M sub(r) 65,000 (pp65), a possible substrate for PKC

In the present study, we have investigated the subcellular localization of pp65, a synaptosomal phosphoprotein of apparent M sub(r) 65,000. The results obtained strongly support that pp65 is localized to synaptic vesicles. The solubility properties of pp65, especially its partitioning into the detergent phase of Triton X-114, indicated that it is tightly associated with the membrane of synaptic vesicles. pp65 is multiply phosphorylated exclusively on serine. By studying the decay of labeled phosphate following incubation of super(32)P-prelabeled synaptosomes in the presence of cold inorganic phosphate, we have found that pp65 shows an unusually higher turnover of phosphate. These results indicated that one of the protein kinases involved in steady state phosphorylation of pp65 is PKC-dependent or is PKC itself.