Similarities between protein IIIa and protein IIIb, two prominent synaptic vesicle-associated phosphoproteins.

Protein IIIa (Mr 74,000) and protein IIIb (Mr 55,000) are 2 major phosphoproteins found in mammalian brain. It was previously shown in intact nerve cells that the phosphorylation state of these 2 proteins could be increased by electrical stimulation, by depolarizing agents in the presence of calcium, and by 8-bromo-cAMP. We now report that protein IIIa and protein IIIb possess significant structural homology, as indicated by immunochemical studies using polyclonal and monoclonal antibodies and by peptide-mapping studies. A quantitative radioimmunoassay using immunolabeling in SDS-polyacrylamide gels has been used to study the tissue distribution and regional and subcellular distribution in the brain of the 2 proteins. The 2 proteins were found only in nervous tissue and the adrenal medulla. Subcellular fractionation studies suggested that both proteins are associated with synaptic vesicles.