Protein Tyrosine Phosphorylation in Synaptic Vesicles

Protein tyrosine phosphorylation in purified synaptic vesicles from rat forebrain has been studied in the presence of Mn2+ and orthovanadate. High levels of endogenous protein tyrosine phosphorylation were observed. Four major phosphoproteins, with apparent molecular masses of 105, 94, 38, and 30 kDa, were shown to contain phosphotyrosine. The 38-kDa phosphoprotein was identified as synaptophysin (p38), a well-characterized integral membrane protein of synaptic vesicles. The three other phosphotyrosine-containing proteins distributed in the same manner as synaptophysin in all subcellular fractions. Like synaptophysin, the two high molecular weight phosphotyrosine proteins (105 and 94 kDa) were found to be glycoproteins by lectin chromatography. Tyrosine phosphorylation of synaptophysin was an intravesicular reaction and reached 50% of maximal level within 3 min. Triton X-100, a nonionic detergent, inhibited tyrosine phosphorylation of endogenous protein substrates but not the phosphorylation of an exogenous substrate, poly(Glu80,Tyr20). Tyrosine phosphorylation of synaptophysin was also demonstrated in synaptosomes, indicating that tyrosine phosphorylation of synaptic vesicle proteins occurs in intact nerve terminals.