Phosphorylation of 25-kDa Synaptosome-associated Protein

Protein kinase C-mediated phosphorylation of a 25-kDa synaptosome-associated protein (SNAP-25) was examined in living PC12 cells. Phorbol 12-myristate 13-acetate treatment enhanced high potassium-induced [3H]-norepinephrine release, and a 28-kDa protein recognized by an anti-SNAP-25 antibody was phosphorylated on Ser residues. The molecular size of the phosphorylated band decreased slightly following treatment with Clostridium botulinum type A neurotoxin, whereas the band disappeared after treatment with botulinum type E neurotoxin, indicating that the 28-kDa protein was SNAP-25. A phosphorylation is likely to occur at Ser187, as this is the only Ser residue located between the cleavage sites of botulinum type A and E neurotoxins. SNAP-25 of PC12 cells was phosphorylated by purified protein kinase C in vitro, and the amount of syntaxin co-immunoprecipitated with SNAP-25 was decreased by phosphorylation. These results suggest that the phosphorylation of SNAP-25 may be involved in protein kinase C-mediated regulation of catecholamine release from PC12 cells.