MULTIPLE KINETIC COMPONENTS OF EXOCYTOSIS DISTINGUISHED BY NEUROTOXINSENSITIVITY

The secretion of synaptic and other vesicles is a complex process invo lving multiple steps. Many molecular components of the secretory appar atus have been identified, but how they relate to the different stages of vesicle release is not clear. We examined this issue in adrenal ch romaffin cells, where capacitance measurements and amperometry allow u s to measure vesicle fusion and hormone release simultaneously. Using flash photolysis of caged intracellular calcium to induce exocytosis, we observed three distinct kinetic components to vesicle fusion, of wh ich only two are related to catecholamine release. Intracellular dialy sis with botulinum neurotoxin E, D or C1 or tetanus-toxin light chains abolishes the catecholamine-related components, but leaves the third component untouched. Botulinum neurotoxin A, which removes nine amino acids from the carboxy(C)-terminal end of SNAP-25, does not eliminate catecholamine release completely, but slows down both catecholamine-re lated components. Thus we assign a dual role to SNAP-25 and suggest th at its nine C-terminal amino acids are directly involved in coupling t he calcium sensor to the final step in exocytosis.