Inhibition of SNARE complex assembly differentially affects kinetic components of exocytosis

In chromaffin cells, an increase in intracellular Ca2+ leads to an exocytot ic burst followed by sustained secretion. The burst can be further resolved into two kinetically distinct components, which suggests the presence of t wo separate pools of vesicles. To investigate how these components relate t o SNARE complex formation, we introduced an antibody that blocks SNARE asse mbly but not disassembly. In the presence of the antibody, the sustained co mponent was largely blocked, the burst was slightly reduced, and one of its kinetic components was eliminated. We conclude that SNARE complexes form b efore Ca2+-triggered membrane fusion and exist in a dynamic equilibrium bet ween a loose and a tight state, both of which support exocytosis. Interacti on of the antibody with preformed SNARE complexes favors the loose state.