Control of fusion pore dynamics during exocytosis by Munc18

Intracellular membrane fusion is mediated by the SNARE (soluble N-ethylmale imide-sensitive factor attachment protein receptor) proteins. All vesicle t ransport steps also have an essential requirement for a member of the Sec1 protein family, including the neuronal Munc18-1 (also known as nSec1) in re gulated exocytosis. Here, in adrenal chromaffin cells, we expressed a Munc1 8 mutant with reduced affinity for syntaxin, which specifically modified th e kinetics of single-granule exocytotic release events, consistent with an acceleration of fusion pore expansion, Thus, Munc18 functions in a late sta ge in the fusion process, where its dissociation from syntaxin determines t he kinetics of postfusion events.