Secretory vesicles dock at the plasma membrane before Ca(2+) triggers their exocytosis. Exocytosis requires the assembly of SNARE complexes formed by the vesicle protein Synaptobrevin and the membrane proteins Syntaxin-1 and SNAP-25. We analyzed the role of Munc18-1, a cytosolic binding partner of Syntaxin-1, in large dense-core vesicle (LDCV) secretion. Calcium-dependent LDCV exocytosis was reduced 10-fold in mouse chromaffin cells lacking Munc18-1, but the kinetic properties of the remaining release, including single fusion events, were not different from controls. Concomitantly, mutant cells displayed a 10-fold reduction in morphologically docked LDCVs. Moreover, acute overexpression of Munc18-1 in bovine chromaffin cells increased the amount of releasable vesicles and accelerated vesicle supply. We conclude that Munc18-1 functions upstream of SNARE complex formation and promotes LDCV docking.

During synaptic vesicle fusion, the SNARE-protein syntaxin-1 exhibits two conformations that both bind to Munc18-1: a ‘closed’ conformation outside the SNARE-complex, and an ‘open’ conformation in the SNARE-complex. Whereas SNARE-complexes containing ‘open’ syntaxin-1 and Munc18-1 are essential for exocytosis, the significance of ‘closed’ syntaxin-1 is unknown. Here, we generated knockin/knockout mice that expressed only ‘open’ syntaxin-1B. Syntaxin-1BOpen mice were viable, but succumbed to generalized seizures at 2-3 months of age. Binding of Munc18-1 to syntaxin-1 was impaired in syntaxin-1BOpen synapses, and the size of the readily-releasable vesicle pool was decreased, whereas the rate of synaptic vesicle fusion was dramatically enhanced. Thus, the closed conformation of syntaxin-1 gates the initiation of the synaptic vesicle fusion reaction, which is then mediated by SNARE-complex/Munc18-1 assemblies.