English: Munc18–1 binds to a partially closed conformation of syntaxin that is organized in clusters and that may (bottom branch) or may not (top branch) be associated with SNAP-25 at a 1:1 stoichiometry. After vesicle docking, synaptobrevin interacts with this complex, thereby displacing Munc18–1. Alternatively, SNAP-25 is not yet associated with the complex, and synaptobrevin binding is associated with the simultaneous recruitment of SNAP-25. As result, SNARE trans-complexes form, leading to exocytosis. It is possible that some syntaxin molecules that are freely diffusing in the membrane are capable of binding Munc18–1 with high affinity in a closed conformation (left) similar to that observed in solution, thereby preventing it from entering SNARE complexes. Such a pool could be reflected by Munc18–1 remaining membrane associated even after extensive washing periods. doi:10.1371/journal.pbio.0040330.g006