Munc13 binds and recruits SNAP25 to chaperone SNARE complex assembly.

Synaptic vesicle fusion is mediated by SNARE proteins - VAMP2 on the vesicle and Syntaxin-1/SNAP25 on the pre-synaptic membrane. Chaperones Munc18-1 and Munc13-1 co-operatively catalyze SNARE assembly via an intermediate 'template' complex containing Syntaxin-1 and VAMP2. How SNAP25 enters this reaction remains a mystery. Here we report that Munc13-1 recruits SNAP25 to initiate the ternary SNARE complex assembly by direct binding, as judged by bulk FRET spectroscopy and single-molecule optical tweezer studies. Detailed structure-function analyses show that the binding is mediated by the Munc13-1 MUN domain and is specific for the SNAP25 'linker' region that connects the two SNARE motifs. Consequently, freely diffusing SNAP25 molecules on phospholipid bilayers are concentrated and bound in ~1:1 stoichiometry by the self-assembled Munc13-1 nano-clusters.