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 cooperatively 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. Neuronal communication involves the controlled release of neurotransmitters stored in synaptic vesicles (SV) into the neuronal synapse [1-4]. This process is tightly regulated to ensure that the message is timely and precise [3, 4]. SV fusion is catalyzed by the synaptic SNARE (soluble N-ethyl maleimide sensitive factor attachment protein) proteins-VAMP2 on the vesicle membrane (v-SNARE) and Syntaxin-1 and SNAP25 on the plasma membrane (t-SNAREs) [5, 6]. When the vesicle approaches the plasma membrane (PM), the helical SNARE motifs of the cognate v-and t-SNAREs constitutively assemble into a ternary complex that initially bridges and ultimately fuses the two membranes [5-7]. The SNARE complex nucleates into a four-helix bundle at its membrane distal end and progressively assembles ("zippers") towards the membranes, exerting a potent force that ultimately drives the membranes together to fuse into one [6, 8, 9]. Though efficient, under in vitro conditions, the fusion process is artificially slow with its rate being limited by the intrinsic rate of nucleation of SNARE complex [10-12]. In vivo, nucleation is greatly accelerated by two cooperating , specialized molecular chaperones, Munc13 and Munc18 [3, 4, 13, 14].