Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.08.19.257261v1?rss=1 Authors: Kalayanasundaram, R. V., Jin, H., Li, F., Shu, T., Coleman, J., Yang, J., Pincet, F., Zhang, Y., Krishnakumar, S. S., Rothman, J. E. Abstract: Synaptic vesicle fusion is mediated by membrane-bridging complexes formed by SNARE proteins - VAMP2 on the vesicle and Syntaxin-1/SNAP25 on the pre-synaptic membrane. Accumulating evidence suggest that chaperones Munc18-1 and Munc13-1 co-operatively catalyze SNARE assembly via an intermediate template complex containing Syntaxin-1 and VAMP2. How SNAP25 is chaperoned into this nascent complex 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 presumably bound in ~1:1 stoichiometry by the self-assembled Munc13-1 nanoclusters. Our data suggest that Munc13-1s capacity to bind all three synaptic SNARE proteins likely underlie its chaperone function. Copy rights belong to original authors. Visit the link for more info