Biophysical Society Thematic Meeting| Padova 2019

Quantitative Aspects of Membrane Fusion and Fission

Tuesday Speaker Abstracts

SEC1/MUNC18 PROTEINS CATALYZE SNARE ASSEMBLY BY TEMPLATING SNARE FOLDING AND ASSOCIATION Yongli Zhang 1 ; Junyi Jiao 1 ; Mengze He 1 ; Sarah A Port 2 ; Richard W Baker 2 ; Yonggang Xu 1 ; Hong Qu 1 ; Yujian Xiong 1 ; Yukun Wang 1 ; Huaizhou Jin 1 ; Travis J Eisemann 2 ; Frederick M Hughson 2 ; 1 Yale School of Medicine, Department of Cell Biology, New Haven, Connecticut, USA 2 Princeton University, Department of Molecular Biology, Princeton, New Jersey, USA Sec1/Munc18-family (SM) proteins are required for SNARE-mediated membrane fusion, but their mechanism(s) of action remain controversial. Using single-molecule force spectroscopy, we found that the SM protein Munc18-1 catalyzes step-wise zippering of three synaptic SNAREs (syntaxin, VAMP2, and SNAP-25) into a four-helix bundle. Catalysis requires formation of an intermediate template complex in which Munc18-1 juxtaposes the N-terminal regions of the SNARE motifs of syntaxin and VAMP2, while keeping their C-terminal regions separated. The template complex is relatively weak, with an unfolding energy of 3.1 kcal/mol, and is stabilized by the N-terminal regulatory domain (NRD) of syntaxin. SNAP-25 binds the templated SNAREs to induce full SNARE zippering. Munc18-1 and SNARE mutations modulate the stability of the template complex in a manner consistent with their effects on membrane fusion, indicating that chaperoned SNARE assembly is essential for exocytosis. Two other SM proteins, Munc18-3 and Vps33, similarly chaperone SNARE assembly via a template complex, suggesting that SM protein mechanism is conserved.

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