Department of Molecular Biology, Princeton University, Princeton, United States
Richard W Baker
Department of Molecular Biology, Princeton University, Princeton, United States; Department of Cellular and Molecular Medicine, University of California, San Diego, San Diego, United States
Yonggang Xu
Department of Cell Biology, Yale University School of Medicine, New Haven, United States
Hong Qu
Department of Cell Biology, Yale University School of Medicine, New Haven, United States
Yujian Xiong
Department of Cell Biology, Yale University School of Medicine, New Haven, United States
Yukun Wang
Department of Cell Biology, Yale University School of Medicine, New Haven, United States
Huaizhou Jin
Department of Cell Biology, Yale University School of Medicine, New Haven, United States
Travis J Eisemann
Department of Molecular Biology, Princeton University, Princeton, United States
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. SNAP-25 binds the templated SNAREs to induce full SNARE zippering. Munc18-1 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.
