SNARE assembly enlightened by cryo-EM structures of a synaptobrevin-Munc18-1-syntaxin-1 complex

Munc18-1 forms a template to recruit and organize assembly of the SNARE complex that triggers neurotransmitter release, binding first to a closed conformation of syntaxin-1 where its N-terminal region interacts with the SNARE motif, and later binding to synaptobrevin. However, the mechanism of SNARE complex assembly remains unclear. Here we report two cryo-EM structures of Munc18-1 bound to cross-linked syntaxin-1 and synaptobrevin. The structures allow visualization of how syntaxin-1 opens and reveal how part of the syntaxin-1 N-terminal region can help nucleate interactions between the N-termini of the syntaxin-1 and synaptobrevin SNARE motifs while their C-termini bind to distal sites of Munc18-1. Mutagenesis, SNARE complex assembly assays and reconstitution experiments support a model whereby these interactions are critical to initiate SNARE complex assembly. One Sentence Summary Cryo-EM structures reveal key insights into the molecular mechanism of neuronal SNARE complex assembly templated by Munc18-1