A dynamic template complex mediates Munc18-chaperoned SNARE assembly

Abstract

Munc18 chaperones assembly of three membrane-anchored soluble N-ethylmaleimide- sensitive factor attachment protein receptors (SNAREs) into a four-helix bundle to mediate membrane fusion between vesicles and plasma membranes, leading to neurotransmitter or insulin release, GLUT4 translocation, or other exocytotic processes. Yet, the molecular mechanism underlying chaperoned SNARE assembly is not well understood. Recent evidence suggests that Munc18-1 and Munc18-3 simultaneously bind their cognate SNAREs to form ternary template complexes - Munc18-1:Syntaxin-1:VAMP2 for synaptic vesicle fusion and Munc18-3:Syntaxin-4:VAMP2 for GLUT4 translocation and insulin release, which facilitate binding of SNAP-25 or SNAP-23 to conclude SNARE assembly. Here, we further investigate the structure, dynamics, and function of the template complexes using optical tweezers. Our results suggest that the synaptic template complex transitions to an activated state with a rate of [~]0.05 s-1 and [~]6.8 kBT higher energy for efficient SNAP-25 binding. The transition depends upon the linker region of syntaxin-1 upstream of its helical bundle-forming SNARE motif. In addition, the template complex is stabilized by a poorly characterized disordered loop region in Munc18-1. While the synaptic template complex efficiently binds both SNAP-25 and SNAP-23, the GLUT4 template complex strongly favors SNAP-23 over SNAP-25, despite similar stabilities of their assembled SNARE bundles. Together, our data demonstrate that a highly dynamic template complex mediates efficient and specific SNARE assembly. SignificanceMunc18-1 chaperones coupled folding and assembly of three synaptic SNAREs, syntaxin-1, VAMP2, and SNAP-25, into a four-helix bundle to mediate membrane fusion and neurotransmitter release. Recent evidence suggests that Munc18-1, syntaxin-1, and VAMP2 first form a weak template complex and then bind to SNAP-25 to complete SNARE assembly. However, the dynamics and function of the template complex are not well understood. Using optical tweezers, we found that the template complex undergoes a conformational change to bind SNAP-25 in a way dependent upon the syntaxin linker region and that Munc18 kinetically proofreads SNARE pairing not governed by its thermodynamic stability. Our study reveals a more dynamic template complex than that seen in its cryo-EM structure.