Syntaxin opening by the MUN domain underlies the function of Munc13 in synaptic-vesicle priming

Abstract

UNC-13-Munc13s play a central function in synaptic vesicle priming through their MUN domains. However, it is unclear whether this function arises from the ability of the MUN domain to mediate the transition from the Munc18-1–closed syntaxin-1 complex to the SNARE complex in vitro. The crystal structure of rat Munc13-1 MUN domain now reveals an elongated, arch-shaped architecture formed by α-helical bundles, with a highly conserved hydrophobic pocket in the middle. Mutation of two residues (NF) in this pocket abolishes the stimulation caused by the Munc13-1 MUN domain on SNARE complex assembly and on SNARE-dependent proteoliposome fusion in vitro. Moreover, the same mutation in UNC-13 abrogates synaptic vesicle priming in C. elegans neuromuscular junctions. These results strongly support the notion that orchestration of syntaxin-1 opening and SNARE complex assembly underlies the central role of UNC-13-Munc13s in synaptic vesicle priming.

Molecular Mechanisms of Synaptic Vesicle Priming by Munc13 and Munc18

“…As expected, the MUN domain was required for the transit of syntaxin-1A into the ternary SNARE complex (Ma et al, 2011;Yang et al, 2015). When the MUN domain concentration was increased, more ternary SNARE complex was formed and, in contrast, BSA control did not lead to ternary complex formation ( Figures 4B, 4F and Table S1).…”

HID-1 is required for homotypic fusion of immature secretory granules during maturation

“…Munc13s, and related neuronal proteins called CAPS (calcium-dependent activator protein for secretion), are essential players during priming, and contain a large module called the MUN domain. It has been proposed that the MUN domain dramatically accelerates the transition from the closed syntaxin 1–Munc18-1 complex to the SNARE complex, probably by extracting the SNARE motif of syntaxin 1 from the closed conformation and providing a template for SNARE complex assembly (Ma et al, 2011; Yang et al, 2015). Structural analyses have shown that the MUN domain has a similar architecture to domains found in vesicle tethering factors such as Sec6, Tip20, and Exo70 (Yang et al, 2015).…”

Molecular Mechanisms of Synaptic Vesicle Priming by Munc13 and Munc18

“…The MUN domain of Munc13-1 (amino acid range 859–1531) catalyzes the transit of syntaxin-1A from the syntaxin-1A / Munc18-1 complex into the ternary SNARE complex (Basu et al, 2005; Ma et al, 2013; Yang et al, 2015). Earlier studies in C. elegans suggested a similar function of full-length unc-13 (Richmond et al, 2001), although the role of the MUN domain has not been directly studied in that system.…”