Neuronal exocytosis is mediated by SNARE proteins, which assemble into a highly stable four-helical bundle in a process that is not well understood. Here, electron paramagnetic resonance spectroscopy was used to examine how the t-SNAREs syntaxin and SNAP25 assemble in the presence and absence of the regulatory protein Munc18-1. Syntaxin and SNAP25 form a 2:1 complex, which is structurally heterogeneous and persists in the presence of excess SNAP25. Munc18-1 dissociates this 2:1 complex, but a 1:1 complex is retained where syntaxin is in a closed state. In the absence of an N-terminal fragment of syntaxin, Munc18-1 also stabilizes a 1:1 complex of sytaxin/SNAP25; however, syntaxin now samples an open state. These data demonstrate that the open-closed syntaxin equilibrium is shifted toward the open state when syntaxin and Munc18-1 are associated with SNAP25, and the results indicate that a syntaxin/SNAP25:Munc18-1 complex is a likely starting point for SNARE assembly.
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