Abstract
The soluble N-ethylmaleimide-sensitive factor-attachment protein receptors (SNAREs) constitute the core machinery for membrane fusion during eukaryotic cell vesicular trafficking. However, how the assembly of the SNARE complex is initiated is unknown. Here we report that Sec3, a component of the exocyst complex that mediates vesicle tethering during exocytosis, directly interacts with the t-SNARE protein Sso2. This interaction promotes the formation of an Sso2-Sec9 ‘binary’ t-SNARE complex, the early rate-limiting step in SNARE complex assembly, and stimulates membrane fusion. The crystal structure of the Sec3-Sso2 complex suggests that Sec3 binding induces conformational changes of Sso2 that are crucial for the relief of its auto-inhibition. Interestingly, specific disruption of the Sec3–Sso2 interaction in cells blocks exocytosis without affecting the function of Sec3 in vesicle tethering. Our study reveals an activation mechanism for SNARE complex assembly, and uncovers a role of the exocyst in promoting membrane fusion in addition to vesicle tethering.
Highlights
The soluble N-ethylmaleimide-sensitive factor-attachment protein receptors (SNAREs) constitute the core machinery for membrane fusion during eukaryotic cell vesicular trafficking
The mechanisms that control the initial assembly of the binary t-SNARE complex, which applies to membrane fusion in most eukaryotic cells, remain elusive
Our study reveals a novel mechanism for the early rate-limiting step of SNARE complex assembly and membrane fusion
Summary
The soluble N-ethylmaleimide-sensitive factor-attachment protein receptors (SNAREs) constitute the core machinery for membrane fusion during eukaryotic cell vesicular trafficking. We report that Sec[3], a component of the exocyst complex that mediates vesicle tethering during exocytosis, directly interacts with the t-SNARE protein Sso[2]. This interaction promotes the formation of an Sso2-Sec9 ‘binary’ t-SNARE complex, the early rate-limiting step in SNARE complex assembly, and stimulates membrane fusion. The exocyst is localized to the growing end of daughter cell (the ‘bud tip’) or mother-daughter cell junction (the ‘bud neck’), site of active exocytosis and surface expansion[19] This pattern of localization stands in contrast to that of the t-SNARE proteins Sso1/2 and Sec[9], which are distributed throughout the plasma membrane[20]. It is thought that the exocyst tethers the vesicles at specific regions of the plasma membrane before SNARE-mediated fusion
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