Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.
Department of Medical Biochemistry, Max F. Perutz Laboratories, Medical University of Vienna, 1030 Vienna, Austria.
Nat Commun. 2017 Jan 23;8:14236. doi: 10.1038/ncomms14236.
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.
可溶性 N-乙基马来酰亚胺敏感因子附着蛋白受体（SNAREs）构成了真核细胞囊泡运输过程中膜融合的核心机制。然而，SNARE 复合物的组装是如何启动的尚不清楚。在这里，我们报告说，Sec3 是外泌体复合物的一个组成部分，该复合物在胞吐作用过程中介导囊泡的锚定，它可以直接与 t-SNARE 蛋白 Sso2 相互作用。这种相互作用促进了 Sso2-Sec9“二元”t-SNARE 复合物的形成，这是 SNARE 复合物组装的早期限速步骤，并刺激了膜融合。Sec3-Sso2 复合物的晶体结构表明，Sec3 的结合诱导了 Sso2 的构象变化，这对于解除其自身抑制至关重要。有趣的是，在细胞中特异性破坏 Sec3-Sso2 相互作用会阻止胞吐作用，而不会影响 Sec3 在囊泡锚定中的功能。我们的研究揭示了 SNARE 复合物组装的激活机制，并揭示了外泌体除了在囊泡锚定中还在促进膜融合方面的作用。
