Department of Molecular, Cellular, and Developmental Biology, University of Colorado, Boulder, CO 80309, USA; Jiangsu Key Laboratory for Molecular and Medical Biotechnology, College of Life Sciences, Nanjing Normal University, Nanjing 210023, China.
Department of Molecular, Cellular, and Developmental Biology, University of Colorado, Boulder, CO 80309, USA.
Cell Rep. 2021 Jan 12;34(2):108611. doi: 10.1016/j.celrep.2020.108611.
Intracellular vesicle fusion is catalyzed by soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs). Vesicle-anchored v-SNAREs pair with target membrane-associated t-SNAREs to form trans-SNARE complexes, releasing free energy to drive membrane fusion. However, trans-SNARE complexes are unable to assemble efficiently unless activated by Sec1/Munc18 (SM) proteins. Here, we demonstrate that SNAREs become fully active when the v-SNARE is split into two fragments, eliminating the requirement of SM protein activation. Mechanistically, v-SNARE splitting accelerates the zippering of trans-SNARE complexes, mimicking the stimulatory function of SM proteins. Thus, SNAREs possess the full potential to drive efficient membrane fusion but are suppressed by a conformational constraint. This constraint is removed by SM protein activation or v-SNARE splitting. We suggest that ancestral SNAREs originally evolved to be fully active in the absence of SM proteins. Later, a conformational constraint coevolved with SM proteins to achieve the vesicle fusion specificity demanded by complex endomembrane systems.
细胞内囊泡融合由可溶性 N-乙基马来酰亚胺敏感因子附着蛋白受体 (SNAREs) 催化。囊泡锚定的 v-SNARE 与靶膜相关的 t-SNARE 配对形成跨 SNARE 复合物，释放自由能以驱动膜融合。然而，除非被 Sec1/Munc18 (SM) 蛋白激活，否则跨 SNARE 复合物无法有效地组装。在这里，我们证明当 v-SNARE 被分裂成两个片段时，SNARE 完全激活，从而消除了对 SM 蛋白激活的需求。从机制上讲，v-SNARE 的分裂加速了跨 SNARE 复合物的拉链运动，模拟了 SM 蛋白的刺激功能。因此，SNARE 具有驱动有效膜融合的全部潜力，但受到构象限制的抑制。这种限制通过 SM 蛋白激活或 v-SNARE 分裂而消除。我们认为，原始 SNARE 最初在没有 SM 蛋白的情况下进化为完全活跃。后来，一种构象限制与 SM 蛋白共同进化，以实现复杂的内体膜系统所要求的囊泡融合特异性。
