Release of neuronal transmitters from nerve terminals is triggered by the molecular Ca2+ sensor Synaptotagmin 1 (Syt1). Syt1 is a transmembrane protein attached to the synaptic vesicle (SV), and its cytosolic region comprises two domains, C2A and C2B, which are thought to penetrate into lipid bilayers upon Ca2+ binding. Prior to fusion, SVs become attached to the presynaptic membrane (PM) by the four-helical SNARE complex, which is thought to bind the C2B domain in vivo. To understand how the interactions of Syt1 with lipid bilayers and the SNARE complex trigger fusion, we performed molecular dynamics (MD) simulations at a microsecond scale. We investigated how the isolated C2 modules and the C2AB tandem of Syt1 interact with membranes mimicking either SV or PM. The simulations showed that the C2AB tandem can either bridge SV and PM or insert into PM with its Ca2+-bound tips, and that the latter configuration is more favorable. Surprisingly, C2 domains did not cooperate in penetrating into PM, but instead mutually hindered their insertion into the bilayer. To test whether the interaction of Syt1 with lipid bilayers could be affected by the C2B-SNARE attachment, we performed systematic conformational analysis of the C2AB-SNARE complex. Notably, we found that the C2B-SNARE interface precludes the coupling of C2 domains and promotes their insertion into PM. We performed the MD simulations of the pre-fusion protein complex positioned between the lipid bilayers mimicking PM and SV, and our results demonstrated in silico that the presence of the Ca2+ bound C2AB tandem promotes lipid merging. Altogether, our MD simulations elucidated the role of the Syt1-SNARE interactions in the fusion process and produced the dynamic all-atom model of the prefusion protein-lipid complex.

中文翻译：

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SNARE 复合物改变 Ca2+ 传感器 Synaptotagmin 1 与脂质双层的相互作用

神经末梢神经递质的释放由分子 Ca2+ 传感器 Synaptotagmin 1 (Syt1) 触发。Syt1 是一种附着在突触小泡 (SV) 上的跨膜蛋白，其胞质区域包含两个结构域 C2A 和 C2B，它们被认为在 Ca2+ 结合后会渗透到脂质双层中。在融合之前，SV 通过四螺旋 SNARE 复合物附着在突触前膜 (PM) 上，该复合物被认为在体内结合 C2B 结构域。为了了解 Syt1 与脂质双层的相互作用以及 SNARE 复合物如何触发融合，我们进行了微秒级的分子动力学 (MD) 模拟。我们研究了孤立的 C2 模块和 Syt1 的 C2AB 串联如何与模拟 SV 或 PM 的膜相互作用。模拟表明，C2AB 串联既可以桥接 SV 和 PM，也可以通过其 Ca2+ 结合尖端插入 PM，并且后一种配置更有利。令人惊讶的是，C2 域在渗透到 PM 中没有合作，而是相互阻碍了它们插入双层。为了测试 Syt1 与脂质双层的相互作用是否会受到 C2B-SNARE 附着的影响，我们对 C2AB-SNARE 复合物进行了系统的构象分析。值得注意的是，我们发现 C2B-SNARE 接口排除了 C2 域的耦合并促进它们插入 PM。我们对位于模拟 PM 和 SV 的脂质双层之间的融合前蛋白复合物进行了 MD 模拟，我们的结果在计算机上证明了 Ca2+ 结合的 C2AB 串联的存在促进了脂质融合。共，