Synaptic vesicles (SVs) fuse with the presynaptic membrane (PM) to release neuronal transmitters. The SV protein synaptotagmin 1 (Syt1) serves as a Ca2+ sensor for evoked fusion. Syt1 is thought to trigger fusion by penetrating the PM upon Ca2+ binding; however, the mechanistic detail of this process is still debated. Syt1 interacts with the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) complex, a coiled-coil four-helical bundle that enables the SV-PM attachment. The SNARE-associated protein complexin (Cpx) promotes Ca2+-dependent fusion, possibly interacting with Syt1. We employed all-atom molecular dynamics to investigate the formation of the Syt1-SNARE-Cpx complex interacting with the lipid bilayers of the PM and SVs. Our simulations demonstrated that the PM-Syt1-SNARE-Cpx complex can transition to a “dead-end” state, wherein Syt1 attaches tightly to the PM but does not immerse into it, as opposed to a prefusion state, which has the tips of the Ca2+-bound C2 domains of Syt1 inserted into the PM. Our simulations unraveled the sequence of Syt1 conformational transitions, including the simultaneous docking of Syt1 to the SNARE-Cpx bundle and the PM, followed by Ca2+ chelation and the penetration of the tips of Syt1 domains into the PM, leading to the prefusion state of the protein-lipid complex. Importantly, we found that direct Syt1-Cpx interactions are required to promote these transitions. Thus, we developed the all-atom dynamic model of the conformational transitions that lead to the formation of the prefusion PM-Syt1-SNARE-Cpx complex. Our simulations also revealed an alternative dead-end state of the protein-lipid complex that can be formed if this pathway is disrupted.

中文翻译：

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蛋白质-脂质预融合复合物的动态形成


突触小泡 （SV） 与突触前膜 （PM） 融合以释放神经元递质。SV 蛋白突触结合蛋白 1 （Syt1） 用作诱发融合的 Ca2+ 传感器。Syt1 被认为通过在 Ca2+ 结合时穿透 PM 来触发融合;然而，这个过程的机制细节仍然存在争议。Syt1 与 SNARE（可溶性 N-乙基马来酰亚胺敏感因子附着蛋白受体）复合物相互作用，SNARE 是一种卷曲螺旋四螺旋束，可实现 SV-PM 附着。SNARE 相关蛋白复合蛋白 （Cpx） 促进 Ca2+ 依赖性融合，可能与 Syt1 相互作用。我们采用全原子分子动力学来研究 Syt1-SNARE-Cpx 复合物与 PM 和 SVs 脂质双层相互作用的形成。我们的模拟表明，PM-Syt1-SNARE-Cpx 复合物可以转变为“死胡同”状态，其中 Syt1 紧紧附着在 PM 上但不浸入其中，这与预融合状态相反，预融合状态将 Syt1 的 Ca2+ 结合的 C2 结构域的尖端插入 PM。我们的模拟揭示了 Syt1 构象转变的序列，包括 Syt1 同时对接 SNARE-Cpx 束和 PM，然后是 Ca2+ 螯合和 Syt1 结构域的尖端渗透到 PM，导致蛋白质-脂质复合物的预融合状态。重要的是，我们发现需要直接的 Syt1-Cpx 相互作用来促进这些转变。因此，我们开发了导致预融合 PM-Syt1-SNARE-Cpx 复合物形成的构象转变的全原子动力学模型。我们的模拟还揭示了如果该途径被破坏，可能会形成蛋白质-脂质复合物的另一种死胡同状态。