Synchronous release at neuronal synapses is accomplished by a machinery that senses calcium influx and fuses the synaptic vesicle and plasma membranes to release neurotransmitters. Previous studies suggested the calcium sensor synaptotagmin (Syt) is a facilitator of vesicle docking and both a facilitator and inhibitor of fusion. On phospholipid monolayers, the Syt C2AB domain spontaneously oligomerized into rings that are disassembled by Ca 2+ , suggesting Syt rings may clamp fusion as membrane-separating “washers” until Ca 2+ -mediated disassembly triggers fusion and release [J. Wang et al., Proc. Natl. Acad. Sci. U.S.A. 111, 13966–13971 (2014)].). Here, we combined mathematical modeling with experiment to measure the mechanical properties of Syt rings and to test this mechanism. Consistent with experimental results, the model quantitatively recapitulates observed Syt ring-induced dome and volcano shapes on phospholipid monolayers and predicts rings are stabilized by anionic phospholipid bilayers or bulk solution with ATP. The selected ring conformation is highly sensitive to membrane composition and bulk ATP levels, a property that may regulate vesicle docking and fusion in ATP-rich synaptic terminals. We find the Syt molecules hosted by a synaptic vesicle oligomerize into a halo, unbound from the vesicle, but in proximity to sufficiently phosphatidylinositol 4,5-bisphosphate (PIP2)-rich plasma membrane (PM) domains, the PM-bound trans Syt ring conformation is preferred. Thus, the Syt halo serves as landing gear for spatially directed docking at PIP2-rich sites that define the active zones of exocytotic release, positioning the Syt ring to clamp fusion and await calcium. Our results suggest the Syt ring is both a Ca 2+ -sensitive fusion clamp and a high-fidelity sensor for directed docking.

中文翻译：

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突触结合蛋白环作为突触小泡对接和融合的高灵敏度调节剂


神经元突触的同步释放是通过感知钙流入并融合突触小泡和质膜以释放神经递质的机制来实现的。先前的研究表明钙传感器突触结合蛋白（Syt）是囊泡对接的促进剂，也是融合的促进剂和抑制剂。在磷脂单层上，Syt C2AB 结构域自发寡聚成环，并被 Ca 分解2+ ，表明 Syt 环可能作为膜分离“垫圈”夹紧融合，直到 Ca 2+介导的分解触发融合和释放[J.王等人，过程。国家。阿卡德。科学。美国111, 13966–13971 (2014)].)。在这里，我们将数学模型与实验相结合来测量 Syt 环的机械性能并测试该机制。与实验结果一致，该模型定量地概括了在磷脂单层上观察到的 Syt 环诱导的圆顶和火山形状，并预测阴离子磷脂双层或含有 ATP 的本体溶液可以稳定环。选定的环构象对膜组成和大量 ATP 水平高度敏感，这一特性可以调节富含 ATP 的突触末端的囊泡对接和融合。我们发现由突触小泡承载的 Syt 分子寡聚成一个光环，未与囊泡结合，但足够接近富含磷脂酰肌醇 4,5-二磷酸 (PIP2) 的质膜 (PM) 结构域，即 PM 结合的反式 Syt 环构象是优选的。因此，Syt 环充当起落架，用于空间定向对接在富含 PIP2 的位点，这些位点定义了胞吐释放的活性区域，定位 Syt 环以夹紧融合并等待钙。 我们的结果表明 Syt 环既是 Ca 2+ - 用于定向对接的灵敏熔接夹和高保真传感器。