Department of Pharmacology, University of Michigan, Ann Arbor, MI.
J Gen Physiol. 2020 Sep 7;152(9). doi: 10.1085/jgp.202012619.
Central to the exocytotic release of hormones and neurotransmitters is the interaction of four SNARE motifs in proteins on the secretory granule/synaptic vesicle membrane (synaptobrevin/VAMP, v-SNARE) and on the plasma membrane (syntaxin and SNAP25, t-SNAREs). The interaction is thought to bring the opposing membranes together to enable fusion. An underlying motivation for this Viewpoint is to synthesize from recent diverse studies possible new insights about these events. We focus on a recent paper that demonstrates the importance of the linker region joining the two SNARE motifs of the neuronal t-SNARE SNAP25 for maintaining rates of secretion with roles for distinct segments in speeding fusion pore expansion. Remarkably, lipid-perturbing agents rescue a palmitoylation-deficient mutant whose phenotype includes slow fusion pore expansion, suggesting that protein-protein interactions have a role not only in bringing together the granule or vesicle membrane with the plasma membrane but also in orchestrating protein-lipid interactions leading to the fusion reaction. Unexpectedly, biochemical investigations demonstrate the importance of the C-terminal domain of the linker in the formation of the plasma membrane t-SNARE "acceptor" complex for synaptobrevin2. This insight, together with biophysical and optical studies from other laboratories, suggests that the plasma membrane SNARE acceptor complex between SNAP25 and syntaxin and the subsequent trans-SNARE complex with the v-SNARE synaptobrevin form within 100 ms before fusion.
激素和神经递质的胞吐释放的核心是蛋白质上四个 SNARE 基序的相互作用，这些基序位于分泌颗粒/突触小泡膜（突触融合蛋白/VAMP，v-SNARE）和质膜上（突触融合蛋白和 SNAP25，t-SNAREs）。这种相互作用被认为可以将相对的膜融合在一起，从而实现融合。这一观点的一个基本动机是从最近的各种研究中综合出关于这些事件的新见解。我们专注于最近的一篇论文，该论文证明了连接神经元 t-SNARE SNAP25 的两个 SNARE 基序的连接区对于维持分泌速率的重要性，其不同的片段在加速融合孔扩张中起作用。值得注意的是，脂质扰动剂挽救了棕榈酰化缺陷突变体，其表型包括融合孔扩张缓慢，这表明蛋白质-蛋白质相互作用不仅在将颗粒或囊泡膜与质膜结合方面发挥作用，而且在协调导致融合反应的蛋白质-脂质相互作用方面也发挥作用。出乎意料的是，生化研究表明，连接区 C 端结构域对于形成突触融合蛋白 2 的质膜 t-SNARE“受体”复合物很重要。这一见解，连同其他实验室的生物物理和光学研究，表明 SNAP25 和突触融合蛋白之间的质膜 SNARE 受体复合物以及随后与 v-SNARE 突触融合蛋白的跨 SNARE 复合物在融合前 100 毫秒内形成。
