Infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is initiated by virus binding to the ACE2 cell-surface receptors 1 – 4 , followed by fusion of the virus and cell membranes to release the virus genome into the cell. Both receptor binding and membrane fusion activities are mediated by the virus spike glycoprotein 5 – 7 . As with other class-I membrane-fusion proteins, the spike protein is post-translationally cleaved, in this case by furin, into the S1 and S2 components that remain associated after cleavage 8 – 10 . Fusion activation after receptor binding is proposed to involve the exposure of a second proteolytic site (S2′), cleavage of which is required for the release of the fusion peptide 11 , 12 . Here we analyse the binding of ACE2 to the furin-cleaved form of the SARS-CoV-2 spike protein using cryo-electron microscopy. We classify ten different molecular species, including the unbound, closed spike trimer, the fully open ACE2-bound trimer and dissociated monomeric S1 bound to ACE2. The ten structures describe ACE2-binding events that destabilize the spike trimer, progressively opening up, and out, the individual S1 components. The opening process reduces S1 contacts and unshields the trimeric S2 core, priming the protein for fusion activation and dissociation of ACE2-bound S1 monomers. The structures also reveal refolding of an S1 subdomain after ACE2 binding that disrupts interactions with S2, which involves Asp614 13 – 15 and leads to the destabilization of the structure of S2 proximal to the secondary (S2′) cleavage site. Cryo-electron microscopy structures of consecutive binding events of ACE2 in complex with the spike protein of SARS-CoV-2 reveal the mechanisms of receptor binding by the spike protein and activation for membrane fusion by the spike protein of SARS-CoV-2.

中文翻译：

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SARS-CoV-2 刺突蛋白的受体结合和启动以实现膜融合


严重急性呼吸综合征冠状病毒 2 (SARS-CoV-2) 的感染是由病毒与 ACE2 细胞表面受体 1 – 4 结合引发的，随后病毒与细胞膜融合，将病毒基因组释放到细胞中。受体结合和膜融合活性均由病毒刺突糖蛋白 5 – 7 介导。与其他 I 类膜融合蛋白一样，刺突蛋白在翻译后被裂解（在这种情况下被弗林蛋白酶裂解）成 S1 和 S2 成分，这些成分在裂解后仍保持关联 8 – 10 。受体结合后的融合激活被认为涉及第二个蛋白水解位点(S2')的暴露，其裂解是融合肽11、12的释放所必需的。在这里，我们使用冷冻电子显微镜分析了 ACE2 与 SARS-CoV-2 刺突蛋白的弗林蛋白酶切割形式的结合。我们对十种不同的分子种类进行了分类，包括未结合的闭合刺突三聚体、完全开放的 ACE2 结合三聚体以及与 ACE2 结合的解离单体 S1。这十个结构描述了 ACE2 结合事件，这些事件破坏了尖峰三聚体的稳定性，逐渐打开和释放各个 S1 组件。打开过程减少了 S1 接触并解除了三聚体 S2 核心的屏蔽，为蛋白质的融合激活和 ACE2 结合的 S1 单体的解离做好了准备。这些结构还揭示了 ACE2 结合后 S1 亚结构域的重新折叠，这会破坏与 S2 的相互作用，其中涉及 Asp614 13 – 15 并导致靠近二级 (S2') 裂解位点的 S2 结构不稳定。 ACE2 与 SARS-CoV-2 刺突蛋白复合物连续结合事件的冷冻电镜结构揭示了刺突蛋白与受体结合以及 SARS-CoV-2 刺突蛋白激活膜融合的机制。