Cell entry mechanisms of coronaviruses : To detect the conformational changes of viral fusion protein.

先前的研究表明，SARS-CoV S蛋白需要通过弹性蛋白酶、组织蛋白酶或TMPRSS2进行蛋白水解裂解，以实现S介导的细胞-细胞或病毒-细胞膜融合。蛋白酶对流感病毒的病毒糖蛋白（GP）活化也有报道。流感病毒和sars冠状病毒之间最显著的区别是病毒复制过程中病毒糖蛋白被蛋白酶裂解的阶段。在流感病毒中，蛋白酶在成熟过程中在GP上做一个简单的切割。相比之下，SARS-CoV S蛋白在受体诱导的构象改变后被蛋白酶切割。S蛋白的蛋白酶裂解位点被认为只有在受体结合后才会暴露。为了支持这一模型，我们报道了小鼠肝炎病毒2型（MHV-2）的S蛋白与SARS-CoV的S蛋白高度相似，需要通过顺序受体结合和蛋白水解介导的两步构象变化来激活膜融合。这种机制通过保护激活的裂解位点不被过早的蛋白水解，同时允许在与靶细胞上的受体结合时进行有效的裂解，从而对融合进行严格的时间控制。

Previous studies have demonstrated that the SARS-CoV S protein requires proteolytic cleavage by elastase, cathepsin or TMPRSS2 for S-mediated cell-cell or virus-cell membrane fusion. Activation of viral glycoprotein (GP) by protease also has been reported for influenza virus. The most distinctive difference between influenza virus and SARS-CoV is the stage during virus replication in which viral glycoproteins are cleaved by proteases. In influenza virus, the protease makes a simple cut in the GP during maturation. In contrast, SARS-CoV S protein is cleaved by the protease following receptor-induced conformational changes. The protease cleavage site in S protein is thought to be exposed only after receptor binding. In support of this model, we reported that the S protein of mouse hepatitis virus type 2 (MHV-2), which is highly similar to the S protein of SARS-CoV, requires two-step conformational changes mediated by sequential receptor binding and proteolysis to be activated for membrane fusion. Such a mechanism allows for tight temporal control over fusion by protecting the activating cleavage site from premature proteolysis yet allowing efficient cleavage upon binding to the receptor on target cells.