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The spike (S) glycoprotein on the surface of the virus particle of SARS-CoV-2 can bind to the host cell receptor, so that the virus membrane and the host cell fusion membrane occurs. The S protein of the virus is related to ACE2, and the binding of the S protein of SARS-CoV-2 to ACE2 may cause changes in the three-dimensional structure of the viral glycoprotein. This structural change can expose a cleavage site and increase viral susceptibility to cathepsin L cleavage, thereby enhancing viral fusion within the host cell. In addition, the S protein of SARS-CoV and SARS-CoV-2 consists of two functional regions called S1 and S2. Different subunits of the S1 domain are responsible for receptor binding and stabilization, while the S2 domain promotes structural rearrangement necessary for the fusion of virus and host cell membranes. For SARS-CoV and SARS-CoV-2, the S1 receptor binding domain (RBD) is required for these virions to bind to ACE2, thus contributing to the ability of these particles to cause host cell infections.