Severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2) is a strain of coronavirus that causes COVID-19, the respiratory illness responsible for the COVID-19 pandemic. SARS-CoV-2 has four structural proteins, known as the S (spike), E (envelope), M (membrane), and N (nucleocapsid) proteins; the N protein holds the RNA genome, and the S, E, and M proteins together create the viral envelope. Coronavirus S proteins are glycoproteins and also type I membrane proteins (membranes containing a single transmembrane domain oriented on the extracellular side). They are divided into two functional parts (S1 and S2). Spike is the protein responsible for allowing the virus to attach to and fuse with the membrane of a host cell; specifically, its S1 subunit catalyzes attachment, the S2 subunit fusion. The S1 region of the spike glycoprotein is responsible for interacting with receptor molecules on the surface of the host cell in the first step of viral entry. S1 contains two domains, called the N-terminal domain (NTD) and C-terminal domain (CTD). The CTD is responsible for the interactions of SARS-CoV-2 with angiotensin-converting enzyme 2 (ACE2).
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