The M protein is the most abundant one on the surface of the virion and together with the E protein, plays an important role in the formation of new virions (Neuman et al., 2011; Schoeman and Fielding, 2019). [...] The S protein is composed of an upper globular structure (S1) which includes the receptor binding domain (RBD; the bit of the key) that engages with the host receptor (the lock), and a lower stalk region (S2) that is responsible for viral fusion and entry (Shang et al., 2020; Wang et al., 2020). [...] The S protein, which is one of the more variable regions of the virus, protrudes from the surface and is under selective pressure from the host’s immune system, as it is an attractive target for antibodies to bind to and prevent entry into cells. [...] Next, the membrane envelope surrounding the virus and the host cell membrane need to fuse and form an opening through which the viral genome can pass to enter the cell. [...] One important protease is called transmembrane protease, serine 2 (TMPRSS2), which snips another part of the S protein (S’), exposing the fusion domain of the S protein and activating its spring-loaded function, turning the key and bringing the viral and 3 cellular membranes close together to ‘open the door’ by forming a channel between the two membranes, liberating the viral genome in the cell.
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