Proteolytic activation of SARS‐CoV‐2 spike protein

AbstractSpike (S) protein cleavage is a crucial step in coronavirus infection. In this review, this process is discussed, with particular focus on the novel coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2). Compared with influenza virus and paramyxovirus membrane fusion proteins, the cleavage activation mechanism of coronavirus S protein is much more complex. The S protein has two cleavage sites (S1/S2 and S2′), and the cleavage motif for furin protease at the S1/S2 site that results from a unique four‐amino acid insertion is one of the distinguishing features of SARS‐CoV‐2. The viral particle incorporates the S protein, which has already undergone S1/S2 cleavage by furin, and then undergoes further cleavage at the S2′ site, mediated by the type II transmembrane serine protease transmembrane protease serine 2 (TMPRSS2), after binding to the receptor angiotensin‐converting enzyme 2 (ACE2) to facilitate membrane fusion at the plasma membrane. In addition, SARS‐CoV‐2 can enter the cell by endocytosis and be proteolytically activated by cathepsin L, although this is not a major mode of SARS‐CoV‐2 infection. SARS‐CoV‐2 variants with enhanced infectivity have bee