Fig. 3.

Two distinct SARS-CoV-2 entry pathways. Left: when transmembrane protease, serine 2 (TMPRSS2, SARS-CoV-2 uses it to activate their entry glycoproteins) is absent on the cell surface or the SARS-CoV-2-ACE2 complex does not bind TMPRSS2, the SARS-CoV-2-ACE2 complex is internalized into lysosomes through endocytosis. Under the action of Cathepsin L (A soluble cysteine proteinase involved in antigen processing in lysosomes), S-protein is cleaved, exposing the S2 site, mediating membrane fusion and releasing viral RNA. Right: when TMPRSS2 is present on the cell surface, the S-protein is cleaved on the cell surface and the S2 site is exposed, mediating membrane fusion and releasing RNA. Adapted from Ref. [52].