4.1.3.1  Hypothesis: SARS-CoV and SARS-CoV2 are ideally similar in the structure and the cell entry receptor and protease
SARS-CoV and SARS-CoV2 share absolutely the same cleavage junctions, almost the same sequence (96%) of their main protease, a high degree (76%) of similarity in the amino acid sequence of their S protein, a similar S2′ cleavage site, a similar spectrum of cells they can enter, and the similarity of the most residues essential for binding ACE2 [16], [17], [18]. Also, both of them utilize the same domain of S1B to interact with the ACE2 receptor. However, they differ in proteolytic processing to some degree. Study [16] of the human embryonic kidney (HEK) cell line, 293 T, has shown that a signal for the S2 subunit is present in cells inoculated with SARS-2-S, but not in cells inoculated with SARS-S.
Two main proteases for both SARS-S and SARS-2-S are endosomal cysteine proteases cathepsin B and L (CatB/L) and the transmembrane protease, serine 2TMPRSS2 [16]. In 293 T cells lacking 2TMPRSS2, blocking CatB/L activity through increasing the endosomal pH by ammonium chloride could significantly limit the entry of both SARS-S and SARS-2-S. In TMPRSS2 + Caco-2 cells, the effect of ammonium chloride existed to a lesser extent. A combination of camostat mesylate, a blocker of TMPRSS2, and E-64d, an inhibitor of CatB/L, yielded the complete inhibition of SARS-2-S entry in TMPRSS2 + Caco-2 cells. In both the human lung cancer cell line Calu-3 and the primary human lung cells, there was a reduction of the entry of both SARS-S and SARS-2-S by camostat mesylate, indicating that SARS-S and SARS-2-S partially require TMPRSS2 for a lung infection.