SARS-CoV-2 requires acidic pH 6.2 to 6.8 to infect cells; sequential steps of S1-fragment dissociation, fusion, & content release from cell surface in TMPRRS2-overexpressing cells ONLY if exposed to
by Paul Alexander
acidic pH; Membrane fusion (viral & host cell) and genome penetration require virion exposure to an acidic milieu of pH 6.2 to 6.8, even when furin and TMPRSS2 cleavages have occurred.
‘Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) cell entry begins with engagement at the cell surface and ends with deposition of the viral contents into the cytosol by membrane fusion. The first step is binding of the viral spike (S) protein with its cellular receptor angiotensin converting enzyme (ACE2) (1–4). The last step delivers the viral genomic RNA in association with the nucleocapsid protein (N), which is removed for translation of the input genome (5, 6). Proteolytic activation of S by additional host-cell factors is necessary for it to function as a fusogen. Cleavage of S by furin in producer cells (7) generates the S1 receptor binding subunit noncovalently associated with the S2 fusion subunit. The S protein is cleaved by cell surface or endosomal proteases during virion entry into host cells, which activate the viral fusion machinery (1, 8–10). This entry-associated proteolysis of S has led to the current model of two routes of infectious cell entry, as follows: fusion of viral and cellular membranes at the host-cell surface or fusion following endosomal uptake (6).
The cellular proteases that are involved in processing S during entry include the transmembrane serine proteases TMPRSS2 or TMPRSS4 found at the cell surface (1, 8) and the endosomal cathepsins that require the acidic milieu of the compartments in which they are enriched (1, 10). Processing of S by TMPRSS proteases or by cathepsins, at a site designated S2', depends on prior cleavage at the furin site in the producer cells (7, 11, 12). TMPRSS cleavage has been thought to result in infection from the plasma membrane and cathepsin cleavage, in cells lacking TMPRSS activity, with infection from endosomes (5, 6). Chemical inhibitors of TMPRSS or cathepsin proteases in cells in culture indeed show that infection of some cell types is more sensitive to inhibition of endosomal cathepsins, whereas others are more sensitive to inhibition of TMPRSS proteases (1, 9). TMPRSS inhibitors such as camostat and nafamostat are in clinical development as SARS-CoV-2 therapeutics, further highlighting the need to understand how entry pathways depend upon specific proteases.’