SARS coronavirus, but not human coronavirus NL63, utilizes cathepsin L to infect ACE2-expressing cells

Viruses require specific cellular receptors to infect their target cells. Angiotensin-converting Enzyme 2 (ACE2) is a cellular receptor for two divergent coronaviruses, SARS coronavirus (SARS-CoV) and human coronavirus NL63 (HCoV-NL63). In addition to hostcell receptors, lysosomal cysteine proteases are required for productive Infection by some viruses. Here we show that SARS-CoV, but not HCoV-NL63, utilizes the enzymatic activity of the cysteine protease Cathepsin L to infect ACE2-expressing cells. Inhibitors of Cathepsin L blocked Infection by SARS-CoV and by a retrovirus pseudotyped with the SARS-CoV spike (S) protein but not Infection by HCoV-NL63 or a retrovirus pseudotyped with the HCoV-NL63 S protein. Expression of exogenous Cathepsin L substantially enhanced Infection mediated by the SARS-CoV S protein and by Filovirus GP proteins but not by the HCoV-NL63 S protein or the vesicular stomatitis virus G protein. Finally, an inhibitor of endosomal acidification had substantially less effect on Infection mediated by the HCoV-NL63 S protein than on that mediated by the SARS-CoV S protein. Our data indicate that two coronaviruses that utilize a common receptor nonetheless enter cells through distinct mechanisms.