Inhibitors of cathepsin L prevent severe acute respiratory syndrome coronavirus entry

Severe acute respiratory syndrome (SARS) is caused by an emergent coronavirus (SARS-CoV), for which there is currently no effective treatment. SARS-CoV mediates receptor binding and entry by its spike (S) glycoprotein, and Infection is sensitive to lysosomotropic agents that perturb endosomal pH. We demonstrate here that the lysosomotropic-agent-mediated block to SARS-CoV Infection is overcome by protease treatment of target-cell-associated virus. In addition, SARS-CoV Infection was blocked by specific inhibitors of the pH-sensitive endosomal protease Cathepsin L. A cell-free membrane-fusion system demonstrates that engagement of receptor followed by proteolysis is required for SARS-CoV membrane fusion and indicates that Cathepsin L is sufficient to activate membrane fusion by SARS-CoV S. These results suggest that SARS-CoV Infection results from a unique, three-step process: receptor binding and induced conformational changes in S glycoprotein followed by Cathepsin L proteolysis within endosomes. The requirement for Cathepsin L proteolysis identifies a previously uncharacterized class of inhibitor for SARS-CoV Infection.