Furin cleaves SARS-CoV-2 spike-glycoprotein at S1/S2 and S2'for viral fusion/entry: indirect role for TMPRSS2

The Spike (S)-protein of SARS-CoV-2 binds host-cell receptor ACE2 and requires proteolytic "priming" (S1/S2) and "fusion-activation" (S2) for viral entry. The S-protein furin-like motifs PRRAR685{downarrow} and KPSKR815{downarrow} indicated that proprotein convertases promote virus entry. We demonstrate that furin and PC5A induce cleavage at both sites, ACE2 enhances S2 processing, and their pharmacological inhibition (BOS-inhibitors) block endogenous cleavages. S1/S2-mutations (S1/S2) limit S-protein-mediated cell-to-cell fusion, similarly to BOS-inhibitors. Unexpectedly, TMPRSS2 does not cleave at S1/S2 or S2, but it can: (i) cleave/inactivate S-protein into S2a/S2b; (ii) shed ACE2; (iii) cleave S1-subunit into secreted S1, activities inhibited by Camostat. In lung-derived Calu-3 cells, BOS-inhibitors and {micro}S1/S2 severely curtail "pH-independent" viral entry, and BOS-inhibitors alone/with Camostat potently reduce infectious viral titer and cytopathic effects. Overall, our results show that: furin plays a critical role in generating fusion-competent S-protein, and indirectly, TMPRSS2 promotes viral entry, supporting furin and TMPRSS2 inhibitors as potential antivirals against SARS-CoV-2.