Receptor binding and priming of SARS-CoV-2 spike proteins for membrane fusion

SARS-CoV-2 infection is initiated by virus binding to ACE2 cell surface receptors1-4, To release the virus genome into the cell after fusion of virus and cell membrane. Both receptor binding and membrane fusion activities are performed by the virus spike glycoprotein, S.5-7. As with other class I membrane fusion proteins, S-is post-transcriptionally cleaved, in this case by furin, into S1 and S2 components that remain attached after cleavage.8-10. Fusion activation after receptor binding is proposed to involve the interaction of a second proteolytic site (S2 ‘), which is required for fusion peptide release.11,12. We have investigated ACE2 for the radical-cleaved form of SARS-CoV-2 S by cryoAM. We classify ten different molecular species including unbound, closed spike trimers, fully exposed ACE2-bound trimers, and monomeric S1 bound to ACE2. Ten constructs describe ACE2 binding events that destabilize the spike trimer, progressively opening and exiting, the individual S1 components. The initial process reduces S1 contacts and shields the fusion activation and dissociation of trimeric S2 cores, ACE2-bound S1 monomers. The constructs also reveal a refleeding of the S1 subdomain after ACE2 binding, which impedes interaction with S2, specifically with Asp61413-15, Leading to destabilizing the structure of the S2 proximal (S2 ‘) cleavage site.