A Cryptic Site of Vulnerability on the Receptor Binding Domain of the SARS-CoV-2 Spike Glycoprotein

M. Gordon Joyce; Rajeshwer S. Sankhala; Wei-Hung Chen; Misook Choe; Hongjun Bai; Agnes Hajduczki; Lianying Yan; Spencer L. Sterling; Caroline E. Peterson; Ethan C. Green; Clayton Smith; Natalia de Val; Mihret Amare; Paul Scott; Eric D. Laing; Christopher C. Broder; Morgane Rolland; Nelson L. Michael; Kayvon Modjarrad

To understand whether CR3022 could bind to SARS-CoV S glycoproteins, we measured binding to stabilized S-2P or non-stabilized versions of S (Figure 4) . We observed robust binding to the nonstabilized S glycoprotein, while binding to SARS S-2P Trimer was low. We then treated the SARS S-2P trimer with trypsin and/or incubation with the ACE2 receptor to assess whether minimal proteolytic action or receptor binding could increase the availability of the "cryptic" CR3022 epitope. Incubation of the stabilized S-2P trimer with human ACE2 did not dramatically affect CR3022 binding, while in contrast, the trypsin treatment of the S-2P protein resulted in increased binding akin to the unstabilized S glycoprotein binding, and the level of binding was titratable, with increasing amounts of S-2P resulting in higher CR3022 binding. Given the prior neutralization and protection studies utilizing CR3022, and its ability to complement potent neutralizing antibodies, it is likely that the CR3022 epitope represents a "cryptic" epitope that becomes exposed during the processes of viral cell entry.