Coronavirus Spike-RBD Variants Differentially Bind to the Human ACE2 Receptor

The SARS-CoV-2 betacoronavirus infects people through binding the human Angiotensin Receptor 2 (ACE2), followed by import into a cell utilizing the Transmembrane Protease, Serine 2 (TMPRSS2) and Furin cofactors. Analysis of the SARS-CoV-2 extracellular spike protein has suggested critical amino acids necessary for binding within a 197-residue portion, the receptor binding domain (RBD). A cell-based assay between a membrane tethered RBD-GFP fusion protein and the membrane bound ACE2-Cherry fusion protein allowed for mutational intersection of both RBD and ACE2 proteins. Data shows Omicron BA.1 and BA.2 variants have altered dependency on the amino terminus of ACE2 protein and suggests multiple epitopes on both proteins stabilize their interactions at the Nt and internal region of ACE2. In contrast, the H-CoV-NL63 RBD is only dependent on the ACE2 internal region for binding. A peptide inhibitor approach to this internal region thus far have failed to block binding of RBDs to ACE2, suggesting that several binding regions on ACE2 are sufficient to allow functional interactions. In sum, the RBD binding surface of ACE2 appears relatively fluid and amenable to bind a range of novel variants.