ABSTRACT
The emergence of new escape mutants of the SARS-CoV-2 virus has escalated its penetration among the human population and has reinstated its status as a global pandemic. Therefore, developing effective antiviral therapy against emerging SARS-CoV variants and other viruses in a short period of time becomes essential. Blocking SARS-CoV-2 entry into human host cells by disrupting the Spike glycoprotein-angiotensin-converting enzyme 2 (ACE2) interaction has already been exploited for vaccine development and monoclonal antibody therapy. Unlike the previous reports, our study used a nine-amino acid peptide from the receptor-binding motif (RBM) of the Spike (S) protein as an epitope. We report the identification of an efficacious nanobody N1.2 that blocks the entry of pseudovirus-containing SARS-CoV-2 Spike as the surface glycoprotein. Moreover, using mCherry fluorescence based reporter assay we observe a more potent neutralizing effect against both the hCoV19 (Wuhan/WIV04/2019) and the Omicron (BA.1) pseudotyped Spike virus with a bivalent version of the N1.2 nanobody. In summary, our study presents a rapid, and efficient methodology to use peptide sequences from a protein-receptor interaction interface as epitopes for screening nanobodies against potential pathogenic targets. We propose that this approach can also be widely extended to target other viruses and pathogens in the future.
ABSTRACT
The entry of the severe acute respiratory syndrome coronavirus 2 virus in human cells is mediated by the binding of its surface spike protein to the human angiotensin-converting enzyme 2 (ACE2) receptor. A 23-residue long helical segment (SBP1) at the binding interface of human ACE2 interacts with viral spike protein and therefore has generated considerable interest as a recognition element for virus detection. Unfortunately, emerging reports indicate that the affinity of SBP1 to the receptor-binding domain of the spike protein is much lower than that of the ACE2 receptor itself. Here, we examine the biophysical properties of SBP1 to reveal factors leading to its low affinity for the spike protein. Whereas SBP1 shows good solubility (solubility > 0.8 mM), circular dichroism spectroscopy shows that it is mostly disordered with some antiparallel ß-sheet content and no helicity. The helicity is substantial (>20%) only upon adding high concentrations (≥20% v/v) of 2,2,2-trifluoroethanol, a helix promoter. Fluorescence correlation spectroscopy and single-molecule photobleaching studies show that the peptide oligomerizes at concentrations >50 nM. We hypothesized that mutating the hydrophobic residues (F28, F32, and F40) of SBP1, which do not directly interact with the spike protein, to alanine would reduce peptide oligomerization without affecting its spike binding affinity. Whereas the mutant peptide (SBP1mod) shows substantially reduced oligomerization propensity, it does not show improved helicity. Our study shows that the failure of efforts, so far, to produce a short SBP1 mimic with a high affinity for the spike protein is not only due to the lack of helicity but is also due to the heretofore unrecognized problem of oligomerization.