Abstract
SummaryThere are as yet no licensed therapeutics for the COVID-19 pandemic. The causal coronavirus (SARS-CoV-2) binds host cells via a trimeric spike whose receptor binding domain (RBD) recognizes angiotensin-converting enzyme 2, initiating conformational changes that drive membrane fusion. We find that the monoclonal antibody CR3022 binds the RBD tightly, neutralizing SARS-CoV-2, and report the crystal structure at 2.4 Å of the Fab/RBD complex. Some crystals are suitable for screening for entry-blocking inhibitors. The highly conserved, structure-stabilizing CR3022 epitope is inaccessible in the prefusion spike, suggesting that CR3022 binding facilitates conversion to the fusion-incompetent post-fusion state. Cryogenic electron microscopy (cryo-EM) analysis confirms that incubation of spike with CR3022 Fab leads to destruction of the prefusion trimer. Presentation of this cryptic epitope in an RBD-based vaccine might advantageously focus immune responses. Binders at this epitope could be useful therapeutically, possibly in synergy with an antibody that blocks receptor attachment.
Highlights
Incursion of animal-derived coronaviruses into the human population has caused several outbreaks of severe disease, starting with severe acute respiratory syndrome (SARS) in 2002 (Menachery et al, 2015)
We report crystallographic analysis to significantly higher resolution, use a different neutralization assay to show that CR3022 does neutralize SARS-CoV-2, and use cryogenic electron microscopy analysis of the interaction of CR3022 with the full spike ectodomain to demonstrate a mechanism of neutralization not seen before for coronaviruses
CR3022 Binds Tightly to the receptor binding domain (RBD) and Allosterically Perturbs angiotensin-converting enzyme 2 (ACE2) Binding To understand how CR3022 works, we first investigated the interaction of CR3022 Fab with isolated recombinant SARSCoV-2 RBD, both alone and in the presence of ACE2
Summary
Incursion of animal (usually bat)-derived coronaviruses into the human population has caused several outbreaks of severe disease, starting with severe acute respiratory syndrome (SARS) in 2002 (Menachery et al, 2015). The heavily glycosylated trimeric surface spike protein mediates viral entry into the host cell. It is a large type I transmembrane glycoprotein (the ectodomain alone comprises over 1,200 residues) (Wrapp et al, 2020). The RBD is predominantly in a ‘‘down’’ state where the receptor binding site is inaccessible; it appears that it stochastically flips up with a hingelike motion transiently presenting the angiotensin-converting enzyme 2 (ACE2) receptor binding site (Roy, 2020; Song et al, 2018; Walls et al, 2020; Wrapp et al, 2020).
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