Unexpected Receptor Functional Mimicry Elucidates Activation of Coronavirus Fusion

Abstract

Graphical Abstract Highlights d MERS-CoV/SARS-CoV S composite glycan shields analyzed by cryo-EM and mass spectrometry d Structures of MERS-CoV/SARS-CoV S with neutralizing antibodies from survivors d LCA60 inhibits receptor binding by interacting with MERS-CoV S protein/glycans d S230 blocks receptor binding and triggers fusogenic rearrangements via functional mimicry In Brief Structural analysis of the SARS-CoV S and MERS-CoV S glycoproteins in complex with neutralizing antibodies from human survivors sheds light into the mechanisms of membrane fusion and neutralization Walls et al., SUMMARYRecent outbreaks of severe acute respiratory syndrome and Middle East respiratory syndrome, along with the threat of a future coronavirus-mediated pandemic, underscore the importance of finding ways to combat these viruses. The trimeric spike transmembrane glycoprotein S mediates entry into host cells and is the major target of neutralizing antibodies. To understand the humoral immune response elicited upon natural infections with coronaviruses, we structurally characterized the SARS-CoV and MERS-CoV S glycoproteins in complex with neutralizing antibodies isolated from human survivors. Although the two antibodies studied blocked attachment to the host cell receptor, only the anti-SARS-CoV S antibody triggered fusogenic conformational changes via receptor functional mimicry. These results provide a structural framework for understanding coronavirus neutralization by human antibodies and shed light on activation of coronavirus membrane fusion, which takes place through a receptor-driven ratcheting mechanism.

Structural insights into coronavirus entry

“…Free 9-O-Ac-Sia, however, did not trigger S conformational changes associated with membrane fusion . This observation contrasts with data for SARS-CoV S, for which addition of the human angiotensin-converting enzyme 2 (ACE2) ectodomain (the proteinaceous receptor) promoted S refolding to the postfusion state (Song et al, 2018;Walls et al, 2019). These findings suggested that either 9-O-Ac-Sia-containing receptors differ from proteinaceous receptors in their mode of action, or that an interaction with a yet unidentified proteinaceous receptor is required before or after virus internalization for HCoV-OC43 entry into target cells.…”

“…Recent cryoEM studies revealed that MERS-CoV S and SARS-CoV S can adopt open and closed conformations in which the receptor binding site of domain B is exposed and occluded, respectively (Gui et al, 2017;Kirchdoerfer et al, 2018;Pallesen et al, 2017;Song et al, 2018;Walls et al, 2019;Yuan et al, 2017). In contrast, the MHV, HCoV-NL63, HCoV-HKU1, PDCoV, IBV and HCoV-OC43 S glycoproteins appear to only adopt a closed conformation (Kirchdoerfer et al, 2016;Shang et al, 2018a, b;Tortorici et al, 2019;Walls et al, 2016a, b;Xiong et al, 2018) and unknown trigger(s), besides proteolytic activation, might be necessary for these viruses to expose their receptor-binding motifs for recognition to occur.…”

“…Recent structural work comparing recombinant S proteins from SARS-CoV and MERS-CoV in isolation and in complex with their cognate receptors or neutralizing antibodies suggested an activation mechanism for coronavirus fusion (Gui et al, 2017;Kirchdoerfer et al, 2018; Song et al, 2018;Walls et al, 2019;Yuan et al, 2017). Specifically, SARS-CoV and MERS-CoV S structures in complex with neutralizing antibodies isolated from survivors showed both antibodies competitively blocked receptor interaction, in agreement with previous surface plasmon resonance data (Corti et al, 2015;Rockx et al, 2008;Traggiai et al, 2004;Walls et al, 2019). The anti-SARS-CoV S230 antibody, however, functionally mimicked the receptor by promoting S fusogenic conformational rearrangements through a molecular ratcheting mechanism (Fig.…”

“…4). These observations suggested that upon receptor recognition, bound B domains are locked in the open state, thereby releasing the constraints imposed on the HR1-central helix hairpin, allowing refolding of the S 2 fusion machinery and membrane fusion to occur (Pallesen et al, 2017;Song et al, 2018;Walls et al, 2019;Yuan et al, 2017) (Fig. 4).…”

“…A common feature observed in the glycosylation patterns of S glycoproteins is the presence of less densely glycosylated regions surrounding the S 1 /S 2 cleavage site and the conserved fusion peptide, near the S 2 ' cleavage site, probably to allow access to activating host proteases and for membrane fusion to take place (Walls et al, 2016b;Walls et al, 2019) (Fig. 5).…”