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Principal Investigator: Adriaan Bax
Organization: NATIONAL INSTITUTE OF DIABETES AND DIGESTIVE AND KIDNEY DISEASES
Fiscal Year: 2024
Award: $658,712
Funding agency: National Institute of Diabetes and Digestive and Kidney Diseases
As applies for HIV-1, entry of SARS-CoV-2 into a host cell is also mediated by a Class I viral fusion protein: Spike. This protein, also referred to as S, is responsible for merging the viral and host cell membranes. Atomic resolution models for both the post-fusion 6-helix bundle (6HB) and the prefusion state of Spike have become available from X-ray and cryo-EM studies. However, a mechanistic understanding of the molecular basis for the intervening structural transition, critical for the design of fusion inhibitors, has remained elusive. Using NMR spectroscopy and other biophysical methods, we demonstrate the presence of alpha-helical, membrane-bound, intermediate states of Spike's heptad repeat (HR1 and HR2) domains that are embedded at the lipid-water interface, while in a slow dynamic equilibrium with the post-fusion 6HB state. These results support a model where the HR domains lower the large energy barrier associated with membrane fusion by destabilizing the host and viral membranes, while 6HB formation actively drives their fusion by forcing physical proximity of the juxtaposed membranes.
The Omicron variants of SARS-CoV-2 may be more easily transmitted compared to the earlier Wuhan and other variants. We aimed to elucidate mechanisms of the altered infectivity associated with the Omicron variants by systemically evaluating mutations located in the S2 sequence of spike and identified mutations that are responsible for altered viral fusion. We demonstrated that mutations near the S1/S2 cleavage site decrease S1/S2 cleavage, resulting in reduced fusogenicity. Mutations in the HR1 and other S2 sequences were also found to affect cell-cell fusion. Based on analysis of NMR data and in silico modeling, these mutations affect fusogenicity possibly at multiple steps of the viral fusion. Our findings revealed that the Omicron variants have accumulated mutations that contribute to reduced syncytial formation which may be responsible for their attenuated pathogenicity.
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