Steric hindrance of antibody binding in an Omicron spike fusion intermediate

Understanding conformational changes of the coronavirus spike protein is critical for developing broad-spectrum therapies. The pan-coronavirus epitope spike residues 815-825 (centred on the S2' site) are buried in the prefusion spike but are transiently exposed upon ACE2 binding^1,2. Here, using integrated functional and structural analyses, we demonstrate that 76E1, an antibody targeting spike residues 815-825, specifically recognizes an open early fusion intermediate conformation in which this epitope adopts a helical conformation, designated the S2'-helix. SARS-CoV-2 Omicron variants evade such antibodies via steric hindrance resulting from S2'-helix shifts and restricted S1-ACE2 distancing in the early fusion intermediate conformation, together with increased reliance on cathepsin-mediated entry that impairs 76E1 inhibition of S2' cleavage. The H655Y mutation is central to this evasion. Antibody size directly affects its access to the S2'-helix. Crucially, antibody size minimization reversed the evasion mechanisms and significantly enhanced neutralizing activity against authentic Omicron variants and other human coronaviruses, including SARS-CoV-1 and HCoV-229E. These findings establish small-molecule targeting of the S2'-helix as a strategy for pan-coronavirus therapies.