Structural Insights into Reovirus sigma 1 Interactions with Two Neutralizing Antibodies

Reovirus attachment protein sigma 1 engages glycan receptors and junctional adhesion molecule-A (JAM-A) and is thought to undergo a conformational change during the proteolytic disassembly of virions to infectious subvirion particles (ISVPs) that accompanies cell entry. The sigma 1 protein is also the primary target of neutralizing antibodies. Here, we present a structural and functional characterization of two neutralizing antibodies that target sigma 1 of serotype 1 (T1) and serotype 3 (T3) reoviruses. The crystal structures revealed that each antibody engages its cognate sigma 1 protein within the head domain via epitopes distinct from the JAM-A-binding site. Surface plasmon resonance and cell-binding assays indicated that both antibodies likely interfere with JAM-A engagement by steric hindrance. To define the interplay between the carbohydrate receptor and antibody binding, we conducted hemagglutination inhibition assays using virions and ISVPs. The glycan-binding site of T1 sigma 1 is located in the head domain and is partly occluded by the bound Fab in the crystal structure. The T1-specific antibody inhibited hemagglutination by virions and ISVPs, probably via direct interference with glycan engagement. In contrast to T1 sigma 1, the carbohydrate-binding site of T3 sigma 1 is located in the tail domain, distal to the antibody epitope. The T3-specific antibody inhibited hemagglutination by T3 virions but not ISVPs, indicating that the antibody- and glycan-binding sites in sigma 1 are in closer spatial proximity on virions than on ISVPs. Our results provide direct evidence for a structural rearrangement of sigma 1 during virion-to-ISVP conversion and contribute new information about the mechanisms of antibody-mediated neutralization of reovirus. IMPORTANCE Virus attachment proteins mediate binding to host cell receptors, serve critical functions in cell and tissue tropism, and are often targeted by the neutralizing antibody response. The structural investigation of antibody-antigen complexes can provide valuable information for understanding the molecular basis of virus neutralization. Studies with enveloped viruses, such as HIV and influenza virus, have helped to define sites of vulnerability and guide vaccination strategies. By comparison, less is known about antibody binding to nonenveloped viruses. Here, we structurally investigated two neutralizing antibodies that bind the attachment protein sigma 1 of reovirus. Furthermore, we characterized the