Virion encapsidation and cell attachment function of the reovirus attachment protein is influenced by its structural flexibility

The reovirus σ1 attachment protein mediates virus interaction with host cell receptors that is critical for cell entry. Reovirus tropism is controlled by properties of σ1. σ1 is present as trimers that are held within turrets at the icosahedral vertices of reovirus virions. However, because σ1 has not been visualized on reovirus virions in high resolution structures and because the fulllength structure of purified σ1 protein has not been solved, it is not clear how σ1 is presented on virions. What properties of σ1 are essential for its incorporation on virions is also not known. In this study, we used ColabFold to model the structure of reovirus serotype 1 (T1) and serotype 3 (T3) σ1 proteins. We find that these proteins fold into similar structures with regions of flexibility between the head and body domains of σ1. We also predicted the structures of chimeric σ1 proteins comprised of domain swaps between T1 and T3 σ1 proteins. Our analyses indicate that chimeric proteins with mismatched body and head domain have increased flexibility in this region. Characterization of particles expressing such chimeric σ1 proteins demonstrated that deviation from the flexibility of parental σ1 leads to a reduction in σ1 incorporation on to the virion. Further, we find that even when incorporation is not affected, virus attachment to host cell receptors is influenced by altered σ1 flexibility. Finally, our work demonstrates that μ1 protein impacts the encapsidation pattern and receptor engagement properties of σ1 and that this effect is influenced by properties of the N-terminal portion of σ1.