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

The reovirus {sigma}1 attachment protein mediates virus interaction with host cell receptors that is critical for cell entry. Reovirus tropism is controlled by properties of {sigma}1. {sigma}1 is present as trimers that are held within turrets at the icosahedral vertices of reovirus virions. However, because {sigma}1 has not been visualized on reovirus virions in high resolution structures and because the fulllength structure of purified {sigma}1 protein has not been solved, it is not clear how {sigma}1 is presented on virions. What properties of {sigma}1 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) {sigma}1 proteins. We find that these proteins fold into similar structures with regions of flexibility between the head and body domains of {sigma}1. We also predicted the structures of chimeric {sigma}1 proteins comprised of domain swaps between T1 and T3 {sigma}1 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 {sigma}1 proteins demonstrated that deviation from the flexibility of parental {sigma}1 leads to a reduction in {sigma}1 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 {sigma}1 flexibility. Finally, our work demonstrates that 1 protein impacts the encapsidation pattern and receptor engagement properties of {sigma}1 and that this effect is influenced by properties of the N-terminal portion of {sigma}1. ImportanceAttachment to host cell receptors is a critical step in initiation of virus infection. Some viruses attach to cellular receptors via dedicated viral proteins. Both the number of attachment factors present on the virus and whether they are present on the virus particle in the correct form can influence cell attachment. Here, using reovirus as a model, we use a protein structure prediction algorithm to model the as yet unknown structure of full-length reovirus attachment protein {sigma}1. We find predicted regions of flexibility in the protein and identify how this flexibility is regulated. We find that the flexibility of {sigma}1 independently regulates whether it is stably incorporated into particles and if can efficiently interact with host receptors.