Protein mismatches caused by reassortment influence functions of the reovirus capsid

ABSTRACT Following attachment to host receptors via σ1, reovirus particles are endocytosed and disassembled to generate infectious subvirion particles (ISVPs). ISVPs undergo conformational changes to form ISVP*, releasing σ1 and membrane-targeting peptides from the viral μ1 protein. ISVP* formation is required for delivery of the viral core into the cytoplasm for replication. We characterized the properties of T3D F /T3D C S1, a S1 gene monoreassortant between two laboratory isolates of prototype reovirus strain T3D: T3D F and T3D C . T3D F /T3D C S1 is poorly infectious. This deficiency is a consequence of inefficient encapsidation of S1-encoded σ1 on T3D F /T3D C S1 virions. Additionally, in comparison to T3D F , T3D F /T3D C S1 undergoes ISVP-to-ISVP* conversion more readily, revealing an unexpected role for σ1 in regulating ISVP* formation. The σ1 protein is held within turrets formed by the λ2 protein. To test if the altered properties of T3D F /T3D C S1 are due to a mismatch between σ1 and λ2 proteins from T3D F and T3D C , properties of T3D F /T3D C L2 and T3D F /T3D C S1L2, which express a T3D C -derived λ2, were compared. The presence of T3D C λ2 allowed more efficient σ1 incorporation, producing particles that exhibit T3D F -like infectivity. In comparison to T3D F , T3D F /T3D C L2 prematurely converts to ISVP* uncovering a role for λ2 in regulating ISVP* formation. Importantly, a virus with matching σ1 and λ2 displayed a more regulated conversion to ISVP* than either T3D F /T3D C S1 or T3D F /T3D C L2. In addition to identifying new regulators of ISVP* formation, our results highlight that protein mismatches produced by reassortment can alter virus assembly and thereby influence subsequent functions of the virus capsid.