The reovirus μ1 protein contributes to the environmental stability of virions

The mammalian orthoreovirus (reovirus) outer capsid is composed of 200 μ1-σ3 heterohexamers and a maximum of 12 σ1 trimers. During cell entry, σ3 is degraded by luminal or intracellular proteases to generate a metastable intermediate, called infectious subviral particle (ISVP). Prior to disassembly, σ3 stabilizes the virion by capping μ1. Reovirus fails to establish a productive infection when σ3 degradation is prevented, suggesting proteolytic priming is required for entry. Once uncovered, ISVPs are converted to ISVP*s, which is accompanied by a μ1 rearrangement. Nonetheless, whether σ3 degradation can be bypassed for virions to adopt an altered conformation is undetermined. In this report, we utilized the T1L/T3D M2 reassortant, which encodes a mismatched outer capsid, to further investigate the determinants of reovirus stability. When μ1-σ3 were derived from different strains, virions resembled wild type in structure and protease sensitivity. Using heat as a surrogate for environmental assault, T1L/T3D M2 ISVPs were more susceptible to inactivation than wild type ISVPs. In contrast, virions of each strain were equally stable. Surprisingly, virion associated μ1 rearranged into an ISVP*-like conformation concurrent with loss of infectivity. Despite the presence σ3, a hyperstable variant of μ1 also contributed to heat resistance. The dual layered architecture of reovirus allowed for differential sensitivity to inactivating agents; the inner capsid (core) displayed exceptional resistance to heating. Together, these findings reveal a previously undefined contribution from μ1 in maintaining virion stability.