The reovirus μ1 protein contributes to the environmental stability of virions

The mammalian orthoreovirus (reovirus) outer capsid is composed of 200 1-{sigma}3 heterohexamers and a maximum of 12 {sigma}1 trimers. During cell entry, {sigma}3 is degraded by luminal or intracellular proteases to generate a metastable intermediate, called infectious subviral particle (ISVP). Prior to disassembly, {sigma}3 stabilizes the virion by capping 1. Reovirus fails to establish a productive infection when {sigma}3 degradation is prevented, suggesting proteolytic priming is required for entry. Once uncovered, ISVPs are converted to ISVP*s, which is accompanied by a 1 rearrangement. Nonetheless, whether {sigma}3 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 1-{sigma}3 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 1 rearranged into an ISVP*-like conformation concurrent with loss of infectivity. Despite the presence {sigma}3, a hyperstable variant of 1 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 1 in maintaining virion stability.