Proteasome activity is required for reovirus entry into cells.

Since viruses have limited coding capacity in their genomes, they use host cell machinery to complete virtually every stage of their replication cycle. Mammalian orthoreovirus (reovirus) is comprised of two concentric protein shells, the inner core and the outer capsid. Following attachment to its receptor, reovirus enters the cell by receptor-mediated endocytosis. Within endosomes, reovirus utilizes host acid-dependent proteases to process the viral outer capsid. Specifically, the outer capsid protein σ3 is degraded, and μ1 is cleaved to form the disassembly intermediate infectious subvirion particles (ISVPs). ISVPs undergo additional conformational changes into ISVP*s that release small peptides, which mediate the penetration of endosomal membranes. Membrane penetration allows for delivery of the remaining viral core into the cytoplasm for subsequent gene expression. Here, we describe that proteasomes control an entry step of reovirus particles. We show that chemically inhibiting the proteasome blocks infection at a stage following ISVP formation but prior to transcriptional activation of cores. Specifically, inhibition of the proteasome prevents conformational changes in μ1 characteristic of ISVP-to-ISVP* conversion. In the absence of these conformational changes, cores are unable to be delivered and become transcriptionally active, thereby blocking viral replication. This work highlights a previously unknown way in which reovirus relies on host factors for successful replication. IMPORTANCE Due to their limited genetic capacity, viruses are reliant on multiple host systems to replicate successfully. Mammalian orthoreovirus (reovirus) is commonly used as a model system for understanding host-virus interactions. In this study, we identify that the proteasome system, which is critical for cellular protein turnover, affects reovirus entry. Inhibition of the proteasome using a chemical inhibitor blocks reovirus uncoating. Blocking these events reduces subsequent replication of the virus. This work identifies that additional host factors control reovirus entry.