Reovirus Replication Protein 2 Influences Cell Tropism by Promoting Particle Assembly within Viral Inclusions

The double-stranded RNA virus mammalian reovirus displays broad cell, tissue, and host tropism. A critical checkpoint in the reovirus replication cycle resides within viral cytoplasmic inclusions, which are biosynthetic centers of genomemultiplication and new-particle assembly. Replication of strain type 3 Dearing (T3) is arrested inMadin-Darby canine kidney (MDCK) cells at a step subsequent to inclusion development and prior to formation of genomic double-stranded RNA. This phenotype is primarily regulated by viral replication protein2. To understand how reovirus inclusions differ in productively and abortively infected MDCK cells, we used confocal immunofluorescence and thin-section transmission electronmicroscopy (TEM) to probe inclu-sion organization and particle morphogenesis. Although no abnormalities in inclusionmorphology or viral protein localization were observed in T3-infectedMDCK cells using confocal microscopy, TEM revealed markedly diminished production of mature progeny virions. T3 inclusions were less frequent and smaller than those formed by T3-T1M1, a productively replicating reovirus strain, and contained decreased numbers of complete particles. T3 replication was enhanced when cells were cultivated at 31°C, and inclusion ultrastructure at low-temperature infectionmore closely resembled that of a productive infection. These results indicate that particle assembly in T3-infectedMDCK cells is defective, possibly due to a temperature-sensitive structural or func-tional property of2. Thus, reovirus cell tropism can be governed by interactions between viral replication proteins and the unique cell environment that modulate efficiency of particle assembly. Acommon strategy shared by numerous viruses is the forma-tion of specialized sites within a host cell to complete vira