Reovirus Nonstructural Protein μNS Binds to Core Particles but Does Not Inhibit Their Transcription and Capping Activities

Previous studies provided evidence that nonstructural protein μNS of mammalian reoviruses is present in particle assembly intermediates isolated from infected cells. Morgan and Zweerink (Virology 68:455–466, 1975) showed that a subset of these intermediates, which can synthesize the viral plus strand RNA transcripts in vitro, comprise core-like particles plus large amounts of μNS. Given the possible role of μNS in particle assembly and/or transcription implied by those findings, we tested whether recombinant μNS can bind to cores in vitro. The μNS protein bound to cores, but not to two particle forms, virions and intermediate subvirion particles, that contain additional outer-capsid proteins. Incubating cores with increasing amounts of μNS resulted in particle complexes of progressively decreasing buoyant density, approaching the density of protein alone when very large amounts of μNS were bound. Thus, the μNS-core interaction did not exhibit saturation or a defined stoichiometry. Negative-stain electron microscopy of the μNS-bound cores revealed that the cores were intact and linked together in large complexes by an amorphous density, which we ascribe to μNS. The μNS-core complexes retained the capacity to synthesize the viral plus strand transcripts as well as the capacity to add methylated caps to the 5′ ends of the transcripts. In vitro competition assays showed that mixing μNS with cores greatly reduced the formation of recoated cores by stoichiometric binding of outer-capsid proteins μ1 and ς3. These findings are consistent with the presence of μNS in transcriptase particles as described previously and suggest that, by binding to cores in the infected cell, μNS may block or delay outer-capsid assembly and allow continued transcription by these particles.