Reovirus nonstructural protein μNS binds to core particles but does not inhibit their transcription and capping activities

Previous studies provided evidence that nonstructural protein mu NS of mammalian reoviruses is present in particle assembly intermediates isolated from infected cells. Morgan and Zweerink (Virology 68:155-466, 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 mu NS. Given the possible role of mu NS in particle assembly and/or transcription implied by those findings, we tested whether recombinant mu NS can bind to cores in vitro. The mu NS 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 mu NS resulted in particle complexes of progressively decreasing buoyant density, approaching the density of protein alone when very large amounts of mu NS were bound. Thus, the mu NS core interaction did not exhibit saturation or a defined stoichiometry. Negative-stain electron microscopy of the mu NS-bound cores revealed that the cores were intact and linked together in large complexes by an amorphous density, which we ascribe to mu NS. The mu NS-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 mu NS with cores greatly reduced the formation of recoated cores by stoichiometric binding of outer-capsid proteins mu 1 and sigma 3, These findings are consistent with the presence of mu NS in transcriptase particles as described previously and suggest that, by binding to cores in the infected cell, mu NS may block or delay outer capsid assembly and allow continued transcription by these particles.