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

Previous studies provided evidence that nonstructural protein mu NS of mamm alian reoviruses is present in particle assembly intermediates isolated fro m infected cells. Morgan and Zweerink (Virology 68:155-466, 1975) showed th at a subset of these intermediates, which can synthesize the viral plus str and RNA transcripts in vitro, comprise core-like particles plus large amoun ts of mu NS. Given the possible role of mu NS in particle assembly and/or t ranscription 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 t wo particle forms, virions and intermediate subvirion particles, that conta in additional outer-capsid proteins. Incubating cores with increasing amoun ts of mu NS resulted in particle complexes of progressively decreasing buoy ant density, approaching the density of protein alone when very large amoun ts of mu NS were bound. Thus, the mu NS core interaction did not exhibit sa turation or a defined stoichiometry. Negative-stain electron microscopy of the mu NS-bound cores revealed that the cores were intact and linked togeth er in large complexes by an amorphous density, which we ascribe to mu NS. T he 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 stoichiom etric binding of outer-capsid proteins mu 1 and sigma 3, These findings are consistent with the presence of mu NS in transcriptase particles as descri bed previously and suggest that, by binding to cores in the infected cell, mu NS may block or delay outer capsid assembly and allow continued transcri ption by these particles.