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Journal of Virology, June 2000, p. 5516-5524, Vol. 74, No. 12
0022-538X/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Reovirus Nonstructural Protein µNS Binds to Core
Particles but Does Not Inhibit Their Transcription and Capping
Activities
Teresa J.
Broering,1,2
Aimee M.
McCutcheon,1,2
Victoria E.
Centonze,3,
and
Max L.
Nibert1,2,*
Department of
Biochemistry,1 Institute for Molecular
Virology,2 and Integrated Microscopy
Resource,3 University of Wisconsin-Madison,
Madison, Wisconsin 53706
Received 23 August 1999/Accepted 22 March 2000
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
3. 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.
*
Corresponding author. Mailing address: Institute for
Molecular Virology, 1525 Linden Dr., Madison, WI 53706. Phone: (608) 262-4536. Fax: (608) 262-7414. E-mail:
mlnibert{at}facstaff.wisc.edu.

Present address: UTHSCSA, San Antonio, TX
78284.
Journal of Virology, June 2000, p. 5516-5524, Vol. 74, No. 12
0022-538X/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
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