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The nonenveloped mammalian orthoreoviruses (MRV), as a prototype member of reovirus in the family of Reoviridae, has a genome composed of 10 double-stranded RNA (dsRNA) that encode 11 proteins. The virion has two concentric protein capsids that form icosahedrons about 75nm in diameter. Of the 11 proteins, the eight viral proteins in the mature virion (also known as structural proteins) have particle-based functions that are critical to initiation of infection and synthesis of the capped viral mRNAs. The structural proteins are consisted of core(λ1, λ2, λ3, σ2, μ2) and outer capsid shell (μ1, σ1, σ3), of which the σ1, λ2, λ3 and μ2 proteins are locate around the icosahedral fivefold axes from the outer to the inner capsid shell. The remaining 4 proteins, called nonstructural proteins, are not presented in the mature virion, but are ex-pressed in reovirus infected cells during the virus replication cycle (9, 13, 14, 23, 26).
he transcription and replication of MRV starts by the internment of the core particle in the target cytoplasm. Initially, [+]RNAs transcription occurs in the intermediate subvirion particle (ISVP) or core that contain viral genomic dsRNA. As soon as the new synthesized RNA has been transcribed, it is released into the cytoplasm from the channel of λ2 at the five-fold axes of icosahedral. Once they have been released, they can function as mRNA for translation of new viral proteins or for packaging into new viral particles with their complement [-] RNAs (10, 13, 15, 29).
When nascent virion protein components emerge, they are found to be assembled in unique cellular constructions called viral factories (21) or viral inclusion bodies (VIB) (24). The factories have a peculiarly dense consistency that is easy to distinguish from the adjacent cytoplasm and causes them to appear highly refractive by phase-contrast microscopy; they are quite small in the early stages of infection and then grow larger and move toward the nucleus as infection proceeds. Later in infection, the inclusion usually anchored to and spreads along stabilized microtubules in many MRV species, but not in all strains (18, 21). Reovirus factories were determined to contain fully and partially assembled viral particles, viral proteins, dsRNA and microtubules, but mem-branebound structures or ribosomes are not found (22, 30).
μNS is determined to play an important role in the formation of viral factory structures. Immunostaining and immunofluorescence (IF) microscopy revealed that μNS can be found to be concentrated in viral factories (21, 22, 25). The features of one or more viral proteins that would make μNS capable of forming such a matrix are not well understood but might involve a variety of different types of intersubunit interactions, as well as interactions with cellular factors. There have been several recent studies which have elucidated the role of μNS in the formation of the matrix of viral factories in virus genome replication and particle assembly by clarifying the μNS protein function and its interaction with other proteins in the virus replication cycle.
Functional Analyses of Mammalian Reovirus Nonstructural Protein μNS*
- Received Date: 10 December 2008
- Accepted Date: 18 December 2008
Abstract: Genome replication of reovirus occurs in cytoplasmic inclusion bodies called viral factories or viroplasms. The viral nonstructural protein μNS, encoded by genome segment M3, is not a component of mature virions, but is expressed to high levels in infected cells and is concentrated in the infected cell factory matrix. Recent studies have demonstrated that μNS plays a central role in forming the matrix of these structures, as well as in recruiting other components to them for putative roles in genome replication and particle assembly.