Silencing and complementation of reovirus core protein μ2: Functional correlations with μ2–microtubule association and differences between virus- and plasmid-derived μ2

A low-copy component of mammalian reovirus particles is μ2, an 83-kDa protein encoded by the M1 viral genome segment and packaged within the viral core. Previous studies have identified μ2 as a nucleoside triphosphate phosphohydrolase (NTPase) as well as an RNA 5′-triphosphate phosphohydrolase (RTPase), putatively involved in reovirus RNA synthesis and/or 5′-capping. Other studies have identified μ2 as a microtubule-binding protein, which also associates with the viral factory matrix protein μNS and thereby anchors the factories to cellular microtubules during infections by most reovirus strains. To extend studies of μ2 functions during infection, we tested a small interfering RNA (siRNA) directed against the M1 plus-strand RNAs of reovirus strains Type 1 Lang (T1L) and Type 3 Dearing (T3D). The siRNA strongly suppressed μ2 expression by either strain and reduced infectious yields in a strain-dependent manner. This first strain difference was genetically mapped to the M1 genome segment and tentatively assigned to a single μ2 sequence polymorphism, Pro/Ser208, which also determines a T1L–T3D strain difference in microtubule association. The siRNA-based defect in μ2 expression was rescued by plasmids, containing silent mutations in the siRNA-targeted sequence, which encoded either T1L or T3D μ2, but the growth defect was rescued only by T1L μ2. This second strain difference was also mapped to Pro/Ser208, in that swapping this one residue between T1L and T3D μ2 reversed the rescue phenotypes. Thus, the T1L–T3D strain difference in μ2–microtubule association was correlated not only with the extent of reduction in infectious yields by the siRNA but also with the extent of rescue by plasmid-derived μ2. In addition, the rescue capacity of T1L μ2 was abrogated by nocodazole treatment, providing independent evidence for the importance of μ2–microtubule association in plasmid-based rescue. In two separate cases, the results revealed functional differences between virus- and plasmid-derived μ2. Ala substitutions within the NTP-binding motif of T1L μ2 also abrogated its rescue capacity, suggesting that the NTPase or RTPase activity of μ2 is additionally required for effective viral growth.