Localization of mammalian orthoreovirus proteins to cytoplasmic factory-like structures via nonoverlapping regions of microNS.

Virally induced structures called viral factories form throughout the cytoplasm of cells infected with mammalian orthoreoviruses (MRV). When expressed alone in cells, MRV nonstructural protein microNS forms factory-like structures very similar in appearance to viral factories, suggesting that it is involved in forming the structural matrix of these structures. microNS also associates with MRV core particles; the core proteins mu2, lambda1, lambda2, lambda3, and sigma2; and the RNA-binding nonstructural protein sigmaNS. These multiple associations result in the recruitment or retention of these viral proteins or particles at factory-like structures. In this study, we identified the regions of microNS necessary and sufficient for these associations and additionally examined the localization of viral RNA synthesis in infected cells. We found that short regions within the amino-terminal 220 residues of microNS are necessary for associations with core particles and necessary and sufficient for associations with the proteins mu2, lambda1, lambda2, sigma2, and sigmaNS. We also found that only the lambda3 protein associates with the carboxyl-terminal one-third of microNS and that viral RNA is synthesized within viral factories. These results suggest that microNS may act as a cytoplasmic scaffolding protein involved in localizing and coordinating viral replication or assembly intermediates for the efficient production of progeny core particles during MRV infection.