As obligate intracellular pathogens, viruses require the host biosynthetic machinery. A fundamental requirement is the host translational machinery for the synthesis of new viral proteins. The mechanisms by which viruses co-opt the host translational machinery and avoid the innate host defenses that act to degrade viral mRNA and diminish cellular protein synthesis to prevent or lessen the synthesis of viral proteins are not understood. The goal of this project is to define mechanisms by which mammalian reoviruses reprogram the host translational machinery to preferentially synthesize viral proteins. In particular, the project will focus on virus-mediated compartmentalization of the translational machinery, a novel pathogenic mechanism by which viral protein synthesis can be maintained during infection-induced stress. The hypotheses to be tested are that reovirus promotes the translation of its viral mRNAs by: (1) compartmentalizing translation factors and ribosomal subunits within defined sites of viral replication in the cytosol called viral factories; (2) by sequestering the GADD34/protein phosphatase 1 complex that dephosphorylates eIF2α within viral factories, thus, protecting translation of viral mRNAs from stress-induced inhibition of protein synthesis; and (3) by directly modifying the ribosome and altering its function to promote translation of non-canonical viral mRNA. Three Aims are proposed: (1) To identify mechanism(s) by which the host cell translational machinery is compartmentalized within viral factories in reovirus-infected cells; (2) To determine whether activation of the integrated stress response and compartmentalization of cellular GADD34 and protein phosphatase 1 within reovirus factories promotes translation of reovirus mRNAs; and (3) To determine the role of the viral nonstructural protein σNS in enhancing the translation of reovirus mRNA in infected cells. Expected outcomes of this work are identification of the viral factors required for compartmentalization of the host translational machinery within viral factories, an understanding of the mechanisms by which reoviruses replicate in the face of ongoing phosphorylated eIF2α and activation of the integrated stress response, and a greater understanding of the role of the reovirus protein σNS in co-opting the host translational machinery and modifying ribosome function. We expect that an important outcome of this proposal will be new basic information regarding viral strategies to overcome innate host defenses.