This proposal will investigate the role severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) N protein phase separation in disrupting cellular translation and RNA metabolism. N protein phase separation creates membraneless organelles and concentrates RNAs in a droplet (dense) phase. While N protein is responsible for encapsidation of the viral genome, N protein also binds hundreds of host messenger RNAs, some of which are necessary for translation and nonsense-mediated decay (NMD). NMD targets diverse families of RNA viruses for RNA decay, including betacoronaviruses, and the N protein from Murine hepatitis virus (MHV) interferes with NMD during the early stages of infection. Since NMD targets 10% of cellular transcripts for degradation, inhibition by N protein likely has profound effects on the host cell transcriptome. While the mechanism underlying the inhibition of NMD by N protein remains unclear, phase separation and translational repression by N could be a predominant factor. The cellular proteins FUS and C9orf72 arginine- rich dipeptide repeats are known to phase separate, repress translation, inhibit NMD, and are associated with Amyotrophic lateral sclerosis (ALS) disease progression. Viral proteins, including N protein, may repress the translation of transcripts necessary for the co-translational NMD pathway, potentially shaping the host transcriptome in a way that favors virus replication. Recent work has shown that SARS-CoV-2 infection induces high expression of inflammatory cytokines but fails to mount a robust interferon response. The underlying factors driving these transcriptional responses remain unknown; however, N protein could provoke this response by interfering with NMD and blocking post-transcriptional regulation. This project will use transcriptome-wide ribosome profiling (Ribo-seq) to identify cellular transcripts that are translationally repressed by N protein. Previously identified NMD targets will be examined to determine if translational repression confers NMD-resistance. Phase-separation deficient N mutants will be expressed in parallel to determine whether phase separation is the driving force in translation and NMD inhibition. Finally, the results generated from this study will be cross-referenced with published RNA-seq datasets from SARS-CoV-2 infected cells to determine the extent that N protein contributes to the large-scale changes in gene expression during infection. These findings will better our understanding of the virus-host interactions that take place during SARS-CoV-2 infection and will support on-going vaccine and antiviral development, especially N-based approaches.