Protein phosphatase 1 suppresses PKR/EIF2α signaling during human cytomegalovirus infection

Human cytomegalovirus (HCMV) is a ubiquitous pathogen that infects the majority of the world's population. Lytic HCMV replication in immunocompromised individuals or neonates can lead to severe disease in multiple organ systems and even death. The establishment of lytic replication is driven by the first viral proteins expressed upon infection, the immediate early proteins, which play a key role in creating an intracellular environment conducive to virus replication. Two immediate early proteins, the functional orthologs pTRS1 and pIRS1, stimulate immediate early gene expression by suppressing antiviral PKR/eIF2α signaling and enhance the translation of viral mRNAs independent of PKR antagonism. To better understand the molecular functions of pTRS1, we used proximity labeling proteomics to identify proteins that interact with pTRS1 in infected cells. Multiple novel host and viral interactors were identified, including the catalytic subunits of the protein phosphatase 1 (PP1) holoenzyme. Mutations to a PP1 catalytic subunit known to disrupt binding to PP1 regulatory subunits decreased binding to pTRS1. pTRS1 immune complexes contained phosphatase activity, and inhibition of phosphatase activity in transfected or infected cells reversed the ability of pTRS1 to inhibit the antiviral kinase PKR. Depletion of individual PP1 catalytic subunits decreased virus replication and increased the phosphorylation of the PKR substrate eIF2α. Taken together, our data suggest potential novel functions for pTRS1 and define a novel role for PP1 as an antagonist of the antiviral PKR/eIF2α signaling axis during HCMV infection.IMPORTANCEThe human cytomegalovirus (HCMV) pTRS1 and pIRS1 proteins are critical regulators of HCMV replication, both during primary infection and during reactivation from viral latency. Thus, defining the molecular functions of pTRS1/pIRS1 is important for understanding the molecular events controlling HCMV replication and viral disease. These data provide new insights into potential pTRS1 functional roles, providing a starting point for others to understand new features of infected cell biology. Another important result of this study is the finding that specific protein phosphatase 1 (PP1) regulatory subunits are required to suppress PKR/eIF2α signaling, a critical cellular innate immune defense to viral infection. These data lay the groundwork for future efforts to discover therapeutics that disrupt pTRS1 interaction with PP1 allowing cellular defenses to limit HCMV replication and disease.