Host Phosphatase PPM1G Governs Influenza vRNP Homeostasis by Orchestrating NP Phosphorylation Dynamics and Autophagy

The influenza viral ribonucleoprotein (vRNP) serves as the machinery for viral RNA replication. It self - assembles from the RNA - dependent RNA polymerase (RdRp), viral RNA, and nucleoprotein (NP). During vRNP replication, NP proteins must rapidly switch between monomeric and polymeric states to sustain dynamic equilibrium, which relies on NP phosphorylation-dephosphorylation cycling and is crucial for viral nucleic acid replication. However, the regulatory mechanisms controlling this process remain poorly understood. In this study, we employed an RdRp/vRNP-targeted affinity mass spectrometry-based differential analysis method, discovered that the cellular protein phosphatase Mg2+/Mn2+-dependent 1G (PPM1G) is a phosphatase of NP and required for maintaining viral polymerase activity. Overexpression of PPM1G decreased viral replication. In ppm1g flox/flox-Sftpc Cre mice, lethal influenza infection led to survival and mild disease, as well as reduced lung viral loads and inflammation. These results suggest that PPM1G is a key host molecule for maintaining steady state of viral replication. PPM1G expression is upregulated upon influenza virus infection. This heightened expression occurs in response to increased viral protein production, simultaneously leading to elevated dephosphorylation of NP, which accelerates NP polymerization to promote viral replication. Meanwhile, surplus NP can be degraded through the ATG7 autophagylysosome pathway. Our research elucidates a mechanism that PPM1G maintains viral replication homeostasis by regulating NP status and fate.