Dephosphorylation of the RNA Sensors RIG-I and MDA5 by the Phosphatase PP1 Is Essential for Innate Immune Signaling

RIG-I and MDA5 have emerged as key cytosolic sensors for the detection of RNA viruses and lead to antiviral interferon (IFN) production. Recent studies have highlighted the importance of posttranslational modifications for controlling RIG-I antiviral activity. However, the regulation of MDA5 signal-transducing ability remains unclear. Here, we show that MDA5 signaling activity is regulated by a dynamic balance between phosphorylation and dephosphorylation of its caspase recruitment domains (CARDs). Employing a phosphatome RNAi screen, we identified PP1 alpha and PP1 gamma as the primary phosphatases that are responsible for MDA5 and RIG-I dephosphorylation and that lead to their activation. Silencing of PP1 alpha and PP1 gamma enhanced RIG-I and MDA5 CARD phosphorylation and reduced antiviral IFN-beta production. PP1 alpha- and PP1 gamma-depleted cells were impaired in their ability to induce IFN-stimulated gene expression, which resulted in enhanced RNA virus replication. This work identifies PP1 alpha and PP1 gamma as regulators of antiviral innate immune responses to various RNA viruses, including influenza virus, paramyxovirus, dengue virus, and picornavirus.