Ubiquitin E3 ligase MID1 inhibits the innate immune response by ubiquitinating IRF3

Summary Interferon regulatory factor 3 (IRF3) is a critical transcription factor for inducing production of type I interferons (IFN‐I) and regulating host antiviral response. Although IRF3 activation during viral infection has been extensively studied, the inhibitory regulation of IRF3 remains largely unexplored. Here, we revealed that Midline‐1 (MID1) is a ubiquitin E3 ligase of IRF3 that plays essential roles in regulating the production of IFN‐I. We found that MID1 physically interacts with IRF3 and downregulates IRF3 protein levels. Next, we demonstrated that MID1 can induce K48‐linked polyubiquitination of IRF3, thus lowing the protein stability of IRF3. Our further studies identified Lys313 as a major ubiquitin acceptor lysine of IRF3 induced by MID1. Finally, MID1‐mediated ubiquitination and degradation of IRF3 restrict IFN‐I production and cellular antiviral response. This study uncovers a role of MID1 in regulating innate antiviral immunity and may provide a potential target for enhancing host antiviral activity.