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

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.