Opposing roles of deubiquitinases in the regulation of IRF7 transcriptional activity

Virus infection rapidly induces the production and secretion of interferons (IFNs), which amplify the antiviral responses in infected and neighboring uninfected cells. IFN regulatory factor 7 (IRF7), the "master transcription factor," is pivotal in IFN induction, particularly in myeloid cells. Ubiquitination of IRF7 is essential for its transcriptional activation; however, the underlying molecular mechanisms remain poorly understood. We hypothesized that deubiquitinases (DUBs) act as endogenous regulators of IRF7 activity, and conducted a genetic screen using an siRNA library of human DUBs. The screen identified USP2 as a positive regulator and OTUD5 as a negative regulator of IRF7 activity. OTUD5, an inducible DUB, interacted with IRF7 and inhibited its K63-linked ubiquitination, thereby suppressing IRF7 activation. Conversely, USP2 promoted IRF7 activity by binding to IRF7 and removing K27-linked ubiquitin chains, which we found to be inhibitory. Specifically, K27-linked ubiquitination impeded phosphorylation of IRF7, a critical step for its activation. Collectively, our independent lines of investigation, coupled with genetic screens and mechanistic studies, uncovered USP2 and OTUD5 as novel modulators of IRF7 function, providing novel insights into the regulation of antiviral immunity. IMPORTANCE: IFN regulatory factor 7 (IRF7) is a central protein that launches type I interferon responses, and its timely activation is essential for antiviral immunity. Our study uncovers a mechanism by which IRF7 activation is controlled by enzymes that specifically remove small molecular tags, known as ubiquitin, from proteins. Through a focused screen, we identified two enzymes with opposing roles in modulating IRF7 activity. OTUD5, one of these enzymes, suppresses IRF7 activity by removing a ubiquitin tag that is essential for its transcriptional function. In contrast, USP2, the other enzyme, activates IRF7 by removing a ubiquitin tag that is inhibitory to IRF7 functions. These findings reveal previously unrecognized layers of IRF7 regulation and highlight how these enzymes can be targeted therapeutically in diseases driven by abnormal IRF7 functions.