USP39 Serves as a Deubiquitinase to Stabilize STAT1 and Sustains Type I IFN-Induced Antiviral Immunity.

Deubiquitinating enzymes (DUBs) are cysteine proteases that reverse the ubiquitination by removing ubiquitins from the target protein. The human genome encodes ∼100 potential DUBs, which can be classified into six families, influencing multiple cellular processes, such as antiviral responses, inflammatory responses, apoptosis, etc. To systematically explore the role of DUBs involved in antiviral immunity, we performed an RNA interference-based screening that contains 97 human DUBs. We identified that ubiquitin-specific protease (USP) 39 expression modulates the antiviral activity, which is, to our knowledge, a previously unknown function of this enzyme. Small interfering RNA knockdown of USP39 significantly enhanced viral replication, whereas overexpression of USP39 had an opposite effect. Mechanistically, USP39 does not affect the production of type I IFN but significantly promotes JAK/STAT downstream of type I signaling by enhancing IFN-stimulated response elements promoter activity and expression of IFN-stimulated genes. Interestingly, USP39, previously considered not to have the deubiquitinase activity, in this study is proved to interact with STAT1 and sustain its protein level by deubiqutination. Furthermore, we found that through novel mechanism USP39 can significantly decrease K6-linked but not K48-linked ubiquitination of STAT1 for degradation. Taken together, these findings uncover that USP39 is, to our knowledge, a new deubiquitinase that positively regulates IFN-induced antiviral efficacy.