Regulations Of Virus-triggered Signaling By OTUB1 And OTUB2

Viral infection triggered a series of signaling events that lead to induction of typeⅠinterferons (IFNs). TypeⅠIFNs then activate the JAK-STAT signal transduction pathways, leading to transcriptional induction of a wide range of downstream antiviral genes and subsequent innate antiviral response. It has been shown that transcriptional activation of the IFNB1 gene requires coordinate and cooperative assembly of an enhanceosome that contains multiple transcription factors such as NF-κB and IRF3.The innate immune system has developed at least two kinds of pathogen recognition receptor (PRRs) for the recognition of viral RNAs. One is mediated by membrane-bound Toll-like receptors (TLRs) such as TLR3. Engagement of TLR3 by dsRNA triggers TRIF (an adaptor of the TLR pathway)-mediated signaling pathway, thereby leading to IRF3 and NF-κB activation. The second one involves the cytosolic RIG-I-like receptor (RLR) family members RIG-Ⅰ, MDA5 and Lgp2. Upon viral infection, the RNA helicase domains of RIG-Ⅰand MDA5 serve as intracellular viral RNA receptors, whereas their CARD modules are associated with the downstream CARD-containing adapter protein VISA (also known as MAVS, IPS-1, and Cardif). Various studies have demonstrated that VISA plays a central role in assembling a complex that activates distinct signaling pathways leading to NF-κB and IRF3 activation respectively. VISA is associated with TRAF2 and TRAF6 through its TRAF interaction motifs. It has been shown that TRAF2 and TRAF6 facilitate K63-linked polyubiquitination of RIP and NEMO/IKKy respectively and these processes cause activation of IKKs and subsequent NF-κB. VISA is also associated with TRAF3, another member of the TRAF protein family. Gene knockout studies have demonstrated that TRAF3 is essential in virus-triggered IRF3 activation and typeⅠIFN induction.Ubiquitination and deubiquitination have emerged as critical post-translational regulatory mechanisms for activation or attenuation of the virus-triggered IFN response pathways. It has been shown that the E3 ubiquitin ligase TRIM25 catalyzes K63-linked ubiquitination of RIG-Ⅰand this ubiquitination is essential for the interaction of RIG-Ⅰwith VISA as well as for its ability to signal. We speculate the unknown ubiquitinases and deubiquitinases involved in IFN-βproduction. In this study, we identified two deubiquitinases, OTUB1 and OTUB2, inhibited virus-triggered IFN induction. Overexpression of OTUB1 and OTUB2 inhibited SeV-induced activation of ISRE, NF-κB, IFN promoter and IFNB1 expression, whereas knockdown of OTUB1 and OTUB2 had opposite effects and inhibited vesicular stomatitis virus (VSV) replication. Coimmunoprecipitations indicate OTUB1 and OTUB2 are associated with the VISA-associated complex, the components in the virus-triggered IFN signaling. We further found that OTUB1 and OTUB2 regulate the antiviral responses by specifically deubiquitinating TRAF3 and TRAF6. These findings suggest that OTUB1 and OTUB2 negativly regulates virus-triggered signaling by their deubiquitinase activity.