The rapid induction of type I IFN is a central event of the innate defense against viral infections and is tightly regulated by a number of cellular molecules. Viral components induce strong type I IFN responses through the activation of toll-like receptors (TLRs) and intracellular cytoplasmic receptors such as an RNA helicase RIG-I and/ or MDA5. According to recent studies, the NF-κB essential modulator (NEMO, also called IKKγ) is crucial for this virus-induced antiviral response. However, the precise roles of signal activation by NEMO adaptor have not been elucidated. Here, we show that virus-induced IRF3 and NF-κB activation depends on the K(lys)-27-linked polyubiquitination to NEMO by the novel ubiquitin E3 ligase triparite motif protein 23 (TRIM23). Virus-induced IRF3 and NF-κB activation, as well as K27-linked NEMO polyubiquitination, were abrogated in TRIM23 knockdown cells, whereas TRIM23 knockdown had no effect on TNFα-mediated NF-κB activation. Furthermore, in NEMO-deficient mouse embryo fibroblast cells, IFN-stimulated response element-driven reporter activity was restored by ectopic expression of WT NEMO, as expected, but only partial recovery by NEMO K165/309/325/326/344R multipoints mutant on which TRIM23-mediated ubiquitin conjugation was substantially reduced. Thus, we conclude that TRIM23-mediated ubiquitin conjugation to NEMO is essential for TLR3-and RIG-I/MDA5-mediated antiviral innate and inflammatory responses.innate immunity | signal transduction | virus infection U pon viral infection, host cells recognize the viral components and activate innate immune signaling to exert antiviral responses (1-4). RIG-I and/or MDA5 sense viral dsRNA (5-8) and are recruited to another antiviral signaling adaptor, IPS-1 (also called MAVS, Cardif, or VISA) (9-12). IPS-1 directly interacts with TRAF3 and triggers auto-ubiquitination of TRAF3, which then activates TBK1 and IKKε, leading to activation of transcription factors NF-κB and IRF3 (13,14). A recent study indicated that NEMO acts upstream of TBK1 and IKKε and is essential for virus-induced TLR3-and RIG-I/MDA5-mediated antiviral activation (15).Because rapid induction of type I IFN expression is the key process in initiating the innate antiviral response, clarification of NEMO-mediated antiviral signaling is important for understanding innate immune signaling; however, NEMO-mediated antiviral signaling is not well elucidated. Recent studies indicate that several ubiquitin E3 ligases are involved in the regulation of innate immune signaling (16-21). We identified the ubiquitin E3 ligase TRIM23 (Triparite motif protein 23), also named ADP ribosylation factor domain protein 1 (ARD1), which was reported to have E3 ligase activity in vitro (22), that functioned as an E3 ligase for NEMO ubiquitin conjugation. TRIM23 exerts a potent antiviral state following its overexpression. Furthermore, we demonstrated that antiviral activity depends not on K(Lys)63-linked but on K27-linked polyubiquitin conjugation to multiple sites of NEMO by TRIM23 expression. Virus-in...