NSD3 keeps IRF3 active

Antiviral innate immune responses are a critical first line of host defense against invading viral pathogens (Akira et al., 2006; Takeuchi and Akira, 2010; Moresco et al., 2011). Viral RNA and DNA is initially recognized by pattern recognition receptors (PRRs) such as TLRs, retinoic acid– inducible gene-I (RIG-I)–like receptors (RLRs), Nod-like receptors (NLRs), and cyclic GMP-AMP (cGAMP) synthase (cGAS). TLRs are transmembrane proteins recognizing microbial components on the cell surface or in the endosomes. Among the TLRs expressed on the endosome, TLR3, TLR7, and TLR9 sense viral double-stranded RNA (dsRNA), single-stranded RNA (ssRNA), and DNA with a CpG motif, respectively. In addition to TLRs, cytoplasmic RLRs, RIG-I and melanoma differentiation-associated gene 5 (MDA5) recognize 5′-triphosphate end dsRNA and long dsRNA, respectively (Yoneyama and Fujita, 2009). Cytosolic DNA sensor, cGAS, senses viral DNAs (Barrat et al., 2016). Extensive studies have revealed that the PRR signaling pathways lead to transcription of type I IFNs via transcription factors, including IFN-regulatory factor 3 (IRF3) and IRF7 (Honda et al., 2006). PRR signaling also activates another transcription factor, NF-κB, which contributes to the transactivation of proinflammatory cytokines as well as type I IFNs. IRF3 and IRF7 are key transcription factors responsible for induction of type I IFNs by viral infection and play a critical role in host antiviral innate immunity (Banchereau and Pascual, 2006; Honda et al., 2006; Sadler and Williams, 2008). IRF3 is constitutively expressed and resides in the cytosol in its latent form. Posttranslational modifications (PTMs), including phosphorylation and polyubiquitination, are key features of signal transduction pathways that allow the modulation of protein function (Deribe et al., 2010; Mowen and David, 2014; Liu et al., 2016). Upon viral infection, PTMs can affect the activation of signaling molecules, as well as their cellular translocation, stabilization, or interaction with other molecules. Indeed, IRF3 undergoes phosphorylation by TBK1 and IKKε after the PRR signaling, which induces IRF3 dimerization and nuclear translocation, resulting in transcription of type I IFN mRNA (Fitzgerald et al., 2003; Takeuchi and Akira, 2009). In addition, unconventional PTMs, including methylation, acetylation, sumoylation, and succinylation, have also been implicated in the regulation of innate immune system (Mowen and David, 2014). However, the role of IRF3 methylation in antiviral responses has not been understood. In this issue of JEM, Wang et al. demonstrate that monomethylation of IRF3 at lysine 366 (K366) is induced by infection with herpes simplex virus (HSV) and vesicular stomatitis virus (VSV). Methylation-defective substitution at K366 (K366A) significantly abolished IRF3-driven Ifnb activation and IFN-β production upon VSV infection. These data suggest that viral infection induces monomethylation of IRF3 at K366, which is responsible for promoting IRF3 activation and IFN-β production. To identify methyltransferases mediating the K366-monomethylation of IRF3, the Wang et al. (2017) performed coimmunoprecipitation and mass spectrometry analysis. They found that a lysine methyltransferase, NSD3, directly binds to IRF3. The K366 methylation of IRF3 was inhibited by VSV infection in infected NSD3-deficient macrophages. Moreover, an in vitro methylation assay showed that NSD3 directly methylates IRF3. NSD3-deficient mice were more susceptible to VSV infection and showed a decreased level of IFN-β production in serum and organs, as well as increased VSV replication and titers in organs compared with control mice. These results show that NSD3 directly methylates IRF3 at K366 upon viral infection, and NSD3 is an essential methyltransferase for the production of type I IFN and antiviral innate responses. Although the authors demonstrated that NSD3 interacts with the IRF3 C-terminal region through its PWWP1 domain and that the NSD3-mediated IRF3 methylation occurs in the nucleus, it is not yet clear how NSD3 specifically methylates IRF3 at K366 upon viral infection. It is interesting to speculate that the activity of NSD3 to methylate IRF3 is also dynamically controlled by viral infection. Wang et al. (2017) subsequently investigated mechanisms of how the NSD3-mediated IRF3 methylation regulates IRF3 activity. Interestingly, VSV-induced IRF3 phosphorylation at Ser388 requires NSD3, and NSD3-mediated IRF3 methylation suppressed the interaction of IRF3 with protein phosphatase 1 (PP1), which is involved in the regulation of IRF3 activity via dephosphorylation (Gu et al., 2014). These data demonstrate that NSD3 decreases the binding of PP1 to IRF3, preventing