Grass carp ( <i>Ctenopharyngodon idella</i> ) SMYD3 negatively regulates antiviral innate immunity by methylating IRF3 and IRF7 during GCRV infection

Interferon regulatory factors 3 and 7 (IRF3 and IRF7, respectively), which serve as key transcription factors in interferon (IFN) activation, are tightly regulated by a variety of mechanisms, including multiple posttranslational modifications, to fulfill their normal physiological functions. Nevertheless, the methylation-mediated regulation of IRF3 and IRF7 in grass carp (Ctenopharyngodon idella) remains poorly understood. In this study, we found that SMYD3, a lysine methyltransferase, is conserved across species and is induced by grass carp reovirus (GCRV) infection in grass carp. Furthermore, overexpression of grass carp SMYD3 exerted a negative regulatory effect on antiviral innate immunity. Conversely, knocking down of SMYD3 in cells enhanced the GCRV-induced antiviral gene expression. Mechanistically, SMYD3 interacts with the transcription factors IRF3 and IRF7, as demonstrated by co-immunoprecipitation and immunofluorescence confocal microscopy assays. Moreover, SMYD3 was found to orchestrate the di- or tri-methylation of the fifth lysine of IRF3 and the 11th lysine of IRF7, as identified by mass spectrometry. Furthermore, treatment with the SMYD3-specific inhibitor BCI121 significantly enhanced resistance to GCRV infection in cells and grass carp. Our results reveal a novel function of the lysine methyltransferase SMYD3 in anti-GCRV immunity and identify SMYD3 as a potential target for breeding new grass carp strains with anti-GCRV ability. In addition, our findings suggest that BCI121, a SMYD3-specific small molecule inhibitor, can be developed as an effective anti-GCRV drug for the treatment of grass carp hemorrhagic disease caused by GCRV.