Identification and characterization of IRF9 from black carp Mylopharyngodon piceus

Abstract Interferon regulatory factor 9 (IRF9) plays a crucial role in JAK-STAT signaling in human and mammal. However, the relationship between IRF9 and STAT1 in teleost fish remains largely unknown. The previous study has elucidated that two STAT1 isoforms (bcSTAT1a and bcSTAT1b) of black carp (Mylopharyngodon piceus) play an important role during the innate immune activation initiated by grass carp reovirus (GCRV). In this paper, black carp IRF9 (bcIRF9) has been identified and characterized. bcIRF9 was distributed majorly in the nucleus and the linker domain (LD) of bcIRF9 was vital for its nuclear localization. bcIRF9 showed ISRE-inducing activity in reporter assay and presented antiviral activity against GCRV in plaque assay, in which both DNA binding domain (DBD) and LD of bcIRF9 were essential for its antiviral signaling. bcIRF9 was identified to interact with both bcSTAT1a and bcSTAT1b in the co-immunoprecipitation assay. It was interesting that bcIRF9-mediated antiviral signaling was up-regulated by bcSTAT1a; however, down-regulated by bcSTAT1b. Thus, our data support the conclusion that bcIRF9 plays an important role in the innate immune defense against GCRV, in which two STAT1 proteins function differently.