Singapore grouper iridovirus VP20 interacts with grouper TBK1 and IRF3 to attenuate the interferon immune response

Singapore grouper iridovirus (SGIV), belonging to genus Ranavirus, family Iridoviridae, is a highly pathogenic agent and causes heavy economic losses in the global grouper aquaculture. Recent studies demonstrated that SGIV infection attenuated antiviral immune and inflammatory response induced by poly (I:C) in vitro. However, little was known about the potential functions of the immune regulatory proteins encoded by SGIV. Here, we identified the detailed roles of VP20 and clarified the potential mechanism underlying its immune regulatory function during SGIV infection. Our results showed that VP20 was an IE gene, and partially co-localized with Golgi apparatus and lysosomes in grouper cells. Overexpression of VP20 enhanced SGIV replication, demonstrated by the increase in the transcription levels of viral core genes and the protein synthesis of MCP. Reporter gene assays showed that SGIV VP20 overexpression significantly reduced the IFN promoter activity induced by poly (I:C), grouper stimulator of interferon genes (EcSTING) and TANK-binding kinase 1 (EcTBK1). Consistently, the transcription levels of IFN related genes were significantly decreased in VP20 overexpressing cells compared to those in control cells. Co -IP assay and confocal microscopy observations indicated that VP20 co-localized and interacted with EcTBK1 and EcIRF3, but not EcSTING. In addition, VP20 was able to degrade EcIRF3 and attenuate the antiviral action of EcIRF3, while had no effect on EcTBK1. Together, SGIV VP20 was speculated to promote viral replication through attenuating the IFN response mediated by TBK1-IRF3 in vitro. Our findings provided new insights into the immune regulatory function of SGIV encoded unknown proteins.