SGIV envelope protein VP088 facilitated virus replication via interacting with other viral proteins and promoting p62-dependent autophagic degradation of TBK1.

Singapore grouper iridovirus (SGIV), a novel member of the genus Ranavirus, family Iridoviridae, frequently causes a severe disease with high mortality in grouper aquaculture. Although previous findings have demonstrated that SGIV envelope protein VP088 was crucial for its infectivity, the underlying mechanism still remained uncertain. Here, we screened the potential viral proteins that interacted with VP088 during SGIV infection using GFP pull-down assay. Co-immunoprecipitation (Co-IP) assays verified the interactions between VP088 and VP018, VP068, or VP156 in vitro. Moreover, confocal microscopy analysis showed that VP088 markedly altered the cellular distribution of exogenously expressed VP018 and VP068 and ultimately translocated into virus assembly sites together upon SGIV infection. Differently, VP088 mostly co-localized with exogenous VP156 in co-transfected cells and almost simultaneously translocated into the virus assembly sites, suggesting that VP088 participated in SGIV replication through interactions with other viral proteins in different ways. Interestingly, VP088 also abrogated IFN response induced by grouper (Epinephelus coioides) EccGAS-EcSTING and EcTBK1 in vitro. Co-IP assays showed that VP088 interacted with EccGAS, EcSTING, EcTBK1, or EcIRF3, while only degraded EcTBK1 via Ecp62-mediated autophagic degradation. Furthermore, VP088 decreased EcTBK1-induced EcIRF3 phosphorylation and nuclear translocation. In addition, the ectopic expression of VP088 attenuated the antiviral function of EcSTING/EcTBK1/EcIRF3 against red-spotted grouper nervous necrosis virus (RGNNV) infection. Thus, our results not only identified the association between SGIV VP088 and other viral proteins during replication, but also for the first time demonstrated that an iridoviral envelope protein could function as an immune evasion protein via abrogating EcTBK1-induced interferon response.IMPORTANCEIridovirus infection frequently causes high levels of morbidity and mortality among commercially and ecologically important fish, crustaceans, amphibians, and reptiles. However, the molecular mechanism of iridovirus pathogenesis still remains largely unknown, and few effective countermeasures have been developed to date. Using the Singapore grouper iridovirus (SGIV) infection model in vitro, we identified the potential viral proteins that interacted with envelope protein VP088 during virus replication. Moreover, for the first time, we demonstrated that VP088 interacted with EccGAS, EcSTING, EcTBK1, and EcIRF3, but only degraded EcTBK1 via Ecp62-mediated autophagic degradation, thereby inhibiting the host IFN response. Thus, our results not only contribute to elucidating the mechanism of SGIV pathogenesis but also provide a novel molecular target for the construction of immunogenic live vaccines against iridoviral diseases in the future.