Singapore grouper iridovirus utilizes Rab1 to facilitate viral trafficking and evade the immune response by targeting the STING-IRF3 pathway

The phylum Nucleocytoviricota comprises diverse nucleocytoplasmic large DNA viruses (NCLDVs) that infect a wide range of eukaryotic hosts, yet the mechanisms underlying their intracellular transport and immune evasion remain poorly understood. Singapore grouper iridovirus (SGIV), a representative NCLDV, is a major pathogen in marine aquaculture. Rab GTPases are central regulators of vesicular trafficking and frequent targets of viral manipulation, but their roles in SGIV infection have not been elucidated. Here, we investigated the roles of Rab1 from Epinephelus coioides (EcRab1) during SGIV infection. Orange-spotted grouper (E. coioides) and grouper spleen (GS) cells were used in this study. The immunoblotting and qRT-PCR were used for measuring the expression of proteins and mRNAs. Viral entry and trafficking were analyzed by single-particle imaging and live-cell imaging, respectively. Immunofluorescence staining was employed to detect the location of proteins and organelle. siRNA or shRNA was used to knock down gene expression in cultured cells in vitro. The interaction between proteins was investigated by co-immunoprecipitation assay and confocal imaging. SGIV infection markedly downregulated EcRab1, and both overexpression and silencing of EcRab1 significantly altered viral replication. EcRab1 promoted viral entry and delivery to early endosomes (EEs) via interaction with Rab5, generating transient Rab1+/Rab5+ vesicles that matured into Rab5+ vesicles. In addition to its role in trafficking, EcRab1 maintained the structural integrity of the Golgi apparatus and mediated the transport of STING from the endoplasmic reticulum to the Golgi, thereby enhancing the production of IFN-I. SGIV counteracted these processes through the action of its protein VP20, which bound both GDP- and GTP-loaded forms of EcRab1, inhibited the association between STING and EcRab1, and prevented the localization of STING to the Golgi. Moreover, the VP20–EcRab1 complex facilitated the recruitment of IRF3 to autophagosomes for degradation, thus attenuating antiviral signaling pathways. Our findings reveal that SGIV hijacks host EcRab1 to promote trafficking to early endosomes while simultaneously disrupting STING–IRF3 signaling, thereby facilitating viral replication and immune evasion. This study reveals a novel mechanism through which Rab1 contributes to large DNA virus infection and suggests that Rab1 could serve as a potential antiviral target.