Jab1 mediates cytoplasmic localization and degradation of West Nile virus capsid protein

The clinical manifestations of West Nile virus (WNV), a member of the Flavivirus family, include febrile illness, sporadic encephalitis, and paralysis. The capsid (Cp) of WNV is thought to participate in these processes by inducing apoptosis through mitochondrial dysfunction and activation of caspase-9 and caspase-3. To further identify the molecular mechanism of the WNV capsid protein (WNVCp), yeast two-hybrid assays were employed using WNV-Cp as bait. Jab1, the fifth subunit of the COP9 signalosome, was subsequently identified as a molecule that interacts with WNVCp. Immunoprecipitation and glutathione S-transferase pulldown assays confirmed that direct interaction could occur between WNVCp and Jab1. Immunofluorescence microscopy demonstrated that the overexpressed WNVCp, which localized to the nucleolus, was translocated to the cytoplasm upon its co-expression with Jab1. When treated with leptomycin B, Jab1-facilitated nuclear exclusion of WNVCp was prevented, which indicated that the CRM1 complex is required for Jab1-facilitated nuclear export of WNVCp. Moreover, Jab1 promoted the degradation of WNVCp in a proteasome-dependent way. Consistent with this, WNVCp-mediated cell cycle arrest at the G₂ phase in H1299 was prevented by exogenous Jab1. Finally, an analysis of WNVCp deletion mutants indicated that the first 15 amino acids were required for interaction with Jab1. Furthermore, the double-point mutant of the WNVCp, P5A/P8A, was incapable of binding to Jab1. These results indicate that Jab1 has a potential protective effect against pathogenic WNVCp and might provide a novel target site for the treatment of disease caused by WNV.