FIG4 is a Hepatitis C Virus particle-bound protein implicated in virion morphogenesis and infectivity with cholesteryl ester modulation potential

There is growing evidence that virus particles also contain host cell proteins, which provide viruses with certain properties required for entry and release. A proteomic analysis performed on double gradient-purified hepatitis C virus from two highly viremic patients identified the Phosphatidylinositol 3,5-bisphosphate 5-phosphatase FIG4 (KIAA0274) as part of the viral particles. We validated the association using immunoelectron microscopy, immunoprecipitation and neutralization assays in vitro as well as patient-derived virus particles. RNAi-mediated reduction of FIG4 expression decreased cholesteryl ester (CE) levels along with intra- and extracellular viral infectivity without affecting HCV RNA levels. Likewise, overexpressing FIG4 increased intracellular CE levels as well as intra- and extracellular viral infectivity without affecting viral RNA levels. Triglyceride (TG) levels and lipid droplets (LD) parameters remained unaffected. The 3,5-bisphosphate 5-phosphatase active site of FIG4 was found to strongly condition these results. While FIG4 was found to localize to areas corresponding to viral assembly sites, at the immediate vicinity of LDs in calnexin+ and HCV core+ regions, no implication of FIG4 in the secretory pathway of the hepatocytes could be found using either FIG4 null mice, in vitro morphometry or functional assays of the ERGIC/Golgi compartments. This indicates that FIG4-dependent modulation of HCV infectivity is unrelated to alterations in the functionality of the secretory pathway. Because of the documented implication of CE in the composition and infectivity of HCV particles, these results suggest that FIG4 binds to HCV and modulates particle formation in a CE-related manner.