Glycoprotein E of the Japanese encephalitis virus forms virus-like particles and induces syncytia when expressed by a baculovirus

The prM glycoprotein is thought to be a chaperone for the proper folding, membrane association and assembly of the envelope protein (E) of flaviviruses. The prM-E and E proteins of the Japanese encephalitis virus (JEV) were expressed in insect cells using both the baculovirus-expression system and the transient expression method. Protein expression was analysed by Western blotting and the cytopathic effect was observed by microscopy. In the baculovirus-expression system the E protein, with or without the prM protein, induced syncytial formation in Sf9 cells. Transient expression of prM-E also induced syncytia in Sf9 cells. Immunofluorescence revealed that in presence of prM, E proteins were endoplasmic reticulum-like in distribution, while in the absence of prM, E proteins were located on the cell surface. Sucrose gradient sedimentation and Western blot analysis indicated that the E protein expressed with or without the prM protein was secreted into the culture medium in particulate form. The formation of virus-like particles (VLPs) in the medium was confirmed by electron microscopy and immunoelectron microscopy. The results suggest that the E protein of JEV in the absence of prM, retained its fusion ability, by either cell surface expression or formation of VLPs. Moreover, based on the observation that co-expression of prM-E in Sf9 cells induced considerable syncytial formation, a novel, safe and simple antiviral screening approach is proposed for studying inhibitory antibodies, peptides or small molecules targeting the JEV E protein.