Membrane fusion activity of tick-borne encephalitis virus and recombinant subviral particles in a liposomal model system

We present a kinetic analysis or the membrane fusion activity of tick-borne encephalitis (TBE) virus and TBE-derived recombinant subviral particles (R SPs) in a liposomal model system. Fusion was monitored using a fluorescence assay involving pyrene-labeled phospholipids. Fusion was strictly dependen t on low pH, with the optimum being at pH 5.3-5.5 and the threshold at pH 6 .8. Fusion did not require a protein or carbohydrate receptor in the target liposomes. Preexposure to tow pH of the virus alone resulted in inactivati on of its fusion activity. At the optimum pH for fusion and 37 degrees C, t he rate and extent of fusion were very high, with more than 50% of the viru s fusing within 2 s and the final extent of fusion being 70%. Lowering of t he temperature did not result in a significant decrease in the rate and ext ent of fusion, suggesting that TBE virus fusion is a facile process with a low activation energy, possibly due to the flat orientation of the E glycop rotein on the viral surface facilitating the establishment of direct interm embrane contact. The fusion characteristics of TBE virus and RSPs were simi lar, indicating that RSPs provide a reliable and convenient model for furth er study of the membrane fusion properties of TBE virus. (C) 2000 Academic Press.