Recombinant subviral particles from tick-borne encephalitis virus are fusogenic and provide a model system for studying flavivirus envelope glycoprotein functions.

Recombinant subviral particles (RSPs) obtained by coexpression of the envelope (E) and premembrane (prM) proteins of tick-borne encephalitis virus in COS cells (S. L. Allison, K. Stadler, C. W. Mandl, C. Kunz, and F. X. Heinz, J. Virol. 69:5816-5820, 1995) were extensively characterized and shown to be ordered structures containing envelope glycoproteins with structural and functional properties very similar to those in the virion envelope. The particles were spherical, with a diameter of about 30 nm and a buoyant density of 1.14 g/cm3 in sucrose gradients. They contained mature E proteins with endoglycosidase H-resistant glycans as well as fully cleaved mature M proteins. Cleavage of prM, which requires an acidic pH in exocytic compartments, could be inhibited by treatment of transfected cells with ammonium chloride, implying a common maturation pathway for RSPs and virions. RSPs incorporated [14C]choline but not [3H]uridine, demonstrating that they contain lipid but probably lack nucleic acid. The envelope proteins of RSPs exhibited a native antigenic and oligomeric structure compared with virions, and incubation at an acidic pH (pH <6.5) induced identical conformational changes and structural rearrangements, including an irreversible quantitative conversion of dimers to trimers. The RSPs were also shown to be functionally active, inducing membrane fusion in a low-pH-dependent manner and demonstrating the same specific hemagglutination activity as whole virions. Tick-borne encephalitis virus RSPs thus represent an excellent model system for investigating the structural basis of viral envelope glycoprotein functions.