RECOMBINANT SUBVIRAL PARTICLES FROM TICK-BORNE ENCEPHALITIS-VIRUS AREFUSOGENIC AND PROVIDE A MODEL SYSTEM FOR STUDYING FLAVIVIRUS ENVELOPEGLYCOPROTEIN 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 charac terized and shown to be ordered structures containing envelope glycopr oteins 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/cm(3) in sucrose gradi ents, They contained mature E proteins with endoglycosidase H-resistan t glycans as well as fully cleaved mature M proteins. Cleavage of prM, which requires an acidic pH in exocytic compartments, could be inhibi ted by treatment of transfected cells with ammonium chloride, implying a common maturation pathway for RSPs and virions, RSPs incorporated [ C-14]choline but not [H-3]uridine, demonstrating that they contain lip id but probably lack nucleic acid. The envelope proteins of RSPs exhib ited a native antigenic and oligomeric structure compared with virions , and incubation at an acidic pH (pH <6.5) induced identical conformat ional changes and structural rearrangements, including an irreversible quantitative conversion of dimers to trimers, The RSPs were also show n to be functionally active, inducing membrane fusion in a low-pH-depe ndent manner and demonstrating the same specific hemagglutination acti vity as whole virions, Tick-borne encephalitis virus RSPs thus represe nt an excellent model system for investigating the structural basis of viral envelope glycoprotein functions.