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extracellular virions respectively. The N-terminal pr segment is predominantlyhydrophilic and contains one to three N-linked glycosylation sites (Chambers er. aI.,1990) and six conserved systemic residues, all of which participate in intramoleculardisulfide bridges (Nowak and Wengler, 1987). The structural protein M located inthe C-tenninal portion of prM is present in mature virions and contains a shortenedectodomain (41 amino acids) followed by two potential membrane-spanningdomains. Antibodies to prM can mediate protective immunity (Kaufman et aI.,1989) perhaps by neutralization of released virions that contain some uncleavedprM.2.2.5.3 The E protei":This is the major envelope protein of the virion. It is believed to play keyroles in a number of important processes including virion assembly, receptor bindingand membrane fusion. It is also the principal target for neutralization antibodies(Heinz, 1986). Not surprisingly, mutations in this protein can result in dramaticeffects on viral pathogenesis. Comparison of deduced E protein sequences showareas of striking homology as well as divergence amongst the Flaviviruses(Chambers et. al., 1990; Heinz et. aI., 1990). All twelve Cys residues in the Eectodomain are strictly conserved and involved in intramolecular disulfide bonds(Nowak and Wengler, 1987). The E protein is glycosylated for some, but not all,Flaviviruses and the role of N-linked glycosylation on E function is unclear(Chambers et. al., 1990)The detergent- solubilized TBE E protein (Hcinz and Kunz, 1980) as well asthe soluble tryptic fragment used for crystallography is a dimer. The X-ray structuredemonstrates that the dimer is a head-to-tail oligomer in the shape of a 170 a rod andpredicts that the dimer is anchored in the bilayer at both dista1 ends, The curvature ofthe dimmer fits with its location on the surface of the SOOA virion. This fittingsuggests that, these oligomers do not form long projections or spikes. Two of thethree-distinct structural entities present in the monomeric unit correspond topreviously defined anti genic domains (Mandl et. aI., 1989). Potential neutralizationsites defined by amino acids substitutions present in monocIonal antibody escapemutants are mostly distributed on the surface and can be present on any of thestructural domains. The role of the highly conserved sequence from the residues 1911