Tick-borne encephalitis virus (TBEV) is an enveloped virus belonging to the family Flaviviridae. It is mainly transmitted by ticks and causes severe disease of central nervous system in humans. Virion surface is covered by envelope proteins (E-protein), that are together with the membrane proteins (M-protein) anchored in virus lipid bilayer. The arrangement of these proteins in the virion is unknown, therefore, detailed structural study of the virus is needed.
We determined structure of mature TBEV virions grown in tissue culture and of complexes of virions with Fab fragments of neutralizing antibodies. Because of fragileness and non-homogeneity of the virions, we used cryo-electron microscopy to determine the structures. The observed particles (~50 nm in diameter) were suitable for reconstruction of the virus envelope. To obtain high-resolution electron density maps, single particle reconstruction techniques were employed, using programs from image-processing packages EMAN2, XMIPP, and RELION. The final reconstructed volume revealed structure in accordance with general structural organization of other flaviviruses including dengue and West Nile viruses. The reconstructions of TBEV particles in complexes with neutralizing antibodies showed attachment of the antibodies to specific sites on the viral surface.
Further improvement of the reconstructions may provide electron density maps of resolutions suitable for de novo model building of structural proteins, detailed structural studies of the virus shell and identification of virus residues constituting the binding site of the neutralizing antibodies. These structural studies may help to get better insight into TBEV particle organization as well as to obtain therapeutic anti-TBEV antibodies.