Asymmetric reconstructions of immature Tick-borne encephalitis virus particles reveal assembly mechanism of flaviviruses

Tibor Füzik1, Lenka Šmerdová1, Petra Pokorná Formanová2, Petra Straková2, Jiří Nováček3, Daniel Růžek2, Pavel Plevka1*

1Structural Biology, Central European Institute of Technology, Masaryk University, Kamenice 5, Brno, Czech Republic, tibor.fuzik@ceitec.muni.cz

2Department of Virology, Veterinary Research Institute, Hudcova 70, Brno, Czech Republic

3Cryo-electron Microscopy and Tomography Core Facility, Central European Institute of Technology, Masaryk University, Kamenice 5, Brno, Czech Republic

 

Tick-borne encephalitis virus (TBEV) is an enveloped virus belonging to the family Flaviviridae, which causes severe disease of central nervous system in humans. The smooth virion surface is covered by envelope proteins (E-protein), that are together with the membrane proteins (M-protein) anchored in the virus lipid bilayer. The immature, non-infectious virus goes through a maturation phase during the viral life cycle. Proteolytic cleavage of prM and a significant rearrangement of the envelope proteins on the viral surface are two aspects of this process.

We isolated immature TBEV particles from infected tissue culture cells and visualized them using cryo-electron microscopy in order to determine their structure. The E-protein-prM-protein complex forms the "spiky" surface of the immature particles. Weused cryo-electron tomography and single-particle analysis to show that the TBEV immature particles are asymmetric. Defects induced during immature particle assembly frequently disturb the symmetric, icosahedral structure of the E-protein-prM-protein spikes on the particle surface. However, these irregularities do not hinder the subsequent maturation process and produce mature particles with vacant patches in the "herring bone" structure of the mature viral surface

The findings shed additional light on the viral assembly and maturation process, which may be the subject of future antiviral medication development.

This work was supported by the project National Institute of Virology and Bacteriology (Programme EXCELES, ID Project No. LX22NPO5103) - Funded by the European Union - Next Generation EU.