Nucleocytoplasmic large dsDNA viruses (NCLDV) are among the largest and most complex viruses known, with the capsid size ranging from 200–800 nm in diameter . Here, we focused on ranaviruses (~200 nm in diameter), global pathogens with increasing importance, which exist in two infectious forms: naked capsids and enveloped virions . In order to determine the structure of the two respective forms of the virus, we purified and imaged virions of the specific type of ranavirus, frog virus 3 (FV3), by cryo-electron microscopy and tomography. Electron micrographs and subtomogram average showed large capsids with three distinguishable layers enclosing the electron-dense core of packaged dsDNA. The inner shell presumably corresponds to an internal membrane, the intermediate layer to a proteinous capsid shell and the outer layer to an external lipid envelope. Overall, the FV3 capsid exhibits similar morphology and structural features as related viruses from the family Iridoviridae, PBCV-1 and CIV . Subtomogram averaging of individual extracted virions was done in Bsoft  using icosahedral symmetry. The average is consistent with the single particle reconstruction and further image analysis are undertaken to identify the putative special vertex for genome ejection, specific for the naked capsid form. Next steps are directed towards determining the structural and mechanistic features of the ranavirus replication cycle and structural basis for cell entry by the enveloped virions, determining the structure of the surface glycoproteins which mediate the entrance.
This research has been supported by the Career Integration Grant (No. 618111) to DN and by the grant from Ministry of Agriculture of the Czech Republic (MZE 0002716202) to TV.