Nucleocytoplasmic large DNA viruses (NCLDVs) blur the line between viruses and cells. Melbournevirus (MelV, family Marseilleviridae) belongs to a new family of NCLDVs. Here we present an electron cryo-microscopy structure of the MelV particle, with the large triangulation number T = 309 constructed by 3080 pseudo-hexagonal capsomers. The most distinct feature of the particle is a large and dense body (LDB) consistently found inside all particles. Electron cryo-tomography of 147 particles shows that the LDB is preferentially located in proximity to the probable lipid bilayer. The LDB is 30 nm in size and its density matches that of a genome/protein complex. The observed LDB reinforces the structural complexity of MelV, setting it apart from other NCLDVs.

Laboratory of Molecular Biophysics Department of Cell and Molecular Biology Uppsala University Husargatan 3 SE 75124 Uppsala Sweden

Laboratory of Molecular Biophysics Department of Cell and Molecular Biology Uppsala University Husargatan 3 SE 75124 Uppsala Sweden; Institute of Physics AS CR v v i Na Slovance 2 18221 Prague 8 Czech Republic

National Institute for Physiological Sciences Okazaki Aichi 444 8585 Japan

Structural and Genomic Information Laboratory UMR 7256 Centre National de la Recherche Scientifique and Aix Marseille University Marseille 13288 France

Structural and Genomic Information Laboratory UMR 7256 Centre National de la Recherche Scientifique and Aix Marseille University Marseille 13288 France; Assistance Publique des Hôpitaux de Marseille La Timone 13005 Marseille France

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