Fig 1.
Genome organization and capsid shape of cultured virophages.
(A) Genome representation of the virophages Sputnik, Zamilon, and mavirus. Homologous genes are colored identically. (B) Electron microscopy images depicting capsids of giant viruses and their associated virophages. (Left) CroV (dark) and mavirus (light); negative stain EM courtesy of U. Mersdorf, MPI for Medical Research, Germany. (Middle) Megavirus vitis (with a visible stargate structure) and Zamilon vitis (inset); negative stain EM courtesy of C. Abergel, Aix-Marseille Université, France. (Right) Acanthamoeba polyphaga mimivirus with two Sputnik virus particles (arrows); thin-section EM courtesy of J.Y. Bou Khalil and B. La Scola, IHU Mediterranée Infection, France. Note that all three virophages have similar capsid sizes but are shown here at different magnifications. EM, electron microscopy; CroV, Cafeteria roenbergensis virus; TIR, terminal inverted repeat.
Fig 2.
Proposed GV and virophage infection cycle in a eukaryotic host cell.
Some GV capsids (e.g., mimivirus) are covered in fibers that allow co-entry of virophages (e.g., Sputnik) by phagocytosis. Other virophages such as mavirus enter cells by receptor-mediated endocytosis. After opening of the GV capsid and fusion of the internal GV membrane with the phagosomal or cytoplasmic membrane, the GV core is released into the cytoplasm and develops into the viral factory. The virophage genome is targeted to the factory, where the GV-encoded transcriptase complex activates virophage genes during the late phase of GV infection. Virophage genome replication is catalyzed by virophage-encoded DNA polymerases and helicases, and virophage particles are assembled within or near the GV factory and are released upon cell lysis. Virophage replication can inhibit GV production. Alternatively, the mavirus genome is able to integrate into the nuclear host genome independently of a GV. The otherwise transcriptionally silent provirophage genes can be activated during infection with a compatible GV, leading to the production of virophage particles in the GV factory. GV, giant virus.