Mimivirus - Structure

Structure

The mimivirus is the second-largest virus, preceded by the recently discovered Megavirus chilensis, and has a capsid diameter of 400 nm. Protein filaments measuring 100 nm project from the surface of the capsid, bringing the total length of the virus up to 600 nm. Variation in scientific literature renders these figures as highly approximate, with the "size" of the virion being casually listed as anywhere between 400 nm and 800 nm, depending on whether total length or capsid diameter is actually quoted. The capsid appears hexagonal under an electron microscope, therefore the capsid symmetry is icosahedral. It does not appear to possess an outer viral envelope, suggesting that the virus does not exit the host cell by exocytosis.

The same team that discovered the mimivirus later discovered a slightly larger virus, dubbed the mamavirus, and the Sputnik virophage that infects it.

Mimivirus shares several morphological characteristics with all members of the NCLDV group of viruses. As an internal lipid layer surrounding the central core is present in all other NCLDV viruses, it has been suggested by M. Suzan-Monti et al. that this may also be present in mimivirus. The condensed central core of the virion appears as a dark region under the electron microscope. The large genome of the virus resides within this area.

Several mRNA transcripts can be recovered from purified virions. Like other NCLDVs, transcripts for DNA polymerase, a capsid protein and a TFII-like transcription factor were found. However, three distinct aminoacyl tRNA synthetase enzyme transcripts and four unknown mRNA molecules specific to mimivirus were also found. These pre-packaged transcripts can be translated without viral gene expression and are likely to be necessary to Mimivirus for replication. Other DNA viruses, such as the Human cytomegalovirus and Herpes simplex virus type-1, also feature pre-packaged mRNA transcripts.