Mimivirus appears most closely related structurally to large algal viruses such as PBCV-1 and other irridoviruses, though it possesses additional features not present in those viruses. The icosahedral capsid consists of 20 large triangular plates joined at their edges to produce the required 12 fivefold vertices. The capsid surface is composed of trimeric major capsid proteins, each subunit of which consists of two jelly roll beta barrels, arranged in a very open hexagonal lattice, with the appearance of a honeycomb. The center-to-center distance of the capsomeres is 14 nm. Because there is some ambiguity as to what lies exactly on the fivefold vertex, there is an uncertainty in the triangulation number T, which could have any one of nine possibilities lying between 972 and 1200.
The fibres that coat particles are reported to have lengths of about 125 nm and are probably anchored to the major capsid proteins or perhaps to an integument layer of protein disposed immediately above the capsid. The most striking and unique feature of mimivirus is a prominent five armed, star-shaped apparatus that occupies one vertex of every virus. While somewhat obscured on the intact virus, it is prominently displayed on particles which lack the coating of fibers. This star shaped assembly, termed a “stargate,” opens up once the virus is inside the host cell to produce a wide opening. The DNA of the virus, which is enclosed inside a membrane sac, then emerges from the interior, fuses with a cellular membrane, and delivers its nucleic acid contents to the cell. Another remarkable is that the DNA does not enter the capsid through the stargate, but through a separate portal, and by a completely different mechanism, in the center of a distal icosahedral face, i.e. at a threefold axis.
Atomic force microscopy investigation of the giant mimivirus. Virology. 2010 404(1): 127-137
Mimivirus was investigated by atomic force microscopy in its native state following serial degradation by lysozyme and bromelain. The 750-nm diameter virus is coated with a forest of glycosylated protein fibers of lengths about 140 nm with diameters 1.4 nm. Fibers are capped with distinctive ellipsoidal protein heads of estimated Mr=25 kDa. The surface fibers are attached to the particle through a layer of protein covering the capsid, which is in turn composed of the major capsid protein (MCP). The latter is organized as an open network of hexagonal rings with central depressions separated by 14 nm. The virion exhibits an elaborate apparatus at a unique vertex, visible as a star shaped depression on native particles, but on defibered virions as five arms of 50 nm width and 250 nm length rising above the capsid by 20 nm. The apparatus is integrated into the capsid and not applied atop the icosahedral lattice. Prior to DNA release, the arms of the star disengage from the virion and it opens by folding back five adjacent triangular faces. A membrane sac containing the DNA emerges from the capsid in preparation for fusion with a membrane of the host cell. Also observed from disrupted virions were masses of distinctive fibers of diameter about 1 nm, and having a 7-nm periodicity. These are probably contained within the capsid along with the DNA bearing sac. The fibers were occasionally observed associated with toroidal protein clusters interpreted as processive enzymes modifying the fibers.