Filovirus Filament Proteins.

Filoviruses are highly filamentous enveloped animal viruses that can cause severe haemorrhagic fevers. The filovirus ribonucleoprotein forms a highly organized double-layered helical nucleocapsid (NC) containing five different virally encoded proteins. The inner layer consists of NP, the RNA binding protein, complexed with the monopartite linear genome. A distinctive outer layer links individual NP subunits with bridges composed of VP24-VP35 heterodimers, which achieves condensation of the NP-RNA into tight helical coils. There are no vertical connections between the outer helical layers, explaining the flexibility of the NC and its ability to bend into tight curves without breaking the genomic RNA. These properties allow the formation of enveloped virions with varying polymorphisms, including single, linear, continuous, linked, comma-shaped and torroidal forms. Virion length is modular so that just one, or two or more genome copies may be present in each virion, producing polyploid particles. The matrix protein VP40, which drives budding and envelopment, is found in a layer adjacent to the inner cytoplasmic side of viral envelope and is arranged in a 5 nm lattice structure, but its exact symmetry is unclear. There is a constant low density gap between VP40 and the nucleocapsid, so that the latter is held rigidly centred on the long axis of the viral filament. This gap likely contains a region of flexible contacts between VP40 and the NC. The unique morphology of filoviruses may be related to high titre replication, their ease of transmission, and abilities to invade a wide range of host cells and tissues.