 | This article is missing information about high-resolution structures & tomography, locus regulation, N-glycosylation and metal binding, flexibility. (April 2019) |
The archaellum (pl.: archaella; formerly archaeal flagellum) is a unique structure on the cell surface of many archaea that allows for swimming motility. The archaellum consists of a rigid helical filament that is attached to the cell membrane by a molecular motor. This molecular motor – composed of cytosolic, membrane, and pseudo-periplasmic proteins – is responsible for the assembly of the filament and, once assembled, for its rotation. The rotation of the filament propels archaeal cells in liquid medium, in a manner similar to the propeller of a boat. The bacterial analog of the archaellum is the flagellum, which is also responsible for their swimming motility and can also be compared to a rotating corkscrew. Although the movement of archaella and flagella is sometimes described as "whip-like", this is incorrect, as only cilia from Eukaryotes move in this manner. Indeed, even "flagellum" (word derived from Latin meaning "whip") is a misnomer, as bacterial flagella also work as propeller-like structures.
Early studies on "archaeal flagella" identified several differences between archaella and flagella, although those differences were dismissed as a possible adaptation of archaella to the extreme ecological environments where archaea were at the time known to inhabit. When the first genomes of archaeal organisms were sequenced, it became obvious that archaea do not code for any of the proteins that are part of the flagellum, thus establishing that the motility system of archaea is fundamentally different from that of bacteria. In order to highlight the difference between these two organelles, the name archaellum was proposed in 2012 following studies that showed it to be evolutionarily and structurally different from the bacterial flagella and eukaryotic cilia.[1]
Archaella are evolutionarily and structurally related to type IV filament systems (TFF).[2] The TFF family seems to have originated in the last universal common ancestor, from where it diversified into archaella, Type IV Pili, Type II Secretion Systems, and the Tad pili.[3]
- ^ Jarrell KF, Albers SV (July 2012). "The archaellum: an old motility structure with a new name". Trends in Microbiology. 20 (7): 307–12. doi:10.1016/j.tim.2012.04.007. PMID 22613456.
- ^ Berry JL, Pelicic V (January 2015). "Exceptionally widespread nanomachines composed of type IV pilins: the prokaryotic Swiss Army knives". FEMS Microbiology Reviews. 39 (1): 134–54. doi:10.1093/femsre/fuu001. PMC 4471445. PMID 25793961.
- ^ Denise R, Abby SS, Rocha EP (July 2019). Beeby M (ed.). "Diversification of the type IV filament superfamily into machines for adhesion, protein secretion, DNA uptake, and motility". PLOS Biology. 17 (7): e3000390. doi:10.1371/journal.pbio.3000390. PMC 6668835. PMID 31323028.