ARCHAEAL FLAGELLA

 ·     The Archaellum (Archaeal flagellum) is a unique, ‘Tail-like’ structure used for Motility.

·       The Archaeal flagellum is thinner (10 - 15 nm) in size.

·    Like bacteria, the structure of the flagellum can be divided into three distinct components: (i) The filament, ii) Hook and iii) Anchoring structure. Each of these components is constructed of specific proteins either encoded by genes residing within flagellar operons or of currently unknown genes.

·   The Archaeal flagellum filament is assembled from Archaellin sub-units, which are N-glycosylated proteins.

·     Flagellation occurs in all the main groupings of the archaea, including Halophiles, Methanogens, Sulfur-dependent thermophiles and Hyperthermophiles.

·       Archaea use the motility apparatus Archaeal flagellum in combination with Chemotaxis systems to bias their direction of movement towards environmental conditions more favourable for cell growth.

·       The Archaellum is considered to be a Type IV pilus-like structure.

·       Pili have been observed in archaea, composed of proteins most likely modified from the bacterial pilin. The resulting tube-like structures have been shown to be used for attachment to surfaces.

Difference between Bacterial and Archaeal Flagella

·     The Archaellum is the functional equivalent of the bacterial flagellum but its architecture, composition and mode of assembly is completely unrelated.

·       The Archaeal flagellum is thinner (10 - 15 nm) compared to the Bacterial flagellum (18 - 24 nm).

·       The rotation of an Archaeal flagellum is powered by ATP, as opposed to the Proton motive force used in bacteria.

·      The proteins making up the Archaeal flagellum are similar to the proteins found in bacterial pili, rather than the bacterial flagellum.

·    Unlike Bacteria, in which a single type of protein makes up the flagellar filament, several different Flagellin proteins are known from Archaea, and their amino acid sequences and genes that encode them bear little relationship to  those of bacterial Flagellin.

·      The Archaeal flagellum filament is not hollow so growth occurs when flagellin proteins are inserted into the base of the filament, rather than being added to the end.

Clockwise rotation pushes an Archaeal cells forward, while Counterclockwise rotation pulls an archaeal cell backwards. An alternation of runs and tumbles is not observed.