Archaellum

An archaellum (plural: archaella) is a new name for the unique whip-like structure on the cell surface of many archaea (formerly called the archaeal flagellum). It can be rotated and is used to swim in liquid environments.

Archaea were first classified as a separate group of prokaryotes in 1977 by Carl Woese and George E. Fox based on the differences in the sequence of ribosomal RNA (16S rRNA) genes. This domain possesses numerous fundamental traits distinct from both the bacterial and the eukaryotic domains. Many archaea possess a rotating motility structure that at first seemed to resemble the bacterial and eukaryotic flagella. The flagellum (Latin for whip) is a lash-like appendage that protrudes from the cell. In the last two decades, it was discovered that the archaeal flagella, although functionally similar to bacterial and eukaryotic flagella, structurally resemble bacterial type IV pili. Bacterial type IV pili are surface structures that can be extended and retracted and are used to adhere to or move on solid surfaces. To underline these differences, Ken Jarrell and Sonja-Verena Albers proposed to change the name of the archaeal flagellum to archaellum.

Most proteins that make up the archaellum are encoded within one genetic locus. This genetic locus contains 7-13 genes which encode proteins involved in either assembly or function of the archaellum. The genetic locus contains genes encoding archaellins (flaA and flaB) - the structural components of the filament - and core components (flaI, flaJ, flaH). The locus furthermore encodes accessory proteins (FlaG, FlaF, FlaX) and signaling components (FlaC, FlaD, FlaE). Genetic analysis in different archaea revealed that each of these components is essential for assembly of the archaellum. Whereas most of the fla-associated genes are generally found in Euryarchaeota, one or more of these genes are absent from the fla-operon in Crenarchaeota. The prepilin peptidase (called PibD in crenarchaeota and FlaK in euryarchaeota) is essential for the maturation of the archaellins and is generally encoded elsewhere on the chromosome.

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