Archaellum
Indeed, even "flagellum" (word derived from Latin meaning "whip") is a misnomer, as bacterial flagella also work as propeller-like structures.
[9] One of the clearer evidences at the time was the observation that archaellins are synthesised in the cytoplasm as pre-proteins, with a signal peptide that needs to be cleaved prior to their insertion, presumably at the base of the growing archaellar filament.
The enzyme responsible for the cleavage of the signal peptide was identified, and so were the other genes thought to be part of the archaella operon (now arl cluster).
During this time it was possible to define a minimal set of components necessary for a mature and functional archaellum: the archaellin (either a single type or several), the prepilin peptidase which cleaves the signal peptide off from the pre-archaellin, and the proteins ArlC/D/E/F/G/H/I/J.
In the phyla Crenarchaetoa the genes for the proteins ArlC/D/E are not found; instead archaellated members of this phylum code for ArlX, thought to have a similar function to ArlC/D/E.
Research of archaella still continues, both related to the basic biology of this organelle, to its ecological roles, and even potential biotechnological applications.
[20][21][22][23][24] The prepilin peptidase (called PibD in crenarchaeota and ArlK (formerly FlaK) in euryarchaeota) is essential for the maturation of the archaellins and is generally encoded elsewhere on the chromosome.
[33] Despite arlH deletion resulting in loss of motiliy, rendering this protein essential for archaellation, its role in the archaellum motor remains unknown.
[34] In Euryarchaeotes, cryo-electron tomograms suggest that ArlCDE form a structure underneath the motor, possibly in the order (from top to bottom) ArlJ-ArlI-ArlH-ArlCDE.
[35] ArlF and ArlG possibly form the stator of this complex, providing a static surface against which the rotor can move, and also anchoring the motor to the cell envelope, thus preventing the membrane from rupturing due to archaellar rotation.
[36][37] The structure of ArlCDE is unknown, but this complex (or variations thereof) have been shown to link the chemotaxis machinery and the archaellum in Haloferax volcanii.
[38] Despite the limited number of details presently available regarding the structure and assembly of archaellum, it has become increasingly evident from multiple studies that archaella play important roles in a variety of cellular processes in archaea.
In spite of the structural dissimilarities with the bacterial flagellum, the main function thus far attributed for archaellum is swimming in liquid[24][39][40] and semi-solid surfaces.
