Editing Flagellum (section)

===Archaeal flagella===
The [[archaellum]] possessed by some species of [[Archaea]] is superficially similar to the bacterial flagellum; in the 1980s, they were thought to be homologous on the basis of gross morphology and behavior.<ref name="Cavalier-Smith ">{{cite journal | last1 = Cavalier-Smith |first1=T | title = The origin of eukaryotic and archaebacterial cells | journal = Annals of the New York Academy of Sciences | volume = 503 | issue = 1 | pages = 17–54 | year = 1987 | pmid = 3113314 | doi = 10.1111/j.1749-6632.1987.tb40596.x | url = http://www.annalsnyas.org/cgi/content/citation/503/1/17 | bibcode = 1987NYASA.503...17C | s2cid = 38405158 }}{{Dead link|date=December 2019 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> Both flagella and archaella consist of filaments extending outside the cell, and rotate to propel the cell. Archaeal flagella have a unique structure which lacks a central channel. Similar to bacterial [[Pilus#Type IV pili|type IV pilins]], the archaeal proteins (archaellins) are made with class 3 signal peptides and they are processed by a type IV prepilin peptidase-like enzyme. The archaellins are typically modified by the addition of N-linked [[glycan]]s which are necessary for proper assembly or function.<ref name="JarrellK"/>

Discoveries in the 1990s revealed numerous detailed differences between the archaeal and bacterial flagella. These include:

*Bacterial flagella rotation is powered by the [[proton motive force]] – a flow of [[hydron (chemistry)|H<sup>+</sup> ion]]s or occasionally by the [[sodium-motive force]] – a flow of [[sodium|Na<sup>+</sup>]] ions; archaeal flagella rotation is powered by [[adenosine triphosphate|ATP]].<ref name="Brock1">{{cite book |last1=Madigan |first1=Michael T. |title=Brock biology of microorganisms |date=2019 |location=NY, NY |isbn=9781292235103 |pages=70–71 |edition=Fifteenth, Global}}</ref> 
*While bacterial cells often have many flagellar filaments, each of which rotates independently, the archaeal flagellum is composed of a bundle of many filaments that rotates as a single assembly.
*Bacterial flagella grow by the addition of flagellin subunits at the tip; archaeal flagella grow by the addition of subunits to the base.
*Bacterial flagella are thicker than archaella, and the bacterial filament has a large enough hollow "tube" inside that the flagellin subunits can flow up the inside of the filament and get added at the tip; the archaellum is too thin (12-15&nbsp;nm) to allow this.<ref name="Archaeal flagella">{{cite journal |last1=Ghosh |first1=Abhrajyoti |last2=Albers |first2=Sonja-Verena |title=Assembly and function of the archaeal flagellum |journal=Biochemical Society Transactions |date=1 February 2011 |volume=39 |issue=1 |pages=64–69 |doi=10.1042/BST0390064 |pmid = 21265748}}</ref>
*Many components of bacterial flagella share sequence similarity to components of the [[type three secretion system|type III secretion systems]], but the components of bacterial flagella and archaella share no sequence similarity. Instead, some components of archaella share sequence and morphological similarity with components of [[type IV pili]], which are assembled through the action of [[secretion#Type II secretion system (T2SS)|type II secretion systems]] (the nomenclature of pili and protein secretion systems is not consistent).<ref name="Archaeal flagella"/>

These differences support the theory that the bacterial flagella and archaella are a classic case of biological [[analogy]], or [[convergent evolution]], rather than [[homology (biology)|homology]].<ref>{{cite journal |last1=Thomas |first1=Nikhil A. |last2=Bardy |first2=Sonia L. |last3=Jarrell |first3=Ken F. |title=The archaeal flagellum: a different kind of prokaryotic motility structure |journal=FEMS Microbiology Reviews |date=April 2001 |volume=25 |issue=2 |pages=147–174 |doi=10.1111/j.1574-6976.2001.tb00575.x |pmid = 11250034 |s2cid=34411164}}</ref><ref>{{Cite journal |last1=Chimileski |first1=Scott |last2=Papke |first2=R. Thane |date=2015 |title=Getting a hold on archaeal type IV pili: an expanding repertoire of cellular appendages implicates complex regulation and diverse functions |journal=Frontiers in Microbiology |volume=6 |page=362 |doi=10.3389/fmicb.2015.00362 |issn=1664-302X |pmc=4419858 |pmid=25999922 |doi-access=free}}</ref><ref>{{Cite journal |last1=de Sousa Machado |first1=J. Nuno |last2=Vollmar |first2=Leonie |last3=Schimpf |first3=Julia |last4=Chaudhury |first4=Paushali |last5=Kumariya |first5=Rashmi |last6=van der Does |first6=Chris |last7=Hugel |first7=Thorsten |last8=Albers |first8=Sonja-Verena |date=2021 |title=Autophosphorylation of the KaiC-like protein ArlH inhibits oligomerization and interaction with ArlI, the motor ATPase of the archaellum |journal=Molecular Microbiology |language=en |volume=116 |issue=3 |pages=943–956 |doi=10.1111/mmi.14781 |issn=0950-382X|doi-access=free |pmid=34219289 }}</ref> Research into the structure of archaella made significant progress beginning in the early 2010s, with the first atomic resolution structure of an archaella protein, the discovery of additional functions of archaella, and the first reports of archaella in Nanoarchaeota and Thaumarchaeota.<ref>{{Cite journal |last1=Nuno de Sousa Machado |first1=João |last2=Albers |first2=Sonja-Verena |last3=Daum |first3=Bertram |date=2022 |title=Towards Elucidating the Rotary Mechanism of the Archaellum Machinery |journal=Frontiers in Microbiology |volume=13 |doi=10.3389/fmicb.2022.848597 |issn=1664-302X |doi-access=free|pmid=35387068 |pmc=8978795 }}</ref><ref>{{Cite journal |last1=Jarrell |first1=Ken F |last2=Albers |first2=Sonja-Verena |last3=Machado |first3=J Nuno de Sousa |date=2021 |title=A comprehensive history of motility and Archaellation in Archaea |url=https://doi.org/10.1093/femsmc/xtab002 |journal=FEMS Microbes |volume=2 |pages=xtab002 |doi=10.1093/femsmc/xtab002 |issn=2633-6685 |pmc=10117864 |pmid=37334237}}</ref>