Editing Flagellum (section)

====Structure and composition====
The bacterial flagellum is made up of [[protein]] subunits of [[flagellin]].<ref name="Alberts1"/> Its shape is a 20-[[nanometer]]-thick hollow tube. It is [[helix|helical]] and has a sharp bend just outside the outer membrane; this "hook" allows the axis of the helix to point directly away from the cell. A shaft runs between the hook and the [[basal body]], passing through protein rings in the cell's membrane that act as bearings. [[Gram-positive]] organisms have two of these basal body rings, one in the [[peptidoglycan]] layer and one in the [[plasma membrane]]. [[Gram-negative]] organisms have four such rings: the [[L ring]] associates with the [[lipopolysaccharides]], the [[P ring]] associates with [[peptidoglycan]] layer, the M ring is embedded in the [[plasma membrane]], and the S ring is directly attached to the [[cytoplasm]]. The filament ends with a capping protein.<ref name=MacNab1>{{cite journal |last1=Macnab |first1=Robert M. |title=How Bacteria Assemble Flagella |journal=Annual Review of Microbiology |date=October 2003 |volume=57 |issue=1 |pages=77–100 |doi=10.1146/annurev.micro.57.030502.090832 |pmid=12730325}}</ref><ref name=dioszeghy1>{{cite journal |last1=Diószeghy |first1=Zoltán |last2=Závodszky |first2=Péter |last3=Namba |first3=Keiichi |last4=Vonderviszt |first4=Ferenc |title=Stabilization of flagellar filaments by HAP2 capping |journal=FEBS Letters |date=18 June 2004 |volume=568 |issue=1–3 |pages=105–109 |doi=10.1016/j.febslet.2004.05.029 |pmid=15196929 |s2cid=33886010 |bibcode=2004FEBSL.568..105D}}</ref>

The flagellar filament is the long, helical screw that propels the bacterium when rotated by the motor, through the hook. In most bacteria that have been studied, including the gram-negative ''[[Escherichia coli]], [[Salmonella typhimurium]], [[Caulobacter crescentus]]'', and ''[[Vibrio alginolyticus]]'', the filament is made up of 11 protofilaments approximately parallel to the filament axis. Each protofilament is a series of tandem protein chains. However, ''[[Campylobacter jejuni]]'' has seven protofilaments.<ref name="SevenProtofilamentsGalkin">{{cite journal |last1=Galkin |first1=Vitold E. |last2=Yu |first2=Xiong |last3=Bielnicki |first3=Jakub |last4=Heuser |first4=John |last5=Ewing |first5=Cheryl P. |last6=Guerry |first6=Patricia |last7=Egelman |first7=Edward H. |title=Divergence of Quaternary Structures Among Bacterial Flagellar Filaments |journal=Science |date=18 April 2008 |volume=320 |issue=5874 |pages=382–385 |doi=10.1126/science.1155307 |pmid=18420936 |bibcode=2008Sci...320..382G |s2cid=7702002}}</ref>

The basal body has several traits in common with some types of [[secretion#In gram-negative bacteria|secretory pores]], such as the hollow, rod-like "plug" in their centers extending out through the plasma membrane. The similarities between bacterial flagella and bacterial secretory system structures and proteins provide scientific evidence supporting the theory that bacterial flagella evolved from the [[type three secretion system|type-three secretion system]] (TTSS).

The atomic structure of both bacterial flagella as well as the TTSS [[injectisome]] have been elucidated in great detail, especially with the development of [[Cryogenic electron microscopy|cryo-electron microscopy]]. The best understood parts are the parts between the inner and outer [[Cell membrane|membrane]], that is, the scaffolding rings of the inner membrane (IM), the scaffolding pairs of the outer membrane (OM), and the rod/needle (injectisome) or rod/hook (flagellum) sections.<ref>{{Cite journal |last1=Worrall |first1=Liam J. |last2=Majewski |first2=Dorothy D. |last3=Strynadka |first3=Natalie C.J. |date=2023-09-15 |title=Structural Insights into Type III Secretion Systems of the Bacterial Flagellum and Injectisome |journal=Annual Review of Microbiology |language=en |volume=77 |issue=1 |pages=669–698 |doi=10.1146/annurev-micro-032521-025503 |pmid=37713458 |s2cid=261963968 |issn=0066-4227|doi-access=free }}</ref>