# Bacterial Flagellar Filament: A Supramolecular Multifunctional Nanostructure
## 3 Flagellins
### Posttranslational Modifications of Flagellins  
The presence of posttranslational modifications in flagellins was suspected early on, but their extent and importance became apparent only recently. Investigating discrepancies between theoretical and observed molecular weights showed that _Pseudomonas_ type A flagellin was glycosylated [55]. A genomic island between _flgL_ and _fliC_ genes containing 14 open reading frames (ORFs) is responsible for glycosylation and it specifically modifies type A flagellin, while having no effect on type B [65]. These glycan chains in the PAK strain are \(O\)-linked to residues T189 and S260 through rhamnose and vary in length (up to 11 monosaccharides) and composition [66].  
The type B flagellin characteristic of the PAO1 strain also contains \(O\)-linked glycans at serine residues 191 and 195, although in this case it is a much simpler modification with a single monosaccharide [67]. The genomic island responsible for type B glycosylation is smaller than its type A counterpart and contains four ORFs. _Pseudomonas_ type A and B mutants in which glycosylation is absent or incomplete do not affect motility in general, although the lack of glycosylation significantly decreased virulence of both PAK and PAO1 strains in a burned-mouse model of infection [68]. Apart from \(O\)-linked glycans, \(N\)-linked glycosylation was also reported in _P. aeruginosa_ strain PA14, with three sites on the D0 and D1 domains proposed as the most likely candidate positions [69].  
_Campylobacter_ flagellin is glycosylated at 19 serine and threonine residues with \(O\)-linked pseudaminic acid or related derivatives [70; 71; 72]. Glycoslation in _C. jejuni_ is responsible for the 6 kDa difference between predicted and experimentally obtained molecular weight. Approximately 50 genes in a locus that is adjacent to the _flaA_ and _flaB_ genes encode the _Campylobacter_ glycosylation machinery [73]. Glycoslation of flagellin in _Campylobacter_ is necessary for filament assembly as mutants that produce unglycoslated flagellin do not have filaments and flagellin accumulates intracellularly [74].  
_H. pylori_ flagellins are also glycosylated with pseudaminic acid with most glycosylation sites in the central core region of the protein [75]. As in _Campylobacter_, glycosylation plays an important role during assembly of the filament; abolishing glycosylation leads to loss of the filament and motility, even though the levels of flagellin mRNA are unaffected [75].  
The flagellin of _Listeria monocytogenes_ has \(O\)-linked \(N\)-acetylglucosamines at up to six sites located in the central surface exposed region of the protein [66]. The flagellin of _P. syringae_ is also glycosylated and these modifications are important for its virulence and swarming [76; 77].  
Another modification that is present in flagellins is methylation. In _S._ Typhimuriumium, the presence of methylated lysines was reported as early as 1959 [78]. Lysine methylation is performed by the enzyme FliB. Abolishing methylation does not affect filament assembly or motility [79]. However, it was shown that it promotes adhesion and host cell invasion [80]. \(N\)-methylation was also detected in _Shewanella oneidensis_[81].