# Bacterial Flagellar Filament: A Supramolecular Multifunctional Nanostructure
## 3 Flagellins  
Flagellins are elongated proteins that constitute the flagellar filament and, apart from the FliD protein at the tip, are the only structural component of the filament. Their molecular weight ranges from 26 kDa in _Bacillus cereus_ to 115 kDa in _Desulfotalea psychrophila_[37; 38]. In _E. coli_ and \(S\). Typhimurium flagellins they are typically 51 kDa, although this varies among strains; for example the symbiotic strain _E. coli_ Nissle 1917 has a flagellin protein of 61 kDa, while in some _Salmonella_ mutants functional flagellins of 41 and 42 kDa have been reported [39; 40]. Terminal regions of flagellins are highly homologous among species and they build the core of the filament [38]. Conversely, a hypervariable region in the middle of the polypeptide sequence makes outer domains significantly different, and in some bacteria such as \(B\). _subtilis_ it is completely absent [38; 41].  
The flagellin genes are known under various names depending on the species, such as _fliC_ in _E. coli_, _P. aeruginosa_, and _Salmonella_, _hag_ in _B. subtilis_, and _flaA_ in _C. jejuni_ and _Helicobacter pylori_. The number of flagellin genes also varies. Approximately 45% of bacteria have more than one flagellin-encoding gene in their genomes, the most extreme case being _Magnetococcus_ with 15 [42]. While the _E. coli_ genome harbors only one flagellin gene, _fliC_, _Salmonella_ has two, _fliC_ and _fljB_[43]. However, only one of them is expressed at any point; the switch between them is called phase variation [44]. These two proteins diverge in the middle region providing distinct antigenicities. The studies on mutants locked in one of the two phases showed the different swimming properties and advantages of the FliC-expressing strains in colonization and infection of the gastrointestinal tract [45; 46].  
In other species with two flagellins such as _H. pylori_, _Campylobacter_ spp. or _Shewanella putrefaciens_, both are present in the filament, the minor flagellin predominantly in the proximal region, while the major flagellin forms the remainder of the filament [47; 48; 49; 50]. Although knocking out the minor flagellin does not usually affect the length of the filament, mutants exhibit changes in motility, especially in more viscous media, suggesting that having two flagellins in the filament provides optimal swimming characteristics in different environments [50; 51]. The minor flagellin of _C. jejuni_ also plays a role in defense against bacteriophage infection [52].  
At the species level, _P. aeruginosa_ has two types of flagellin genes, A and B, which differ in molecular weight and recognition by antibodies [53; 54]. They are not, however, simultaneously present in the genome, but rather they are strain-specific with strains such as PAK containing type A FliC, while strains like PAO1 have type B FliC. While their N- and C-termini are highly conserved, the central region of the type A FliC is significantly shorter compared to the type B [55].  
The number of flagellins in one cell can be as high as six (_Caulobacter crescentus_ and _Vibrio vulnificus_) or even seven (_Rhizobium leguminosarum_) [56; 57; 42]. In _V. vulnificus_, mass spectrometry confirmed the presence of five distinct flagellins in the filament, with three having higher impact on motility, adhesion and cytotoxicity [56]. Some of these are redundant, such as for _C. crescentus_ in which five of six flagellins can make filaments alone [42].