# Bacterial Flagellar Filament: A Supramolecular Multifunctional Nanostructure
## 4 Filament  
The filament is a tubular structure made of more than 20,000 copies of the flagellin protein arranged in a helical fashion [82]. Flagellin is rotated and translated 11 times along the screw axis, making two turns, before it reaches its original position. Such an arrangement results in 11 stacks of flagellin along the axis called protofilaments, which are the basic functional units of the filament (Figure 2a). Depending on the interaction between flagellin molecules in a protofilament, it switches between two different states, left-handed (L) and right-handed (R) relative to the longitudinal axis, independently of its neighboring protofilament (Figure 2b) [83]. During swimming, native filaments adopt a superhelical or corkscrew shape and their rotation creates thrust. Depending on the number of L- and R-protofilaments, the pitch of the superhelix changes, modulating the swimming properties [84]. If all protofilaments are in the L- or R-state, the filament is straight and cannot create a thrust and, thus, cells are immotile. Apart from handedness, L- and R-filament also differ in length, with the R-filament being 1.5% shorter. During the normal smooth swimming of S. Typhimurium and _E. coli_, two R- and nine L-protofilaments form a filament that is a left-handed superhelix with a pitch of around 2-3 \(\upmu\)m. The increased number of R-protofilaments eventually results in right-handed curly flagella with a shorter pitch typical for tumbling movements [84]. Straight filaments in which all protofilaments are locked in either L- or R-state are much more amenable for structural studies than the native wavy forms in which, due to the presence of a mixture of L- and R-protofilaments, the strict helical symmetry is disturbed [85]. Several mutations of the _filic_ gene in _Salmonella_ result in formation of straight filaments and were used in X-ray and EM studies [83]. Additionally, these mutations served as a basis to design the equivalent L- and R-mutants in other bacteria like _P. aeruginosa_ and C. _jejuni_ and to study their structures [64,86]. A cross-section of the _Salmonella_ filament reveals a densely packed helical core 115 A in diameter around a central channel that is \(\sim\)25 A wide, and an outer region exposed to the surface where domains of the individual molecules are clearly separated [87,88]. The diameter of the whole filament is \(\sim\)230 A. The core is organized into two tubes, inner and outer, connected through eleven short spokes, while the outer region contains two domains that are clearly defined. Using cryo-EM, [PERSON] et al. [62] obtained an electron density map of \(\sim\)4 A resolution that allowed them, in combination with the already known structure of FiC, to build the first complete atomic model of the R-filament from _Salmonella_. This model greatly contributed to understanding of the filament architecture, especially of its core, where the boundaries between domains of individual molecules were not obvious. Figure 2: Cryo-EM structure of the flagellar filament from _P. aeruginosa_ PAO1. (**a**) Cross-section of the left-handed filament consisting of 11 protofilaments. The inner tube (red) made of D0 domains and outer tube (cyan) together form the densely packed inner core of the filament. Outer domains are shown in green; (**b**) Left- and high-handed filaments with all protofilaments tilted to the left and right from the central axis; (**c**) Helical symmetry lines in the filament. EMD: 8855, 8856. In regard to geometry, the _Salmonella_ R-filament is a helical structure in which the FliC molecule is rotated 65.8\({}^{\circ}\) to the right followed by the translation of 4.7 A. Protofilaments are tilted to the right by 3.5\({}^{\circ}\) relative to the longitudinal axis. The inner and outer tube are made of the D0 and D1 domains, respectively. In the inner tube, interactions between subunits are found along 11-start and 5-start helices and they are mostly hydrophobic, stabilizing the filament.