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Flagellin is a globular protein that arranges itself in a hollow cylinder to form the filament in bacterial flagellum. It has a mass of about 30,000 to 60,000 daltons. Flagellin is the principal substituent of bacterial flagellum, and is present in large amounts on nearly all flagellated bacteria.
The structure of flagellin is responsible for the helical shape of the flagellar filament, which is important for its proper function.
The N- and C-termini of flagellin form the inner core of the flagellin protein, and is responsible for flagellin's ability to polymerize into a filament. The central portion of the protein makes up the outer surface of the flagellar filament. While the termini of the protein is quite similar among all bacterial flagellins, the central portion is wildly variable.
Immune response 
In mammals 
Mammals often have acquired immune responses (T-cell and antibody responses) to flagellated bacterium, which occur frequently to flagellar antigens. Some bacteria are able to switch between multiple flagellin genes in order to evade this response.
The propensity of the immune response to flagellin may be explained by two facts:
- Flagellin is an extremely abundant protein in flagellated bacteria.
- There exists a specific innate immune receptor that recognizes flagellin, Toll-like receptor 5 (TLR5).
In plants 
In addition, a 22-amino acid sequence (flg22) of the conserved N-terminal part of flagellin is known to activate plant defence mechanisms. Flagellin perception in Arabidopsis thaliana functions via the receptor-like-kinase FLS2 (flagellin-sensitive-2)). Mitogen-activated-protein-kinases (MAPK) acts as signalling compounds, and more than 900 genes are affected upon flg22 treatment.
Pre-stimulation with a synthetic flg22-peptide led to enhanced resistance against bacterial invaders.
- Bacterial flagellin and plant disease resistance, published by Zipfel. et al. (2004) Abstract Article
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