Flagellin is a globular

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

that arranges itself in a hollow cylinder to form the filament in a

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...

flagellum A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates. A microorganism may have f ...

. It has a mass of about 30,000 to 60,000 daltons. Flagellin is the principal component of bacterial flagella, and is present in large amounts on nearly all flagellated bacteria.

Structure

The structure of flagellin is responsible for the

helical Helical may refer to: * Helix, the mathematical concept for the shape * Helical engine, a proposed spacecraft propulsion drive * Helical spring, a coilspring * Helical plc, a British property company, once a maker of steel bar stock * Helicoil A t ...

shape of the flagellar filament, which is important for its proper function. It is transported through the center of the filament to the tip where it

polymerases A polymerase is an enzyme ( EC 2.7.7.6/7/19/48/49) that synthesizes long chains of polymers or nucleic acids. DNA polymerase and RNA polymerase are used to assemble DNA and RNA molecules, respectively, by copying a DNA template strand using bas ...

spontaneously into a part of the filament. It is unfolded by the FliS () flagellar secretion chaperone during transport. The filament is made up of eleven smaller "protofilaments", nine of which contains flagellin in the L-type shape and the other two in the R-type shape. The helical N- and C-termini of flagellin form the inner core of the flagellin protein, and is responsible for flagellin's ability to

polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...

ize into a filament. The middle residues make up the outer surface of the flagellar filament. While the termini of the protein are quite similar among all bacterial flagellins, the middle portion is wildly variable and can be absent in some species. The flagellin domains are numbered from the helical core (D0/D1) to the outside (D2, ...); when viewed from the amino-acid sequence, D0/D1 appears on the two termini. Flagellin-like structural proteins are found in other portions of the flagellum, such as the hook (flgE; ), the rod at the base, and the cap at the top. The middle part of ''E. coli'' (and related) flagellin, D3, displays a beta-folium fold and appears to maintain flagellar stability.

Immune response

In mammals

Mammals often have acquired immune responses (

T-cell A T cell is a type of lymphocyte. T cells are one of the important white blood cells of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell r ...

and

antibody An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the ...

responses) to flagellated bacterium, which occur frequently to flagellar antigens. Flagellin has also been shown to directly interact with

TLR5 Toll-like receptor 5, also known as TLR5, is a protein which in humans is encoded by the ''TLR5'' gene. It is a member of the toll-like receptor (TLR) family. TLR5 is known to recognize bacterial flagellin from invading mobile bacteria. It has bee ...

on T cells and

TLR11 Toll-like receptor 11 (TLR11) is a protein that in mice and rats is encoded by the gene ''TLR11'', whereas in humans it is represented by a pseudogene. TLR11 belongs to the toll-like receptor (TLR) family and the interleukin-1 receptor/toll-like r ...

. Some bacteria are able to switch between multiple flagellin

gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...

s 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 An immune receptor (or immunologic receptor) is a receptor, usually on a cell membrane, which binds to a substance (for example, a cytokine) and causes a response in the immune system. Types The main receptors in the immune system are pattern r ...

that recognizes flagellin,

Toll-like receptor Toll-like receptors (TLRs) are a class of proteins that play a key role in the innate immune system. They are single-pass membrane-spanning receptors usually expressed on sentinel cells such as macrophages and dendritic cells, that recognize ...

5 (

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 SENSING 2). Upon flg22 detection, FLS2 quickly binds to BAK1 (BRI1-associated kinase 1) to initiate signalling by reciprocal transphosphorylation of their kinase domains. Both flagellin and

UV-C Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...

act similarly to increase

homologous recombination Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in cellular organisms but may ...

, as demonstrated by Molinier et al 2006. Beyond this somatic effect, they found this to extend to subsequent generations of the plant. Mitogen-activated-protein-kinases (MAPK) acts as downstream signalling compounds, leading ultimately to PAMP-triggered immunity in which more than 900 genes are up-/down-regulated upon flg22 treatment. Pre-stimulation with a synthetic flg22-peptide led to enhanced resistance against bacterial invaders.

References

External links

* {{MeshName, Flagellin * Bacterial flagellin and plant disease resistance, published by Zipfel. et al. (2004
Abstract Article
Bacterial proteins