Flagelline

Flagellins are a group of proteins that occur in flagella (= flagella) of bacteria and archaea. In conjunction with the motor proteins at the base of the flagella, they are used for locomotion in liquids.

Flagellins are glycosylated globular proteins, the main component of flagella, which are composed of 20,000 to 30,000 helically arranged flagellin units. The molecular size of flagellin ranges from 20 to 60 kDa. Flagellins are glycosylated globular proteins that spontaneously assemble into hollow cylinders with an inner diameter of about 2 nm. The outer diameter is 20 nm.

One of the most characteristic features of flagellin is already evident in the primary structure of the molecule: the amino acid sequences of the two terminal regions are well conserved, while that of the central region itself is highly variable between species or subspecies of the same genus. This hypervariability of the central region is the reason for the emergence of hundreds of serotypes of Salmonella spp. Flagellin can be modified post-translationally. Salmonella flagellin is methylated at several Lys residues by a methylase encoded by the fliB gene, which is adjacent to the fliC gene. Pseudomonas syringae flagellin is glycosylated at six Ser residues.

The protein domains of flagellin are numbered from the inside to the outside (D0 to D3), with both termini located in the D0 domain. The ratio of long and short protofilaments determines the winding of the flagellum.

For example, molecular biology data in human pathogenic Borrelia species such as Borrelia burgdorferi sensu stricto, Borrelia garinii and Borrelia afzelii indicate that the flagellin genes are useful for Borrelia taxonomy as they contain species-specific restriction sites (Fukunaga M et al. 1996).

In mammals, Toll-like receptor 5 (TLR5) binds to flagellins, activating the innate immune response in epithelial and dendritic cells. In dendritic cells, the formation of T helper cells of the adaptive immune response is also stimulated. Flagellins are therefore being investigated as an adjuvant (Mizel SB et al. 2010).

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