flagellins

Flagellins are the principal protein subunits of bacterial flagella. They assemble into the filament that acts as a rotary propeller, enabling motility in many Gram-negative and some Gram-positive species. Most flagellins are 30–60 kDa and contain conserved N-terminal and C-terminal regions that form the core of the filament, flanking a central, hypervariable region that projects outward and determines antigenic properties.

Flagellin monomers are exported through the flagellar type III secretion system to the tip, where they assemble into long, hollow protofilaments. The N- and C-terminal regions form the inner helical core, while the central D2/D3 domains are surface exposed and highly variable. Proper assembly requires chaperones (such as FliS) and export signals in the N-terminus; filament growth proceeds by addition of subunits at the distal end.

Flagellin genes are typically named fliC or flaA/flaB and are part of a hierarchical flagellar regulon. Expression is controlled by master regulators such as FlhDC and the late-stage sigma factor FliA (sigma-28). The flagellar apparatus, including the basal body and hook, is assembled sequentially, with secretion of flagellin coordinated by the type III secretion system and its cognate anti-sigma factor FlgM.

Flagellins provide locomotion and serve as pathogen-associated molecular patterns detected by host receptors, notably Toll-like receptor 5 (TLR5). The conserved N-terminus is often recognized, while the central hypervariable region influences antigenicity. Some bacteria alter or glycosylate flagellin to modulate immune detection or to adapt to niches.

Due to central-domain variability, flagellins yield many serotypes across species. In research and medicine, flagellin and its conserved regions are explored as vaccine components and adjuvants, and as models for protein polymerization and secretion. Post-translational modifications, including glycosylation, occur in several taxa and affect function and immune interactions.