flagellin

Flagellin is the protein subunit that makes up the filament of bacterial flagella. It is produced from multiple flagellin genes (for example fliC in many species; some bacteria also carry additional flagellin genes such as fljB). Flagellin monomers are synthesized in the cytoplasm, chaperoned, and exported by the flagellar type III secretion system through the basal body and hook, where they polymerize at the distal end to form the helical filament that propels the cell.

Structurally, flagellin has a conserved N-terminal and C-terminal region that assemble to form the D0 and D1 core of the filament, essential for polymerization and mechanical stability. The central portion of the molecule is more variable and typically exposed on the filament surface, contributing to antigenic diversity. The central region length and composition can differ among species, and in some bacteria flagellin is glycosylated.

Genetically, many bacteria harbor more than one flagellin gene, enabling phase variation or the production of distinct flagellin types. For example, Salmonella encodes two flagellin genes, fliC and fljB, allowing alternating expression of different flagellins. The conserved termini enable filament assembly, while the variable central domain drives antigenic variation recognized by the host immune system.

In addition to motility, flagellin is a key pathogen-associated molecular pattern recognized by Toll-like receptor 5 in the host, triggering innate immune responses. Because of its immunostimulatory properties, flagellin and its fragments are explored as vaccine adjuvants and in nanotechnology and structural biology research.