Flagellin is the subunit protein which polymerizes to form the filaments of bacterial flagella. Heteromer of FlaA and FlaB. A flagellar filament composed exclusively of FlaA is indistinguishable in length from that of the wild-type and shows a slight reduction in motility. The flagellar filament composed exclusively of the FlaB is severely truncated in length and greatly reduced in motility. Thus, while both flagellins are not necessary for motility, both are required for a fully active flagellar filament. Belongs to the bacterial flagellin family.