Required for the morphogenesis and for the elongation of the flagellar filament by facilitating polymerization of the flagellin monomers at the tip of growing filament. Forms a capping structure, which prevents flagellin subunits (transported through the central channel of the flagellum) from leaking out without polymerization at the distal end. Essential for motility. Responsible for adhesion to mucin, which is the initial event in colonization by this organism of the airways of cystic fibrosis patients.