In vitro reconstitution of flagellar filaments onto hooks of filamentless mutants of Salmonella typhimurium by addition of hook-associated proteins.

An in vitro system for reconstituting flagellar filaments onto hooks of filamentless mutants of Salmonella typhimurium was used to investigate the role played in filament formation by the three hook-associated proteins (HAPs, products of the flaW, flaU, and flaV genes). These proteins--FlaW, FlaU, and FlaV--are believed to be assembled in this order at the distal end of the hook. When the recipient hooks were provided by flaU mutants, whose hook tips contained FlaW only, exogenous FlaU was essential for polymerization of both exogenous and endogenous flagellin, whereas exogenous FlaV inhibited such polymerization. When the recipients were flaV-mutant hooks, whose tips contained FlaW and FlaU but not FlaV, exogenous FlaV inhibited polymerization of exogenous flagellin. FlaV also inhibited polymerization of exogenous flagellin at the tips of filament fragments. In contrast, FlaV was essential for polymerization of endogenous flagellin onto flaV-mutant hooks, and onto short filaments that had been made (in the absence of FlaV) by polymerization of exogenous flagellin on the tips of flaV-mutant hooks. These results suggest that FlaV acts not only at the tip of the hook to initiate growth of the filament, but also at the tip of the growing filament, and that FlaV is essential for polymerization of endogenous flagellin--i.e., for the normal process of filament assembly in vivo.