[ODP-066/W3-7] Multiple roles of flagellar export chaperones for flagellar filament formation in Salmonella
The bacterial flagellum is composed of the basal body acting as a reversible rotary motor, the filament functioning as a helical propeller and the hook connecting the basal body and filament to act as a universal joint. Flagellar structural subunits are transported via the flagellar type III secretion system (fT3SS) from the cytoplasm to the distal end of the growing structure where their assembly occurs. The fT3SS requires export chaperones, FlgN, FliS and FliT, to efficiently transport structural subunits responsible for filament assembly at the tip of the hook. These chaperones facilitate the docking of their cognate substrates to a transmembrane export gate protein, FlhA, to facilitate their subsequent unfolding and export by the fT3SS. Dynamic interactions of the chaperones with FlhA are thought to determine the substrate export order. To clarify the role of the flagellar export chaperones in filament assembly, we constructed cells lacking FlgN, FliS and/or FliT and analyzed their efficiency of filament formation. We show that flagellar export chaperones not only assist the export of their cognate substrates but also contribute to efficient and robust filament formation. We also show that an interaction between FliS and FlhA becomes essential for efficient unfolding and transport of FliC by the fT3SS to form a long filament after FliC synthesis is complete.