The FlhA linker mediates flagellar protein export switching during flagellar assembly

The flagellar protein export apparatus switches substrate specificity from hook-type to filament-type upon hook assembly completion, thereby initiating filament assembly at the hook tip. The C-terminal cytoplasmic domain of FlhA (FlhA C ) serves as a docking platform for flagellar chaperones in complex with their cognate filament-type substrates. Interactions of the flexible linker of FlhA (FlhA L ) with its nearest FlhA C subunit in the FlhA C ring is required for the substrate specificity switching. To address how FlhA L brings the order to flagellar assembly, we analyzed the flhA(E351A/W354A/D356A) Δ flgM mutant and found that this triple mutation in FlhA L increased the secretion level of hook protein by 5-fold, thereby increasing hook length. The crystal structure of FlhA C (E351A/D356A) showed that FlhA L bound to the chaperone-binding site of its neighboring subunit. We propose that the interaction of FlhA L with the chaperon-binding site of FlhA C suppresses filament-type protein export and facilitates hook-type protein export during hook assembly. Inoue and Kinoshita et al. report an important role for the flexible linker of the major flagellar export apparatus protein FlhA (FlhA L ) in regulating the switch from secretion of hook-type to filament-type components. The data presented here improve our understanding of the complex process of bacterial flagellar assembly.