The FlgN chaperone activates the Na+-driven engine of the flagellar protein export apparatus

The bacterial flagellar protein export machinery promotes H + -coupled translocation of flagellar proteins to the cell exterior. When the cytoplasmic ATPase complex does not function, the transmembrane export gate complex opens its Na + channel and continues protein transport. However, it remains unknown how. Here we report that the FlgN chaperone acts as a switch to activate a backup export mechanism for the ATPase complex by activating the Na + -driven engine. Impaired interaction of FlhA with the FliJ subunit of the ATPase complex increased Na + -dependence of flagellar protein export. Deletion of FlgN inhibited protein export in the absence of the ATPase complex but not in its presence. Gain-of-function mutations in FlhA restored not only the FlgN defect but also the FliJ defect. We propose that the interaction of FlgN with FlhA opens the Na + channel in the export engine, thereby maintaining the protein export activity in the absence of the active ATPase complex.