Bacterial flagella are assembled from thousands of protein subunits that are unfolded and exported via a specialized type III secretion system. Subunit export is fuelled by the proton motive force (PMF) facilitated by a cytoplasmic ATPase complex comprising FliH, FliI and FliJ, which are evolutionarily related to components of the F1 ATPase. The FliJ stalk component of the ATPase binds the export gate protein FlhA, allowing it to utilise ΔΨ to drive highly efficient subunit export. What is unclear is how FliJ activation of FlhA is regulated to prevent constitutive proton influx when there are no subunits available. FliJ-mediated export gate activation could be regulated by other proteins that bind FliJ. We have shown that FliJ recruits unladen export chaperones, transferring them to their cognate subunits to create a local cycle of chaperone-subunit binding. To investigate whether chaperones also regulate FliJ activation of FlhA, we sought to isolate chaperone variants that were defective in FliJ binding but retained their ability to bind subunits and other export components. Disruption of chaperone-FliJ binding attenuated motility and cognate subunit export. To test whether chaperones blocked the FlhA-FliJ interaction, we developed in vitro and in vivo competition assays. Our data showed that chaperones and FlhA compete for a common binding site on FliJ, and that unladen chaperones, which would be present in the cell when subunit levels are low, disrupt the FliJ-FlhA interaction, preventing activation of the export gate. This provides a mechanism whereby the export gate is only activated when subunits are available.

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