Interaction between FliI ATPase and a flagellar chaperone FliT during bacterial flagellar protein export

FliT is a flagellar type III export chaperone specific for the filament-capping protein FliD. The FliT/FliD complex binds to the FliI ATPase of the flagellar export apparatus. The C-terminal a4 helix of FliT controls its interaction with FliI but it remains unknown how it does so. Here, we analysed the FliIFliT interaction by pull-down assays using GST affinity chromatography. FliT94, missing the C-terminal a4 helix, bound to the extreme N-terminal region of FliI (FliIEN) with high affinity and to the C-terminal ATPase domain (FliICAT) with low affinity. The C-terminal a4 helix of FliT suppressed the interaction with FliIEN. FliH and FliT94 bound to a common binding site on FliIEN and hence FliH induced the release of FliI from FliT94 in an ATP-independent manner. FliD increased the binding affinity of FliICAT for FliT. These results raise a possible hypothesis that the FliH/FliI complex binds to the FliT/FliD complex through FliICAT to escort it from the cytoplasm to the export gate made up of six integral membrane proteins and that, upon dissociation of FliD from FliT, FliT94 may bind to FliIEN and then FliI may transfer from FliT94 to FliH by the direct competition of FliT94 and FliH for FliIEN.