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For self-assembly of the bacterial flagellum, most of the flagellar component proteins synthesized in the cytoplasm are exported by the flagellar type Ill export apparatus to the growing, distal end. Flagellar protein export is highly organized and well controlled in every step of the flageilar assembly process. Flagellar-specific chaperones not only facilitate the export of their cognate proteins, as well as prevent their premature aggregation in the cytoplasm, but also play a role in fine- tuning flagellar gene expression to be coupled with the flagellar assembly process. FliT is a flagellar-specific chaperone responsible for the export of the filament-cappWig protein FliD and for negative control of flagellar gene expression by binding to the FIhDC complex. Here we report the crystal structure of Salmonella FliT at 3.2-A resolution. The structural and biochemical analyses clearly reveal that the C-terminal segment of FliT regulates its interactions with the FIhDC complex, Flil ATPase, and Fiji (subunits of the export apparatus), and that its conformational change is responsible for the switch in its binding partners during flagellar protein export. [ABSTRACT FROM AUTHOR]