Roles of the extreme N-terminal region of FliH for efficient localization of the FliH-FliI complex to the bacterial flagellar type III export apparatus.

Most bacterial flagellar proteins are exported by the flagellar type III protein export apparatus for their self-assembly. FliI ATPase forms a complex with its regulator FliH and facilitates initial entry of export substrates to the export gate composed of six integral membrane proteins. The FliH-FliI complex also binds to the C ring of the basal body through a FliH-FliN interaction for efficient export. However, it remains unclear how these reactions proceed within the cell. Here, we analysed subcellular localization of FliI-YFP by fluorescence microscopy. FliI-YFP was localized to the flagellar base, and its localization required both FliH and the C ring. The ATPase activity of FliI was not required for its localization. FliI-YFP formed a complex with FliHDelta1 (missing residues 2-10) but the complex did not show any localization. FliHDelta1 did not interact with FliN, and alanine-scanning mutagenesis revealed that only Trp-7 and Trp-10 of FliH are essential for the interaction with FliN. Overproduction of the FliH-FliI complex improved the export activity of the fliN mutant whereas neither of the FliH(W7A)-FliI nor FliH(W10A)-FliI complexes did, suggesting that Trp-7 and Trp-10 of FliH are also required for efficient localization of the FliH-FliI complex to the export gate.