Interactions between C ring proteins and export apparatus components: a possible mechanism for facilitating type III protein export

Summary The flagellar switch proteins of Salmonella, FliG, FliM and FliN, participate in the switching of motor rotation, torque generation and flagellar assembly/export. FliN has been implicated in the flagellar export process. To address this possibility, we constructed 10‐amino‐acid scanning deletions and larger truncations over the C‐terminal domain of FliN. Except for the last deletion variant, all other variants were unable to complement a fliN null strain or to restore the export of flagellar proteins. Most of the deletions showed strong negative dominance effects on wild‐type cells. FliN was found to associate with FliH, a flagellar export component that regulates the ATPase activity of FliI. The binding of FliM to FliN does not interfere with this FliN–FliH interaction. Furthermore, a five‐protein complex consisting of FliG, His‐tagged FliM, FliN, FliH and FliI was purified by nickel‐affinity chromatography. FliJ, a putative general chaperone, is bound to FliM even in the absence of FliH. The importance of the C ring as a possible docking site for export substrates, chaperones and FliI through FliH for their efficient delivery to membrane components of the export apparatus is discussed.