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

Structural similarity between the flagellar type III ATPase FliI and F 1 -ATPase subunits

“…Oligomerization of FliI is also strongly controlled by a small number of N-terminal residues, and variants of FliI missing those N-terminal residues cannot form the hexamer ring (13, 15), although they still retain a low-level ATPase activity (13). The N-terminal 18 residues, which are missing in the present structure, include the binding region for FliH, which inhibits the FliI oligomerization but apparently promotes the formation of the active export complex by helping the docking of FliI to the membrane components of the export apparatus (7,(17)(18)(19). These observations suggest that the conformation of the short N-terminal stretch is important for the stability of the hexamer.…”

“…However, FliH facilitates docking of FliI on to the platform of the export apparatus and recruitment of export substrates to the export apparatus, probably through binding of the FliH 2 FliIsubstrate-chaperone complex to the basal body C ring (7,18,19). Thus, FliI displays dynamic interactions with other components in driving flagellar protein export.…”

Subunit Organization and Reversal-associated Movements in the Flagellar Switch of Escherichia coli

“…The hydrophobic patch is centered on the symmetry axis of the FliN dimer, and so the FliN tetramer has two patches, displayed on opposite faces of the donut. The hydrophobic patch was shown to bind to the flagellar export protein FliH, which presumably accounts for its importance in assembly (11,13). Several counterclockwise-biased mutations in and around the patch pointed to a role in switching.…”

“…The E. coli FliN protein was shown to form stable tetramers in solution (30), and targeted disulfide cross-linking experiments indicated a donut-like organization for the protein in the cell (36). A conserved hydrophobic patch on FliN, centered on the dimer 2-fold axis, forms a site of interaction with the flagellar export protein FliH (11,13). The hydrophobic patch and adjoining regions are also important for the switch to clockwise rotation and accordingly were suggested to interact with the signaling protein CheY-P (13).…”

Crystal structure of the C‐terminal domain of the Salmonella type III secretion system export apparatus protein InvA

“…It has been shown to interact with several soluble flagellar apparatus proteins including the ATPase FliI (InvC/YscN), its regulator FliH (OrgB/YscL), and FliJ (InvI/YscO); and is associated with a dominant-negative multicopy effect, inhibiting flagellar action by sequestering soluble components of the assembly apparatus in non-productive complexes. 1,[5][6][7][8] It is proposed that the C-terminal domain of FlhA provides one of the main sites of interaction at the base of the F-T3SS for the soluble assembly complex before subsequent substrate insertion into the inner membrane portal of the F-T3SS. 7 Structural data for the conserved core inner membrane components of the T3SS export apparatus is limited to the C-terminal cytoplasmic domain of SpaS/ YscU/FlhB from several NF-T3SS homologues [10][11][12][13] and the ATPase from both NF-T3SS 14 and F-T3SS 15 homologues.…”