Molecular basis of the interaction between the flagellar export proteins FliI and FliH from Helicobacter pylori.

Bacterial flagellar protein export requires an ATPase, FliI, and presumptive inhibitor, FliH. We have explored the molecular basis for FliI/FliH interaction in the human gastric pathogen Helicobacter pylori. By using bioinformatic and biochemical analyses, we showed that residues 1-18 of FliI very likely form an amphipathic alpha-helix upon interaction with FliH, and that residues 21-91 of FliI resemble the N-terminal oligomerization domain of the F1-ATPase catalytic subunits. A truncated FliI-(2-91) protein was shown to be folded, although the N-terminal 18 residues were likely unstructured. Deletion and scanning mutagenesis showed that residues 1-18 of FliI were essential for the FliI/FliH interaction. Scanning mutation of amino acids in the N-terminal 10 residues of FliI indicated that a cluster of hydrophobic residues in this segment was critical for the interaction with FliH. The interaction between FliI and FliH has similarities to the interaction between the N-terminal alpha-helix of the F1-ATPase alpha-subunit and the globular domain of the F1-ATPase delta-subunit, respectively. This similarity suggests that FliH may function as a molecular stator.