Functional Transfer of an Essential Aspartate for the Ion-binding Site in the Stator Proteins of the Bacterial Flagellar Motor

Rotation of the bacterial flagellar motor exploits the electrochemical potential of the coupling ion (H+ or Na+) as its energy source. In the marine bacterium Vibrio alginolyticus, the stator complex is composed of PomA and PomB, and conducts Na+ across the cytoplasmic membrane to generate rotation. The transmembrane (TM) region of PomB, which forms the Na+-conduction pathway together with TM3 and TM4 of PomA, has a highly conserved aspartate residue (Asp24) that is essential for flagellar rotation. This residue contributes to the Na+-binding site. However, it is not clear whether residues other than Asp24 are involved in binding the coupling ion. We examined the possibility that loss of the negative charge of Asp24 can be suppressed by introduction of negatively charged residues in TM3 or TM4 of PomA. The motility defect associated with the D24N substitution in PomB could be rescued only by a N194D substitution in PomA. This result suggests that there must be a negatively charged ion-binding pocket in the stator complex but that the presence of a negatively charged residue at position 24 of PomB is not essential. A tandemly fused PomA dimer containing the N194D mutation either in its N-terminal or C-terminal half with PomB-D24N was functional, suggesting that PomB-D24N can form an ion-binding pocket with either subunit of PomA dimer. The findings obtained in this study provide important clues to the mechanism of ion binding in the stator complex.