The MinD homolog FlhG regulates the synthesis of the single polar flagellum of Vibrio alginolyticus.

FlhG, a MinD homolog and an ATPase, is known to mediate the formation of the single polar flagellum of Vibrio alginolyticus together with FlhF. FlhG and FlhF work antagonistically, with FlhF promoting flagellar assembly and FlhG inhibiting it. Here, we demonstrate that purified FlhG exhibits a low basal ATPase activity. As with MinD, the basal ATPase activity of FlhG can be activated and the D171A residue substitution enhances its ATPase activity sevenfold. FlhG-D171A localizes strongly at the cell pole and severely inhibits motility and flagellation, whereas the FlhG K31A and K36Q mutants, which are defective in ATP binding, do not localize to the poles, cannot complement a flhG mutant and lead to hyperflagellation. A strong polar localization of FlhF is observed with the K36Q mutant FlhG but not with the wild-type or D171A mutant FlhG. Unexpectedly, an Ala substitution at the catalytic residue (D60A), which abolishes ATPase activity but still allows ATP binding, only slightly affects FlhG functions. These results suggest that the ATP-dependent polar localization of FlhG is crucial for its ability to downregulate the number of polar flagella. We speculate that ATP hydrolysis by FlhG is required for the fine tuning of the regulation.