ELECTROSTATIC INTERACTIONS BETWEEN ROTOR AND STATOR IN THE BACTERIAL FLAGELLAR MOTOR

Bacterial flagellar motors rotate, obtaining power from the membrane g radient of protons or, in some species, sodium ions. Torque generation in the flagellar motor must involve interactions between components o f the rotor and components of the stator. Sites of interaction between the rotor and stator have not been identified. Mutational studies of the rotor protein FliG and the stator protein MotA showed that both pr oteins contain charged residues essential for motor rotation. This sug gests that functionally important electrostatic interactions might occ ur between the rotor and stater. To test this proposal, we examined do uble mutants with charged-residue substitutions in both the rotor prot ein FliG and the stator protein MotA, Several combinations of FliG mut ations with MotA mutations exhibited strong synergism, whereas others showed strong suppression, in a pattern that indicates that the functi onally important charged residues of FliG interact with those of MotA. These results identify a functionally important site of interaction b etween the rotor and stator and suggest a hypothesis for electrostatic interactions at the rotor-stator interface.