Mutational analysis of charged residues in the cytoplasmic loops of MotA and MotP in the Bacillus subtilis flagellar motor.

Bacterial flagellar motors are energized by a proton (H(+)) or sodium ion (Na(+)) motive force. The motor torque is generated by the interactions between a rotor and about a dozen stators at the interface. MotAB-type stators use H(+), whereas MotPS- and PomAB-type stators use Na(+) as the coupling ion. In Escherichia coli, the cytoplasmic loop of MotA contains charged residues that interact with conserved charged residues in a rotor protein FliG. Bacillus subtilis has two distinct stator elements MotAB and MotPS. Both stator elements contribute to torque generation by the flagellar motor. To clarify the roles of conserved charged residues in the cytoplasmic loops of MotA and MotP in flagellar rotation, we performed site-directed mutagenesis and analysed motility as well as the relative expression levels of mutant Mot proteins. The motility of the majority of these mutants was reduced compared with that of the wild-type, but was observed at a significant level compared with that of a ΔmotAB ΔmotPS mutant. From the expression levels and the decrease in the motility, we propose that MotA-E98, MotA-E102, MotP-R94, MotP-K95 and MotP-E107 may be responsible for flagellar rotation.