Coupling ion specificity of chimeras between H+- and Na+-driven motor proteins, MotB and PomB, in Vibrio polar flagella

We have shown that a hybrid motor consisting of proton-type Rhodobacter sphaeroides MotA and sodium-type Vibrio alginolyticus PomB, MotX and MotY, can work as a sodium-driven motor in Vibrio cells. In this study, we tried to substitute the B subunits, which contain a putative ion-binding site in the transmembrane region. Rhodobacter sphaeroides MotB did not work with either MotA or PomA in Vibrio cells. Therefore, we constructed chimeric proteins (MomB), which had N-terminal MotB and C-terminal PomB, MomB proteins, with the entire transmembrane region derived from the H+-type MotB, gave rise to an Na+ motor with MotA. The other two MomB proteins, in which the junction sites were within the transmembrane region, also formed Na+ motors with PomA, but were changed for Na+ or Li+ specificity. These results show that the channel part consisting of the transmembrane regions from the A and B subunits can interchange Na+- and H+-type subunits and this can affect the ion specificity. This is the first report to have changed the specificity of the coupling ions in a bacterial flagellar motor.