Na⁺ conductivity of the Na⁺-driven flagellar motor complex composed of unplugged wild-type or mutant PomB with PomA

PomA and PomB form the stator complex, which functions as a Na⁺ channel, in the Na⁺-driven flagellar motor of Vibrio alginolyticus. The plug region of PomB is thought to regulate the Na⁺ flow and to suppress massive ion influx through the stator channel. In this study, in order to measure the Na⁺ conductivity of the unplugged stator, we over-produced a plug-deleted stator of the Na⁺-driven flagellar motor in Escherichia coli. The over-production of the plug-deleted stator in E. coli cells caused more severe growth inhibition than in Vibrio cells and that growth inhibition depended on the Na⁺ concentration in the growth medium. Measurement of intracellular Na⁺ concentration by flame photometry and fluorescent analysis with a Na⁺ indicator, Sodium Green, revealed that over-production of the plug-deleted stator increased the Na⁺ concentration in cell. Some mutations in the channel region of PomB or in the cytoplasmic region of PomA suppressed both the growth inhibition and the increase in intracellular Na⁺ concentration. These results suggest that the level of growth inhibition correlates with the intracellular Na ⁺ concentration, probably due to the Na⁺ conductivity through the stator due to the mutations.