The Function of the Na + -Driven Flagellum of Vibrio cholerae Is Determined by Osmolality and pH

Abstract

ABSTRACT Vibrio cholerae is motile by its polar flagellum, which is driven by a Na + -conducting motor. The stators of the motor, composed of four PomA and two PomB subunits, provide access for Na + to the torque-generating unit of the motor. To characterize the Na + pathway formed by the PomAB complex, we studied the influence of chloride salts (chaotropic, Na + , and K + ) and pH on the motility of V. cholerae . Motility decreased at elevated pH but increased if a chaotropic chloride salt was added, which rules out a direct Na + and H + competition in the process of binding to the conserved PomB D23 residue. Cells expressing the PomB S26A/T or D42N variants lost motility at low Na + concentrations but regained motility in the presence of 170 mM chloride. Both PomA and PomB were modified by N , N ′-dicyclohexylcarbodiimide (DCCD), indicating the presence of protonated carboxyl groups in the hydrophobic regions of the two proteins. Na + did not protect PomA and PomB from this modification. Our study shows that both osmolality and pH have an influence on the function of the flagellum from V. cholerae . We propose that D23, S26, and D42 of PomB are part of an ion-conducting pathway formed by the PomAB stator complex.