Cloning and characterization of motX, a Vibrio alginolyticus sodium-driven flagellar motor gene

Four motor proteins, MotX, MotY, PomA, and PomB, have been identified as constituents of the Na(+)-driven flagellum of Vibrio species. In this study, the complete motX gene was cloned from Vibrio alginolyticus and shown to complement three mot mutations, motX94, motX115, and motX119, as well as a V. parahaemolyticus motX mutant. The motX94 mutant contains a frameshift at Val86 of MotX, while the motX115 and motX119 mutations comprise substitutions of Ala146 to Val and Gln 194 to amber, respectively. When MotX was overexpressed in Vibrio cells, the amount of MotY detected in the membrane fraction increased, and vice versa, suggesting that MotX and MotY mutually stabilize each other by interacting at the membrane level. When a plasmid containing the motX gene was introduced into motY mutants NMB117 (motY117) and VIO542 (motY542), the mutations were suppressed. In contrast, motY could not cause the recovery of any swarm-defective motX mutants studied. Considering the above evidence, we propose that MotX is more directly involved than MotY in the mechanical functioning of the Na(+)-type flagellar motor, and that MotY may stabilize MotX to support its interaction with other Mot proteins.