MotP Subunit is Critical for Ion Selectivity and Evolution of a K(+)-Coupled Flagellar Motor

The bacterial flagellar motor is a sophisticated nanomachine embedded in the cell envelope. The flagellar motor is driven by an electrochemical gradient of cations such as H(+), Na(+), and K(+) through ion channels in stator complexes embedded in the cell membrane. The flagellum is believed to rotate as a result of electrostatic interaction forces between the stator and the rotor. In bacteria of the genus Bacillus and related species, the single transmembrane segment of MotB-type subunit protein (MotB and MotS) is critical for the selection of the H(+) and Na(+) coupling ions. Here, we constructed and characterized several hybrid stators combined with single Na(+)-coupled and dual Na(+)- and K(+)-coupled stator subunits, and we report that the MotP subunit is critical for the selection of K(+). This result suggested that the K(+) selectivity of the MotP/MotS complexes evolved from the single Na(+)-coupled stator MotP/MotS complexes. This finding will promote the understanding of the evolution of flagellar motors and the molecular mechanisms of coupling ion selectivity.