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 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.