Marine Vibrio has a sodium-driven polar flagellum for motility. The force-generating stator unit of the motor consists of PomA and PomB. PomA contains four-transmembrane regions and a cytoplasmic domain of approximately 100 residues which interacts with the rotor protein, FliG, to be important for the force generation. Recently reported, three-dimensional structure of the stator, shows that the cytosolic interface (CI) helix of PomA is located parallel to the inner membrane. We investigated the

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... function of CI helix by systematic proline mutagenesis. Results showed that three residues near the end of CI helix were important for ion-conduction and assembly around the rotor. The CI helix may play important roles in various processes, such as the hoop-like function in securing the stability of the stator complex and the ion conduction pathway. Meanwhile, the stator structure inspired a model in which the A subunit ring rotates around the B subunit dimer in response to ion influx. Our site-directed photoor disulfide-crosslinking results support this model: the plug helix in the B subunit interacts with the extracellular short loop region of PomA and this inter-subunit interaction interferes the stator function. The plug region terminates the ion influx by blocking the rotation of the rotor as a spanner. Further updates of structure-guided functional studies will be discussed.