[W1-1] Functional mechanism of the flagellar stator revealed by the structure-based functional studies
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 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 photo- or 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.