Structure of the C-terminal domain of FliG, a component of the rotor in the bacterial flagellar motor

Many motile species of bacteria are propelled by flagella, which are rigid helical filaments turned by rotary motors in the cell membrane(1-3). The mo tors are powered by the transmembrane gradient of protons or sodium ions. A lthough bacterial flagella contain many proteins, only three-MotA, MotB and FliG-participate closely in torque generation. MotA, and MotB are ion-cond ucting membrane proteins that: form the stator of the motor. FliG is a comp onent of the rotor, present in about 25 copies per flagellum. It is compose d of an amino-terminal domain that functions in flagellar assembly and a ca rboxy-terminal domain (FliG-C) that functions specifically in motor rotatio n. Here we report the crystal structure of FliG-C from the hyperthermophili c eubacterium Thermotoga maritima, Charged residues that are important for function, and which interact with the stator protein MotA(4,5), duster alon g a prominent ridge on FliG-C. On the basic; of the disposition of these re sidues, we present a hypothesis for the orientation of FliG-C domains in th e flagellar motor, and propose a structural model for the part of the rotor that interacts with the stator.