Thermodyanmic Control on the Torque Generation of Bacterial Flagellar Motors

Abstract

Bacterial flagellar motor is an ion-driven molecular machine that is composed of a rotor and stator units. At least 11 stator units simultaneously associate with a single rotor, and the rest units diffuse in the inner membrane. The stator units exchange rapidly with the rotor and are concerned in the torque generation. Little is understood about the detailed energy conversion mechanism. Here, we demonstrate a novel assay that controls the intermolecular interaction between rotor and stator units with high-pressure techniques [1]. The strain was enclosed in the specially designed high-pressure chamber, which could be available up to 2,000 bar. The torque-speed relationship of the motor was measured by tracking of polystyrene beads attached to flagellar filaments at pressure range of less than 800 bar. At ambient pressure (1 bar), the torque is approximately constant (at ∼1500 pN nm) from stall up to a “knee” speed of ∼150 Hz, and then falls linearly with speed, extrapolating to zero torque at ∼280 Hz. As the pressure increased, both the knee and zero-load speeds decreased significantly, while the zero-speed torque was not affected. Similar relations were obtained by decreasing intracellular pH [2]. Thus, our results suggest that applied pressure decreases the rate of proton translocation in the mechanochemical energy translation, but not the actual torque generation step within the cycle by the stator-rotor interactions.[1] Nishiyama et al., Biophys. J. 96(3) 1142-1150 (2009).[2] Nakamura et al., J. Mol. Biol. 386(2), 332-338 (2009).