Intact Flagellar Motor Architecture Revealed by Cryo-Electron Tomography

Abstract

Motility is an important component of the pathogenesis of bacteria and the bacterial flagellar motor is considered to be the most proficient biological machine for this purpose. The influx of protons through the motor produces either counterclockwise or clockwise flagellar rotation, resulting in translational motion or ‘tumbling’, respectively. Although the rotor portion of the bacterial flagellar motor has been purified and studied for decades, very little is known about the stator portion of the motor and its relationship to the rotor. We used high-throughput Cryo-Electron Tomography (Cryo-ET) and cutting edge image analysis to obtain 3-D structures of the intact flagellar motor assembly associated with native snap frozen bacteria at a level of detail that has not been previously observed. By averaging the 3-D volumes of ∼1280 flagellar motors, we obtained a detailed model of the intact flagellar motor showing both stator and rotor assembly in its native cellular environment at about 3 nm resolution. We have also been able to identify distinctive structural changes resulting from the mutation of a flagellar gene. This is direct mapping of a single genetic code change into the 3-D structure of a functioning molecular machine in situ. Our results provide new insights into the motor structure and the molecular basis for bacterial motility.