Structural Insight into Torque Generation Mechanism of the Bacterial Flagellar Motor

Abstract

The bacterial flagellum is made of a rotary motor and a long helical filament by means of which bacteria swim. The flagellar motors of Salmonella and E. coli rotate at around 300 Hz and drive rapid rotation of each flagellum to propel cell movements. The mechanism of torque generation by the motor has been intensively studied by many groups based on the torque-speed relationships and the structure, but it has been elusive due to the lack of high-resolution structural information of the rotor and stator and their interactions. X-ray crystal structures are available for some part of the rotor and switch complex component proteins but no detailed information is yet available for their arrangement in the motor assembly and also for the transmembrane and cytoplasmic domains of the stator. A currently proposed 3D model of the rotor for torque generation does not satisfy the stepping behavior of the motor observed by single motor nanophotometry. We have been trying to solve the structure of the flagellar hook-basal body by electron cryomicroscopy and single particle image analysis in its isolated form from the membrane as well as by electron cryotomography of the cell to visualize the basal body in situ. I will report our current progress and discuss some structural insights into the torque generation mechanism.