TCBG Seminar

Molecular Mechanisms of Bacterial Swimming and Tumbling

Professor Keiichi Namba
Graduate School of Frontier Biosciences
Osaka University
Osaka Japan

Wednesday, June 1, 2005
10:00 am (CT)
3269 Beckman Institute


The bacterial flagellum is made of a rotary motor and a long helical filament by means of which bacteria swim. The flagellar motor rotates at around 300 Hz and drives the rapid rotation of each flagellum to propel the cell movements. The long helical filament, which is a tubular structure with a diameter of about 20 nm, is made of a single protein flagellin. The filament switches between left- and right-handed helical forms in response to the twisting force produced by reversal of the motor rotation, allowing bacteria to alternate their swimming pattern between running and tumbling for taxis. The flagellum also has a short, highly curved segment called hook, which connects the motor and the helical propeller. Its bending flexibility makes it work as a nano-scale universal joint, while the filament is relatively more rigid to function as a propeller. A very short segment made of proteins HAP1 and HAP3 connects these two mechanically distinct structures. The flagellum is constructed by self-assembly of proteins translocated from the cytoplasm through the narrow central channel to the distal end of the growing structure, where one of three different cap complexes is attached to help efficient self-assembly of particular proteins that need to be assembled at each specific stage of the assembly process. We have solved most part of the structures in the cell exterior by X-ray crystallography, fiber diffraction and electron cryomicroscopy. These structures present interesting implications for the function of each molecule and subcomplex, demonstrating the importance of dual nature of protein molecules, dynamic flexibility and subatomic level precision.

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