MECHANISMS OF SELF-ASSEMBLY AND SWITCHING OF THE BACTERIAL FLAGELLUM

Keiichi Namba

Graduate School of Frontier Biosciences, Osaka University Protonic NanoMachine Project, ERATO, JST, & Dynamic NanoMachine Project, ICORP, JST, 3-4 Hikaridai, Seika, Kyoto 619-0237 Japan (keiichi@fbs.osaka-u.ac.jp)

The bacterial flagellum is made of a rotary motor and a long helical filament by means of which bacteria swim. The size of the bacterial cell body is about 1 mm by 2 mm, but the flagellum grows to about 15 mm long. The flagellar motor at its base rotates at around 300 Hz and drives the rapid rotation of each flagellum to propel the cell movements in viscous environments. The diameter of the flagellar motor is 30 to 40 nm, ant it consists of many proteins including membrane spanning proteins: a rotor ring, made of about 25 copies of FliF/FliG complex; about eight stator units, made of MotA/MotB complex; other parts such as the rotation switch regulator, bushing, and drive shaft, all made of different proteins. The long helical filament, which is a tubular structure with a diameter of about 20 nm, is made of 20,000 to 30,000 copies of a single protein flagellin, and yet the filament can form left-handed or right-handed helical forms and switch between these two in response to the twisting force produced by quick reversal of the motor rotation. This allows bacteria to alternate their swimming pattern between running and tumbling, which is essential for their tactic behavior. The flagellum also has a short, highly curved segment that connects the motor and the helical propeller, and this segment is called hook. Its bending flexibility makes it function as a universal joint, while the filament is relatively more rigid to work as a propeller. There is a very short segment called the hook-filament junction, which is made of HAP1 and HAP3. This junction is thought to play a mechanical buffer to connect the two mechanically distinct structures. The flagellum is constructed through various self-assembly processes, in which all the axial structures growing towards the cell exterior are constructed by the flagellar component proteins translocated from the cytoplasm to the distal end of the growing structure, where three cap complexes help efficient self-assembly of these proteins in different stages.

We have been trying to visualize the structure of the flagellum in atomic detail to understand how it self-assembles and works. We solved crystal structures of core fragments of the flagellar axial component proteins by X-ray crystallography. X-ray fiber diffraction gave high-resolution structural information. Electron cryomicroscopy also visualized the structures of the filament, cap and cap-filament complex. All these structures present interesting implications for the function of each molecule, demonstrating the importance of dual nature of protein molecules, flexibility and precision.