W0239

Structural Mechanisms of Self-assembly and Polymorphic SuperCoiling of the Bacterial Flagellum. Keiichi Namba, Protonic NanoMachine Project, ERATO, JST & Advanced Technology Research Laboratories, Matsushita Electric Industrial Co., Ltd.

The bacterial flagellum is a helical filament by means of which bacteria swim. The flagellar motor at its base rapidly rotates each flagellum to propel the cell movements. The flagellar filament is made of a single protein flagellin, and yet the tubular structure of the filament can form left-handed or right-handed helical forms, and these forms are switchable in response to the twisting force produced by quick reversal of the motor rotation for running and tumbling in the tactic behavior of bacteria. I will present the structures of the filament and the cap-filament complex to explain how the distal cap complex promotes the self-assembly of flagellin and how chemically identical molecules can form these switchable helical tubular structures.

The distal cap complex is a pentamer of a protein FliD. The structure of the cap-filament complex deduced by electron cryomicroscopy showed interesting binding interactions between the pentamer cap and the distal end of the filament with 11 protofilaments forming a tubular structure. The symmetry mismatch is used to prepare just one binding site for a flagellin subunit at a time, suggesting a rotary cap mechanism for efficient promotion of flagellin self-assembly.

The two different straight filament structures that represent the two distinct conformational or packing states of flagellin to form helical tubes were analyzed by electron cryomicroscopy and X-ray fiber diffraction. The difference between the two states was an axial shift of about 2 Å in the lateral packing of the protofilaments and a change in the subunit repeat distance along the protofilament. The repeat distances of the two states were 51.9 Å and 52.7 Å, the difference being only 0.8 Å, indicating that the flagellin molecule has a very fine mechanical switch function. The crystal structure of a core fragment of flagellin at 2.0 Å resolution revealed the protofilament structure of one state with a repeat distance of 51.9 Å. This atomic model of the protofilament allowed us to see its mechanical response to forced extension by simulation, by which we identified a structural motif responsible for the lengthwise mechanical switch of the flagellar protofilament.