X-ray crystallography of a flagellin fragment
     The bacterial flagellar filament is a helical filament that functions as the propeller for bacterial swimming. Its rapid rotation (about 20,000 rpm) is driven by the rotary motor at its base. The filament is formed by self-assembly of a single protein, flagellin. The filament is supercoiled to be a left-handed helical propeller, but it is not simply a rigid propeller; it quickly switches into a right-handed helix upon reversal of the motor rotation to allow disintegration of the filament bundle that drives the cell's tumbling motion.
     To understand the mechanisms of self-assembly and polymorphic supercoiling of the flagellar filament, it is necessary to solve the three dimensional structure of the filament at high resolution. X-ray fiber diffraction and electron cryomicroscopy are potentially powerful tools to deduce filamentous structures with helical symmetry, but a crystal structure of flagellin would also be essential to obtain the high resolution structure of the filament. However, flagellin is difficult to crystallize because of its strong tendency to aggregate into the filament. Flagellin is composed of 494 amino acid residues, and the filament formation occurs by interactions of the NH₂- and the COOH-terminal regions of flagellin. A proteolytic fragment of flagellin missing 52 NH₂- and 44 COOH-terminal residues, named F41, does not aggregate into the filament and therefore suitable for crystallization. Crystallization of F41 was not an easy task either, but we finally obtained relatively large, but very thin plate crystals reproducibly. We collected diffraction data from the native crystal and its heavy atom derivatives using synchrotron radiation. We used beamline ID14 at ESRF and beamlines BL41XU and BL45XU at SPring8. Some of the crystals diffracted beyond 2-Å resolution. Now, chain tracing is underway to build an atomic model of F41.