Structure analysis of the flagellar filament by X-ray fiber diffraction
     Bacteria, such as E.Coli and S. Typhimurium, swim in liquid media by rotating helical filaments, and the filament is called bacterial flagellum. The major portion of the flagellum is composed of a single protein flagellin. In contrast to the fact that other filamentous macromolecular assemblies, such as the actin filament and TMV, form straight filaments, the flagellar filament forms various supercoils as well as two types of straight forms. The wild-type supercoil is left handed, but it switches into a right-handed one upon quick reversal of the motor rotation. It also turns into various supercoils depending on the ionic strength or pH of the solvent. Point mutations in the flagellin molecule also result in the formation of various types of supercoils. This feature of the flagellar filament is called "polymorphism". The mechanism is explained by "two-state model", in which the supercoiled filaments are constructed by a regular arrangement of two distinct types of inter- or intra-subunit interactions with an elasticity that allows shortening and elongation of the inter-subunit repeat distance to form curved tubular structures.
     Our previous work showed that the difference of the inter-subunit distance along the protofilament between the two distinct states is only 0.8 Å (I. Yamashita et al., 1998). Model simulation of supercoiled filaments based on the helical parameters obtained from the two straight filaments showed that elastic shortening or elongation within 0.6 Å are sufficient to produce various types of supercoiled forms ( K. Hasegawa et al., 1998).
     Our current aim is to study the structure of the flagellar filament by X-ray fiber diffraction to understand the mechanism of the polymorphic supercoiling at atomic resolution.

Yamashita, I., Hasegawa, K., Suzuki, H., Vonderviszt, F., Mimori-Kiyosue, Y. & Namba, K. (1998). Structure and switching of bacterial flagellar filament studied by X-ray fiber diffraction. Nature Struct. Biol. 5, 125-132.
Hasegawa, K., Yamashita, I. & Namba, K. (1998). Quasi-equivalence and non-equivalence in the structure of bacterial flagellar filament. Biophys. J. 74, 569-575.