Section 19.5.8.5. Other large assemblies

Low-resolution X-ray fibre-diffraction data have been successfully used to model the structural details of a number of complex assemblies. For example, the structure of the F-actin helix at 8 Å resolution has been described by combining the single-crystal structure of the G-actin monomer with fibre-diffraction data (Holmes et al., 1990). This structure, in turn, has been used to model the muscle thin filament, composed of F-actin monomers and tropomyosin, at about 25 Å resolution, both in the resting and activated states, and hence to understand the movement of tropomyosin in muscle function (Squire et al., 1993). The structure of the microtubule has been determined at 18 Å resolution using information from electron microscopy and fibre diffraction (Beese et al., 1987). A similar but more sophisticated approach was used for bacterial flagellar filaments at 9 Å resolution (Yamashita, Hasegawa et al., 1998); the diffraction patterns obtained from these filaments are of such high quality that prospects for a complete molecular structure are excellent.