The structure of F−actin calculated from X−ray fibre diagrams and the 0.6nm crystal structure

The structure of the g-actin monomer complexed with DNaseI has been solved by x-ray crystallography to 0.45nm resolution, Fig.l (Kabsch, Mannherz, & Suck, 1985). In the following we describe the structure of f-actin arrived at by a search procedure: the structure obtained from crystallography at 0.6nm is placed in all possible orientations in the F-actin helix; the fibre diffraction pattern is computed from the resulting structure and compared with the x-ray diffraction data from orientated gels of F-actin (Popp, Lednev, & Jahn, 1986) measured to 0.8 nm resolution. This process yielded five possible solutions at low resolution (2.0nm) only one of which successfully refined to high resolution (0.8nm). A full account of this study is in preparation (Holmes, et al. 1989). The best of the five possible solutions resulting from the low resolution search is shown in Fig. 2. The (intensity) R-factor is 0.12. To refine at high resolution we adopted an iterative least squares procedure. However, no solution would refine satisfactorily. The resulting R-factors ranged from 0.37–0.45