Actin filaments (F-actin) complexed to various nucleotides (ADP (adenosine diphosphate), ADP-P-i (P-i, inorganic phosphate), and ADP-BeF3- (BeF3-, beryllium fluoride)) have been studied by small angle X-ray scattering and electron cryomicroscopy. The small angle X-ray scattering data show that the value of the cross-radius of gyration (2.55 +/- 0.15 nm) and the mean helical symmetry of the filaments are independent of the nature of the bound nucleotide. The scattering profiles of all the F-actin suspensions exhibit secondary maxima located at similar positions. However, the intensities of the secondary maxima depend upon both the protein concentration and the type of nucleotide bound to the F-actin. These variations indicate that F-actin has different properties (such as its tendency to aggregate and the structure of the filament), which depend upon the type of nucleotide bound, Electron cryomicroscopy of vitrified suspensions shows that F-actin may have different conformations. The nonequatorial layer lines, which constitute the Fourier transforms of the filament images, have different signal to noise ratios depending on the type of nucleotide bound to the actin. The 3-D reconstructions of the actin filaments show an inner and outer domain. In the maps, the visibility of the outer domain depends upon the type of nucleotide bound to the actin. It respectively increases for ADP-P-i, ADP, and ADP-BeF3-. We attribute these differences to the disorder of the filaments, which in turn depends upon the nucleotide bound. These variations in disorder imply that there are structural changes (contacts, structure, or orientation) in the actin subunits forming the filaments. These changes may have an important role in the motility processes in which actin is involved. (C) 1994 Academic Press, Inc.