Towards atomic interpretation of F-actin filament three-dimensional reconstructions

We have recorded dark field images of negatively stained F-actin filaments polymerized with 2 mM MgCl2 and 50 mM KCl with a scanning transmission electron microscope and computed 3-D reconstructions using a helical parameter search to optimize simultaneously the helical repeat length, the radial position of the filament axis, and the helical selection rule. The resulting optimized averaged filament 3-D reconstruction at 2.5 nm resolution is remarkably similar to an atomic model of the F-actin filament. By comparison, several structural features of the reconstruction can be interpreted at the level of distinct secondary structure elements, and predictions made by the atomic model could be verified: for instance, the density connecting the two long-pitch helical strands in our reconstruction co-localizes with an extended beta-hairpin, the "hydrophobic loop" (i.e. residues 262 to 274), which according to the atomic model establishes the major intersubunit contact between the two long-pitch helical strands. The most pronounced structural variations among individual filament 3-D reconstructions were observed in (1) the details of the intersubunit contact pattern between the two long-pitch helical strands, and (2) the exact size and shape of subdomain 2 of the F-actin molecule, which appears rather flexible and easily deformed. In addition, we found that all phenotypes of F-actin filament 3-D reconstructions that arise from small deviations from the optimal helical parameters or from lowering the nominal resolution exhibited stronger intersubunit contacts between than along the two long-pitch helical strands, a structural feature that has been emphasized for a number of F-actin filament 3-D reconstructions in the past. Since this is clearly at variance with the relative strength of the intersubunit contacts as predicted by the atomic model, it may represent an artifactual structural feature arising from low-resolution data or suboptimal helical data processing, and should therefore be interpreted with caution in terms of indicating chemical, mechanical or conformational states of the F-actin filament.