Structure of the actin molecule determined from electron micrographs of crystalline actin sheets with a tentative alignment of the molecule in the actin filament.

Electron microscopy and image processing of negatively stained crystalline sheets induced from Acanthamoeba actin have been used to yield a three-dimensional reconstruction of the actin molecule, including data to a maximum resolution of 15 A. This model shows actin to be an asymmetric, wedge-shaped molecule. A three-dimensional reconstruction of an averaged, polar actin filament from negatively stained polylysine-induced actin filament paracrystals has also been computed. We show two possible ways in which the wedge-shaped actin molecule from the sheets can be placed into such a filament reconstruction. In both, the major intermolecular contacts are formed on complementary surfaces of the actin subunit and follow the left-handed genetic helix of the filament, a feature also found in the filament reconstruction.