X-ray diffraction evidence that actin is a 100 Å filament

Abstract

The shape of the actin monomer has been determined by X-ray crystallography¹ and image analysis² to be a prolate ellipsoid consisting of two domains. A key structural question is how this monomer is assembled into the biologically important actin filament. Two completely different classes of models of F-actin have recently appeared. In one, the long axis of the monomer is oriented parallel to the filament axis^1,2, while in the other it is nearly perpendicular to that axis^3–5. Fowler and Aebi⁶ have argued that a proper assessment of the diameter of the actin filament can distinguish between the correctness of these models, since the ‘perpendicular’ model is about 95 Å in diameter, while the ‘parallel’ model is about 75 Å in diameter. Using electron microscopy, they have determined the diameter to be between 70 and 80 Å, consistent with their model. Egelman and DeRosier³ also provided evidence from electron microscopy that the filament was about 95 Å, consistent with their perpendicular model. However, all specimens prepared for electron microscopy are susceptible to various artefacts induced by the preparation procedure. X-ray diffraction of specimens in solution allows one to examine objects in their native state. Data from X-ray diffraction of actin filaments in live muscle are presented here which show that a reasonable value for the diameter of F-actin is between 95 and 100 Å.