Determination of the 'true' width of the actin filament

Abstract

Although the structure of the actin filament has been studied extensively both, by X-ray diffraction analysis and by electron microscopy and structure reconstruction, molecular models of actin have been obtained only recently. However none of the crystalline forms used in these latter studies are made directly from filaments. Since the resolution of current filament models is insufficient to allow unambiguous tracing of the actin subunits within them [e.g.2], the orientation of molecular models within such filament models is speculative. A primary constraint on the overall orientation of the elongated and polar actin subunit model within the actin filament is the 'true' width of the filament: reported values as obtained by electron microscopy range from 6 to 8nm for negatively stained specimens and to 9.5nm or more for rotary shadowed filaments [reviewed in 3]. Furthermore, X-ray solution scattering of actin filaments has revealed an average width of 7nm. Using a variety of different specimen preparation methods, we have endeavoured to set realistic upper and lower limits for this crucial parameter.