A Conformational Change in the Actin Subunit Can Change the Flexibility of the Actin Filament

The mechanical properties of F-actin are very significant, given the central structural role played by actin filaments within muscle and the cytoskeleton. We have determined that actin can exist in a state that has a fourfold increase in flexibility over normal F-actin, and nucleotide. Three-dimensional reconstructions from electron micrographs suggest that this increased flexibility arises from a rotation of subdomain-2, the smallest subdomain, of the actin subunit. The modulation of actin's flexibility by Ca2+ and Mg2+ may have important physiological consequences within the cell. Further, since it has been shown that myosin-decorated actin filaments are more flexible than pure F-actin, it is possible that myosin induces this more flexible state in actin.