Electron microscopy and three-dimensional reconstructions have been used to show that F-actin can exist in multiple states. We have also been able to visualize large allosteric effects involving the C-terminus, the nucleotide binding site, the high-affinity metal-binding site, and the DNase I-binding loop. Further, there exists a large degree of cooperativity in F-actin, such that conformational changes at one end of a filament may be transmitted to the other, distant, end. While many of these allosteric and cooperative effects have been previously suggested to exist based upon biochemical observations, we have been able to observe that these effects probably involve the movement of a large number of residues over significant distances. In systems such as muscle some of these conformational changes in F-actin may play an important role. (C) 1995 Academic Press, Inc. Press, Inc.