Pulling Forces Dampen Torsional Fluctuations of Actin Filaments and Reduce Cooperative Cofilin Binding

A variety of potential biological roles of mechanical forces have been proposed in the field of cell biology. In particular, mechanical forces alter the mechanical conditions within cells and their environment, exerting a strong effect on the reorganization of the actin cytoskeleton. Single-molecule imaging studies have provided evidence that an actin filament may act as a mechanosensor. An increase in the tension within actin filaments causes changes in their conformation and affinity to their regulatory proteins. However, our current understanding of the molecular mechanisms of the tension sensing and the affinity change of regulatory proteins is still incomplete. In this study, we employed fluorescence polarization microscopy and magnetic tweezers to directly quantify the torsional fluctuations of single actin filaments and cofilin binding to the filament under several distinct mechanical conditions. When an actin filament was severed by scratching the filament with a pipette tip, the amplitude of twisting/torsional fluctuations and the rate of cooperative cofilin binding increased. On the other hand, when a piconewton force was applied to single actin filaments by magnetic tweezers, the amplitude of twisting fluctuations and the rate of cooperative cofilin binding decreased. This may be involved in the molecular mechanism behind the mechanical force-dependent severing of actin filaments in cells.