Cooperativity in F-actin: binding of gelsolin at the barbed end affects structure and dynamics of the whole filament.

We have studied the effect of gelsolin, a Ca-dependent actin-binding protein, on the microsecond rotational dynamics of actin filaments, using time-resolved phosphorescence (TPA) and absorption anisotropy (TAA) of erythrosin iodoacetamide attached to Cys374 on actin. Polymerization of actin in the presence of gelsolin resulted in substantial increases in the rate and amplitude of anisotropy decay, indicating increased rotational motion. Analysis indicates that the effect of gelsolin cannot be explained by increased rates of overall (rigid-body) rotations of shortened filaments, but reflects changes in intra-filament structure and dynamics. We conclude that gelsolin induces (1) a 10 degrees change in the orientation of the absorption dipole of the probe relative to the actin filament, indicating a conformational change in actin, and (2) a threefold decrease in torsional rigidity of the filament. This result, which is consistent with complementary electron microscopic observations on the same preparations, directly demonstrates long-range cooperativity in F-actin, where a conformational change induced by the binding of a single gelsolin molecule to the barbed end is propagated along inter-monomer bonds throughout the actin filament.