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-binding1Department of Biochemistry University of Minnesota protein, on the microsecond rotational dynamics of actin filaments, using Medical School, Minneapolis time-resolved phosphorescence (TPA) and absorption anisotropy (TAA) of erythrosin iodoacetamide attached to Cys374 on actin. Polymerization ofMN 55455, USA actin in the presence of gelsolin resulted in substantial increases in the rate2Division of Experimental and amplitude of anisotropy decay, indicating increased rotational motion. Medicine, Harvard Medical Analysis indicates that the effect of gelsolin cannot be explained by School, Boston, MA 02115 increased rates of overall (rigid-body) rotations of shortened filaments, but USA reflects changes in intra-filament structure and dynamics. We conclude that gelsolin induces (1) a 10 ° change in the orientation of the absorption dipole of the probe relative to the actin filament, indicating a con-formational change in actin, and (2) a threefold decrease in torsional rigidity of the filament. This result, which is consistent with complemen-tary 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