Fluorescence anisotropy of labelled F-actin: Influence of Ca2+ on the flexibility of F-actin

Abstract We measured the fluorescence static anisotropy and the time-resolved fluorescence anisotropy decay of F-actin labelled with N -iodoacetyl- N '-(5-sulfo-1-naphthyl)ethylenediamine at 20°C in solutions containing 100 mM KCl and free Ca 2+ at various concentrations. The average fluorescence anisotropy and the fluorescence rotational correlation time of actin decreased in the presence of micromolar concentrations of free Ca 2+ . The change of the rotational correlation time of labelled actin could not be explained by a variation of the actin critical concentration. We concluded therefore that F-actin undergoes a conformational change induced by Ca 2+ binding. The binding constant was 6 × 10 6 M −1 .