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

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 degrees C in solutions containing 100 mM KCl and free Ca2+ at various concentrations. The average fluorescence anisotropy and the fluorescence rotational correlation time of actin decreased in the presence of micromolar concentrations of free Ca2+. 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 Ca2+ binding. The binding constant was 6 X 10(6) M-1.