Polymerization of actin induced by a molar excess of ATP in a low salt buffer

The polymerization of actin induced by dilution has previously been reported, where a 1000-fold molar excess of ATP over actin resulted when actin was diluted to 4.0 microg/ml in low salt buffer A (0.1 mM ATP, 0.1 mM CaCl2, 2 mM Tris-HCl, pH 8.0, 5 mM 2-mercaptoethanol, 1 mM NaN3). Filaments formed by the addition of ATP to a 1000-fold molar excess over actin in buffer B (0.1 mM CaCl2, 2 mM Tris-HCl, pH 8.0, 1 mM NaN3) were then separated by gel-filtration. When ATP was removed from these filaments using Dowex-1, depolymerization occurred. Thus, the reversible polymerization induced by the dilution of actin or by addition of ATP can be ascribed to the binding of ATP at the low affinity site of actin.