Effects of KCl, MgCl2 and CaCl2 Concentrations on the Monomer-Polymer Equilibrium of Actin in the Presence and Absence of Cytochalasin D1

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Abstract
Critical metal concentrations of KCl, MgCl 2 and CaCl 2 below which actin cannot exist in filamentous form at equilibrium were estimated to be approximately 8 mM 0.2 mM and 0.4 mM respectively, when actin, 0.5 mg/ml, was incubated for a long time at pH 8.0 and 25°C. These values were obtained by viscosity, UV absorbance and pyrene-labeled actin fluorescence measurements. In the case of CaCl 2 viscosity measurements resulted in a larger critical concentration, 0.7 mM. It turned out that actin filaments formed in a low concentration of CaCl 2 were easily fragmented by shearing force. The higher the actin concentration, the lower the critical metal concentration was. The critical concentrations of actin above which actin can exist as a polymer at equilibrium became larger as the KCl, MgCl 2 and CaCl 2 concentrations were lower. It is advisable to add 0.05–0.1 mM CaCl 2 to a depolymerization solution to obtain a concentrated monomeric solution. In the presence of cytochalasin D, which preferentially blocks the elongation at the barbed end of actin filaments, the critical concentrations of KCl and CaCl 2 were the same as in its absence. On the other hand, the critical concentration of MgCl 2 was increased and the extent of polymerization was decreased by cytochalasin D. In the presence of 50 mM KCl and 1 μM cytochalasin D, 0.01–0.1 mM MgCl 2 markedly decreased the extent of polymerization of actin at equilibrium, where the critical actin concentration was tenfold increased. It is suggested that Mg 2+ enhances dissociation of actin monomers at the pointed end of an actin filament.