12-fold difference between the critical monomer concentrations of the two ends of actin filaments in physiological salt conditions.

We determined the critical monomer concentrations at which association and dissociation reactions are balanced at the two ends of actin filaments. For measurement of the critical concentration of the pointed end, interference with the high dynamics of the barbed end was excluded by capping the barbed ends with an actin filament capping protein isolated from bovine brain. The critical concentration of the pointed end (1.5 microM) was found to be 12- to 15-fold higher than the critical concentration of the barbed end (0.10-0.12 microM) at a temperature of 37 degrees C and physiological salt concentrations (100 mM KCl/1-2 mM MgCl2/0.3 mM EGTA or 0.2 mM CaCl2, pH 7.5).