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

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Abstract
The critical monomer concentrations at which association and dissociation reactions are balanced at the 2 ends of [rabbit] actin filaments were determined. 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 .mu.M) was 12- to 15-fold higher than the critical concentration of the barbed end (0.10-0.12 .mu.M) at a temperature of 37.degree. C and physiological salt concentrations (100mM KCl/1-2 mM MgCl2/0.3 mM EGTA [ethylene glycol bis(.beta.-aminoethyl ether)-N,N,N'',N''-tetraacetic acid] or 0.2 mM CaCl2 , pH 7.5).