Ca-Dependent Regulation of the Subunit Exchange of F-Actin under the Influence of Tropomyosin and Troponin

Abstract

A quasi-steady state rate of actin subunit incorporation into a complex of F-actin, tropomyosin, and troponin was studied in a solution containing MgCl₂ 0.5mM, ATP 0.2mM, Tris-HCl 5mM (pH 8.0), and either CaCl₂ 64 μM or EGTA 644 μM at 33°C by adding a small amount (less than 1 μM) of fluorescence-labeled actin monomer. Incorporation of the labeled actin monomer was remarkably much slower in the presence of EGTA, but the rate recovered instantaneously upon addition of an excess amount of CaCl₂. The rate was very much reduced by the presence of cytochalasin D (1 μM) and again showed Ca-dependence.