Binding of aldolase to actin-containing filaments. Evidence of interaction with the regulatory proteins of skeletal muscle

Abstract

The interactions of aldolase with regulatory proteins of rabbit skeletal muscle were investigated by moving-boundary electrophoresis. A salt-dependent interaction of troponin, tropomyosin and the tropomyosin-troponin complex with aldolase was detected, the tropomyosin-troponin complex displaying a greater affinity for the enzyme than did either regulatory protein alone. The results indicate that aldolase possesses multiple binding sites (three or more) for these muscle proteins. Quantitative studies of the binding of aldolase to actin-containing filaments showed the interaction to be influenced markedly by the presence of these muscle regulatory proteins on the filaments. In imidazole/HCl buffer, I 0.088, pH 6.8, aldolase binds to F-actin with an affinity constant of 2 x 10(5) M-1 and a stoicheiometry of one tetrameric aldolase molecule per 14 monomeric actin units. Use of F-actin-tropomyosin as adsorbent results in a doubling of the stoicheiometry without significant change in the intrinsic association constant. With F-actin-tropomyosin-troponin a lower binding constant (6 x 10(4) M-1) but even greater stoicheiometry (4:14 actin units) are observed. The presence of Ca2+ (0.1 mM) decreases this stoicheiometry to 3:14 without affecting significantly the magnitude of the intrinsic binding constant.