Effect of troponin on the binding between tropomyosin and F-actin

Abstract

The flow birefringence of the complex of troponin, tropomyosin and F-actin was studied in a solution containing urea. Optical rotatory dispersion (ORD) was used to detect conformational changes in troponin and tropomyosin resulting from the interaction between them. 1. 1. Tropomyosin, once dissociated from F-actin on the addition of urea, could bind again to F-actin on the addition of troponin, provided that the urea concentration was lower than the critical value. This meant that troponin made the binding between tropomyosin and F-actin stronger. 2. 2. The critical urea concentration necessary for the dissociation of tropomyosin from F-actin became higher when free Ca 2+ was removed. The Ca 2+ concentration necessary for the shift of the critical urea concentration was only 1 μM. 3. 3. In the presence of 3 mM MgCl 2, a higher concentration of urea was needed for the dissociation of tropomyosin in the presence of Ca 2+. 4. 4. A sharp decrease of the helix contents of troponin and tropomyosin was observed at about 3 and 2 M urea, respectively. At urea concentrations higher than 3 M, the helix content of troponin depended on the concentration of Ca 2+ in a micromolar range. 5. 5. In a urea concentration range from 2 to 3 M, the helix content of the complex of troponin and tropomyosin was higher than the simple sum of their helix contents. 6. 6. A possible mechanism for the regulation of muscular contraction will be discussed in the light of the present results.