The helix content of tropomyosin and the interaction between tropomyosin and F-actin under various conditions

Abstract

The interaction between tropomyosin and F-actin was studied by means of flow birefringence and optical rotatory dispersion methods. An analysis of the results suggests that the conformation of tropomyosin plays an important role in its binding to F-actin, i.e. there exists a threshold value for the helix content of tropomyosin necessary for binding. Investigations of the binding under various conditions also showed that (1) the lowering of pH and the addition of ethanol induced, as a primary effect, the stabilization of the helical structure of tropomyosin, while (2) the addition of small amounts of divalent cations made the polymer structure of F-actin more rigid. Both conditions made the binding of tropomyosin to F-actin stronger. Urea affected both F-actin and tropomyosin, making their structures more flexible or looser, thus causing a weakening of the binding. The effects of pH, ethanol, urea and divalent cations on structure of the proteins and on their interactions are discussed from the point of view of the conformational change of protein structure. Finally, the effect of salt (KCl) on this interaction is also discussed.