Studies on the F-actin.tropomyosin.troponin complex. III. Effects of troponin components and calcium ion on the binding affinity between tropomyosin and F-actin.

We studied the effects of the troponin components TN-T, TN-I and TN-C, and of Ca2+ concentration on the dissociation temperature of the F-actin.tropomyosin complex. Troponin components and their complexes increased the dissociation temperature in the following sequence of effectiveness: TN-C(+/- Ca2+) less than TN-I less than TN-I.TN-C(+/-Ca2+) less than TN-T.TN-C(+/- Ca2+) less than TN-T.TN-C(--Ca2+) approximately TN-T less than TN-T.TN-I.TN-C(+Ca2+) less than TN-T.TN-I.TN-C(--Ca2+) approximately TN-T.TN-I; TN-C has no effect. The effect of TN-I is small. TN-T is the most important component for increasing the dissociation temperature. The shifting by Ca2+ of the dissociation temperature to lower values is only seen when both TN-T and TN-C are present.