Calcium-dependent interactions of F-actin filaments under the influence of troponin components

The effect of three components of troponin (TN C, I, T) on the gelation of F-actin was investigated by measuring the increase in viscosity at a very low velocity gradient in a rotating viscometer. TN I or TN T greatly enhanced the generation of F-actin. The effect of TN I-C or T-C complex became Ca-dependent: in the absence of Ca, the complex increased the rate of viscosity rise of F-actin, but in its presence this enhancing effect was almost absent. For these actions, the presence of tropomyosin or heat treatment at 45 degrees was not required. These results can be explained in terms of strengthened interactions of F-actin particles bound with TN T or TN I and the release of TN I-C or TN T-C in the presence of Ca.