Rheology of F-actin II. Effect of tropomyosin and troponin

Abstract

Effect of tropomyosin and troponin on the rheological properties of F-actin in solution was investigated by the use of a rotational viscometer and an apparatus for measuring dynamic viscoelasticity. Tropomyosin reduced the rate of gelation of F-actin as measured in a very slowly rotating viscometer, although the final value of the viscosity of the tropomyosin-F-actin complex was higher than that of F-actin alone. The effect of tropomyosin was discussed in relation to the change in flexibility of F-actin filaments by tropomyosin. The dynamic rigidity and loss moduli of F-actin were increased by the tropomyosin binding. Structural viscosity, dynamic rigidity modulus and loss modulus of the tropomyosin-F-actin complex was remarkably increased by the addition of troponin. The rheological properties of the troponin-tropomyosin-F-actin complex became dependent on free Ca 2+ concentrations at micromolar level, after treated at 45 °C. In the presence of more than 10 −5 M Ca 2+, the values of dynamic rigidity and loss moduli were very low, whereas in the absence of Ca 2+ (< 10 −8 M) the values were as high as the initial ones before the heat treatment. The Ca 2+-dependent action of troponin was substituted for by its components, TN-C and TN-T.