Effects of troponin on thermal unfolding of actin-bound tropomyosin.

Differential scanning calorimetry (DSC) was used to study the effect of troponin (Tn) and its isolated components on the thermal unfolding of skeletal muscle tropomyosin (Tm) bound to F-actin. It is shown that in the absence of actin the thermal unfolding of Tm is expressed in two well-distinguished thermal transitions with maxima at 42.8 and 53.8 degrees C. Interaction with F-actin affects the character of thermal unfolding of Tm leading to appearance of a new Tm transition with maximum at about 48 degrees C, but it has no influence on the thermal denaturation of F-actin stabilized by aluminum fluoride, which occurs within the temperature region above 70 degrees C. Addition of troponin leads to significant increase in the cooperativity and enthalpy of the thermal transition of the actin-bound Tm. The most pronounced effect of Tn was observed in the absence of calcium. To elucidate how troponin complex affects the properties of Tm, we studied the influence of its isolated components, troponin I (TnI) and troponin T (TnT), on the thermal unfolding of actin-bound Tm. Isolated TnT and TnI do not demonstrate cooperative thermal transitions on heating up to 100 degrees C. However, addition of TnI, and especially of TnT, to the F-actin-Tm complex significantly increased the cooperativity of the thermal unfolding of actin-bound tropomyosin.