Thermally induced chain exchange of smooth muscle tropomyosin dimers studied by differential scanning calorimetry.

The thermal unfolding of duck gizzard tropomyosin dimers, alphabeta, alphaalpha, and betabeta, and of a 1:1 mixture of alphaalpha and betabeta homodimers was studied by differential scanning calorimetry (DSC). Both alphaalpha and betabeta homodimers demonstrated a broad thermal transition with maxima at 37.4 degrees C and 44.6 degrees C, respectively. However, a sharp cooperative thermal transition at 41.5 degrees C characteristic for alphabeta heterodimer appeared on the thermogram of the mixture of homodimers. The appearance of this transition was prevented by disulfide cross-linking of polypeptide chains in the homodimers. Thus, DSC studies clearly demonstrate formation of tropomyosin heterodimers during heating of the mixture of homodimers and in agreement with earlier published reports indicate thermally induced chain exchange between tropomyosin dimers.