CONFORMATIONAL STABILITIES OF TROPOMYOSINS

The stability to denaturation by heat and guanidine hydrochloride of 7 vertebrate (including skeletal, cardiac and smooth muscle) tropomyosins [rabbit, bovine, dog, chicken] and 3 invertebrate tropomyosins [oyster, abalone, scallop] was examined. The transition profiles were discontinuous and in many cases distinct plateaux were observed which indicated the presence of unique partially unfolded states at intermediate temperatures and guanidine hydrochloride concentrations. The denaturation by guanidine hydrochloride was described in the majority of cases by a model in which the native state unfolds to a partially unfolded stable intermediate which then unfolds to the completely denatured state. The free energies of unfolding in water were estimated. Part of the .alpha.-helical structure of tropomyosin was only marginally stable and the free energy of unfolding in water of this segment was less than values found for globular proteins, whereas another segment (or segments) had a stability comparable to that found for globular proteins. The stepwise unfolding were explained in terms of the coiled-coil interactions in tropomyosin. There were differences in stability between tropomyosins from different muscles of the same species and between species, no 2 tropomyosins gave the same denaturation profiles. The invertebrate tropomyosins showed a wider range of stabilities, that from scallop striated muscle being far more easily denatured than all the others. No correlation was found between the stability of tropomyosin and the type of regulatory system of the muscle. There was no selection for proteins of higher or lower stability during the evolutionary time scale.