Stability of two beta-tropomyosin isoforms: effects of mutation Arg91Gly.

In order to comprehend the domain structure of two beta-tropomyosin (beta-Tm) isoforms (skeletal muscle and smooth muscle beta-Tm) and the influence of the disease-causing mutation Arg91Gly on it, we studied the thermal unfolding of these tropomyosin species by means of differential scanning calorimetry (DSC). Our results show that the studied point mutation dramatically decreases thermal stability of a significant part of both beta-Tm isoforms (about a half of the molecule) that unfolds as a cooperative unit (calorimetric domain). We have assigned this domain to the N-terminal part of the molecule combining, in the case of smooth muscle beta-Tm, DSC studies with measurements of temperature dependence of pyrene excimer fluorescence, whose decrease reflects dissociation of two beta-Tm chains in the region of pyrene-labeled Cys-36. Interestingly, the destabilizing effect of the mutation spreads along the coiled-coil reflecting the high extent of cooperativity within this part of the beta-Tm molecule.