The structure of the myosin subfragment-1 (SI) from rabbit skeletal muscle was studied using differential scanning microcalorimetry. Three independently melting regions (domains) were revealed in S1. Selective denaturation of the middle 50 kDa segment of the S1 heavy chain resulted in the disappearance of the heat sorption peak corresponding to the melting of the first, the most thermolabile domain without any effect on the thermally induced blue shift of the intrinsic tryptophan fluorescence spectrum which occurs within the temperature region of melting of the second domain. It is concluded that the most thermolabile domain seems to correspond to the N-terminal part of the 50 kDa segment devoid of tryptophan residues.
