Substructure of skeletal myosin subfragment 1 revealed by thermal denaturation

The stability of myosin subfragment 1 (S1) to thermal denaturation has been followed by limited tryptic proteolysis. Digestions done during the thermal denaturation show that at temperatures at and above 37 degrees C there is a marked increase in the susceptibility of S1 to tryptic degradation, as evidenced by the loss of all bands corresponding to the normally trypsin-resistant fragments of 50, 27, and 21 kDa of the heavy chain and to the light chain. The enhanced digestion of S1 appears to be due to a general unfolding of all segments of S1, although the 50-kDa segment appears to unfold at a lower temperature than the remainder of the S1 structure. Digestions done after 30-min exposure to higher temperatures or after subsequent cooling to 25 degrees C show marked differences in the susceptibility of the S1 to trypsin. This suggests that, on cooling, a substantial portion of the S1, but not the 50-kDa segment, is capable of refolding to a state corresponding closely to that in the native S1. These data indicate that in terms of thermal denaturation the S1 behaves as though it is comprised of two domains--an unstable 50-kDa domain and a more stable domain comprised of the 27- and 21-kDa segments of the heavy chain interacting with the light chain, as proposed recently by Setton and Muhlrad [Setton, A., & Muhlrad, A. (1984) Arch. Biochem. Biophys. 235, 411-417]. The rates of thermal inactivation of the ATPase of S1 are found to correspond closely to the decay rates for the 50-kDa fragment, suggesting that this segment in S1 is closely associated with the ATPase function of the protein.