High-resolution differential scanning calorimetric study of myosin, functional domains, and supramolecular structures

High-resolution differential scanning calorimetry (DSC) has been employed to study the thermal stability of myosin, its major constitutive fragments (S-1, light chains, and rod), and also reconstituted thick filaments. The thermal denaturation of soluble myosin was complex and was characterized by a multistep endothermic process for the temperature range from 41 to 60 degrees C. The shape of the endotherm was highly dependent on the pH and the ionic strength of the solution, although the delta Hcal (calorimetric enthalpy) of denaturation (1715 +/- 75 kcal/mol) was insensitive to these changes for the solvent conditions used in this study. This value also agrees, within experimental error, with the sum of the denaturation enthalpies obtained for isolated fragments (1724 +/- 79 kcal/mol). In identical conditions of ionic strength, pH, and heating rate, the computer-calculated differential endotherms of domains belonging to S-1 and light chains were superimposable with those of the isolated fragments. Their responses to changes in the solvent condition were also similar. We suggest that the observed functional independence of the major domains in myosin reflects also the independence of their structural stability. The thermal unfolding of the isolated rod was multiphasic and readily reversible (95%). It occurred between 41 and 60 degrees C, with an delta Hcal of 1058 +/- 59 kcal/mol. The melting of S-1 showed a single peak at 46.3 +/- 0.1 degrees C with an delta Hcal of 255 +/- 12 kcal/mol. Light chains melted at 51.0 +/- 0.2 degrees C with an delta Hcal of 85 +/- 15 kcal/mol.(ABSTRACT TRUNCATED AT 250 WORDS)