Differential scanning calorimetry of the unfolding of myosin subfragment 1, subfragment 2, and heavy meromyosin

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Abstract
The thermal unfolding of rabbit skeletal heavy meromyosin (HMM), myosin subfragment 1, and subfragment 2 has been studied by differential scanning calorimetry (DSC). Two distinct endotherms are observed in the DSC scan of heavy meromyosin. The first endotherm, with a Tm of 41.degree. C at pH 7.9 in 0.1 M KCl, is assigned to the unfolding of the subfragment 2 domain of HMM based on scans of isolated subfragment 2. The unfolding of the subfragment 2 domain is reversible both in the isolated form and in HMM. The unfolding of subfragment 2 in HMM can be fit as a single two-state transition with a .DELTA.Hvh and .DELTA.Hcal of 161 kcal/mol, indicating that subfragment 2 exists as a single domain in HMM. The unfolding of subfragment 2 is characterized by an extraordinarily large .DELTA.Cp of approximately 30,000 cal/deg.cntdot.mol). In the presence of nucleotides, the high-temperature HMM endotherm with a Tm of 48.degree. C shifts to higher temperature, indicating that this peak corresponds to the unfolding of the subfragment 1 domain. This assignment has been confirmed by comparison with isolated subfragment 1. The stabilizing effect of AmPPNP was significantly greater than that of ADP. The vanadate-trapped ADP species was slightly more stable than M.cntdot.AMPPNP with a Tm at 58.degree. C. The unfolding of subfragment 1, both in the isolated form and in HMM, was irreversible. Only a single endotherm was noted in the DSC scans of the subfragment 1 domain of HMM and in freshly prepared subfragment 1 complexes. There is no evidence in the DSC data for preferential stabilizing of any subdomain of the head by the binding of nucleotides.