Conformational stability of the myosin rod

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Abstract
Chymotryptic cleavage patterns of myosin rods from pig stomach, chicken gizzard and rabbit skeletal muscle indicate that short (.apprxeq. 45 nm) heavy meromyosin subfragment 2 (SF2) is a consistent product of all 3 rods, whereas long (.apprxeq. 60 nm) SF2 is derived only from skeletal muscle myosin. Differential scanning calorimetry was used to follow the thermally induced melting transition of the rods and certain of their subfragments. In 0.12 M KCl, sodium phosphate buffer, pH 6.2-7.6, the light meromyosin (LMM) and SF2 domains of each rod had essentially identical conformational stabilities. Temperature midpoints for the melting transitions were 54-56.degree. C for the 2 smooth muscle myosin rods and 50-53.degree. C for the skeleltal muscle myosin rod. In 0.6 M KCl buffer, melting transitions for the smooth muscle myosin rods were essentially unchanged, but skeletal muscle myosin rods showed multiphase melting, with major transitions at 43.degree. C and 52.degree. C. The first of these was tentatively attributed to LMM, and the second to SF2. In 0.12 M KCl buffer, the LMM transition was stabilized so that it superimposed on that of SF2. No melting was observed in any of the rods at physiological temperature. These results indicate that, excluding a possible but only narrow hinge region, the entire myosin rod has essentially uniform conformational stability at physiological pH and ionic strength, and thus that the contractile and elastic properties of the cross-bridge exist in the heavy meromyosin subfragment 1 (SF1) domains of the molecule.