The optical rotatory dispersion of myosin a IV. Conformational changes in meromyosins

The effect of 8% dioxane on the optical rotatory dispersion of myosin A purified by treatment with DEAE-cellulose was measured in 0.6 M KCl and at pH 7.0 and 20°. The increase in the —b₀ term by 5% was followed by a gradual decrease to 0.88 of the original. The —b₀ term of L₁-meromyosin in 0.6 M KCl and at pH 7.0 and 20% was 575 and was unchanged by the addition of dioxane, ATP of PP₁. On the other hand, the —b₀ term of H₂-meromyosin in 0.3 M KCl and at pH 7.0 and 20% was 263. It increased by several percent immediately after the addition of 8% dioxane and decreased gradually with time. It increased by several percent on the addition of 4 moles p-chloromercuribenzoate, but decreased by several percent on the addition of 8 moles p-chloromercuribenzoate per 10⁵ g protein. It was also decreased by the binding of H₂-meromyosin with F-actin. ATP increased the —b₀ term of H₂-meromyosin by several percent, though PP₁ decreased the term by several percent. Thus these reagents change the helical content of H₂-meromyosin in ways similar to the case of myosin A. The extents of the changes in the helical content of H₂-meromyosin by F-actin and ATP were, however, higher than those observed in myosin A.