Actin-Induced Local Conformational Change in the Myosin Molecule: II. Conformational Change around the S2 Thiol Group Related to the Essential Intermediate of ATP Hydrolysis

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Abstract
The effect of F-actin on the conformation around a specific thiol group, S2, in heavy meromyosin was investigated. The extent of the change was estimated from the residual activity of Ca2+-ATPase after modifying the thiol with N-ethylmaleimide (NEM). Experiments were carried out with a modified heavy meromyosin (HMM*), in which S1 had been blocked with NEM, to observe the reactivity of S2 alone. 1. F-Actin markedly increased the ATP-induced conformational change around S2, but did not affect the adenylyl imidodiphosphate (AMPPNP)-induced change, and markedly supressed the adenylyl methylenediphosphate (AMPPCP)-induced change. 2. The initial burst of HMM ATPase was retained after the modification of S1. Replacement of Mg2+ with Mn2+ in the medium reduced the cooperative action of F-actin and ATP with concomitant loss of the initial burst. 3. Nevertheless, F-actin was capable of activating the steady-state ATPase activity of HMM* even in the presence of Mn2+. 4. The degree of activation by F-actin of the ATP-induced increase in the reactivity of S2 did not parallel that of the steady-state ATP splitting, when the KCl concentration of the medium was varied. The results indicate that the actin-induced local conformational change in the S2 region is related to an energized state of the myosin molecule caused by Mg2+- ATP, but is apparently not related to the actin-activated steady-state ATPase activity.