Actin-Induced Local Conformational Change in the Myosin Molecule

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Abstract
Experiments were carried out with [rabbit muscle] heavy meromyosin (HMM), S1 (thiol group) of which had been blocked with NEM (N-ethylmaleimide), to observe the reactivity of S2 alone. In the presence of F-actin, the Ca2+-ATPase activity was measured using the HMM fraction after actin was removed by centrifugation and gel filtration. ATP and other nucleotides activated the reactivity of S2 in the presence of Mg2+. F-actin without free nucleotide (Asakura''s actin) scarcely affected its reactivity. F-actin markedly activated the reactivity of S2 which was increased by ATP, but not by the other nucleotides. The above cooperative action of F-actin with ATP was not observed in the presence of Ca2+ instead of Mg2+, or above 0.2 M KCl. The S2 region of the myosin molecule was a key region in the molecular interaction of the actin-myosin-ATP system under physiological conditions.