Actin-induced local conformational change in the myosin molecule. I. Effect of metal ions and nucleotides on the conformational change around a specific thiol group (S2) of heavy meromyosin.:I. Effect of Metal Ions and Nucleotides on the Conformational Change around a Specific Thiol Group (S<sub>2</sub>) of Heavy Meromyosin

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  • KAMEYAMA Tsuneo
    Department of Biochemistry, School of Medicine, Juntendo University
  • KATORI Tokue
    Department of Biochemistry, School of Medicine, Juntendo University
  • SEKINE Takamitsu
    Department of Biochemistry, School of Medicine, Juntendo University

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説明

As previously reported, when a specific thiol group, S2, of myosin reacts with N-ethylmaleimide (NEM), its Ca2+-ATPase activity is decreased. Therefore, the reactivity of S2 can be estimated by measuring the decrement of the enzymatic activity. Using the change in the reactivity as a structural probe, we investigated whether F-actin affects the conformation around the region containing S2 under physiological conditions (at neutral pH and low ionic strength).<br> 1. Experiments were carried out with heavy meromyosin (HMM*), Sl of which had been blocked with NEM, to observe the reactivity of S2 alone. In the experiments done in the presence of F-actin, the Ca2+-ATPase activity was measured using the heavy meromyosin fraction after actin had been removed by centrifugation and gel filtration.<br> 2. ATP and other nucleotides activated the reactivity of S2 in the presence of Mg2+. On the other hand, F-actin without free nucleotide (Asakura's actin) scarcely affected its reactivity. However, F-actin markedly activated the reactivity of S2 which had been increased by ATP, but not by the other nucleotides.<br> 3. The above cooperative action of F-actin with ATP was not observed in the presence of Call instead of Mg2+, or above 0.2 M KCl.<br> These results suggest that the S2 region of the myosin molecule is a key region in the molecular interaction of the actin-myosin-ATP system under physiological conditions.

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