Local Conformational Changes of the Myosin Molecule Induced by Mg<sup>2+</sup> and ATP

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  • MIMURA TOSHIKO
    Department of Biochemistry, Juntendo University, School of Medicine.

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  • マグネシウムイオンおよびATPによるミオシン分子の微細な構造変化について
  • マグネシウム イオン オヨビ ATP ニヨル ミオシン ブンシ ノ ビサイ ナ

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In order to detect the local conformational changes in the myosin molecule induced by Mg2+ and nucleotides, we studied the changes through tryptic digestion of myosin in its ATPase activity, in its interaction with actin and in the pattern of its digestion products. The following results were obtained. 1) Mg2+-and Ca2+-ATPase activities of myosin showed a biphasic response-- first activated and then inhlbited--during the course of digestion. 2) Mg2+ slightly accelerated tryptic peptide-bond cleavage occuring in the active site of ATPase. ATP and ITP markedly increased this effect of Mg2+. 3) The activation of the Mg2+-ATPase activitiy by actin was quickly lost by tryptic digestion regardless of the degree of inactivation of myosin ATPase itself. Mg2+-ATP in the digestion medium slightly increased the change mentioned above. 4) The ability of myosin to accelerate G-F transformation of actin was found to be more resistant to tryptic digestion than its catalytic activity. In this respect Mg2+-ATP protected myosin against digestion. 5) Subfragments 1 and 2 were formed by tryptic digestion from myosin in the absence of Mg2+, but hardly obtained in the presence of Mg2+. Nucleotides, KCl concentration and actin added to the system did not affect this cutting-off of the head at all. Judging from these experimental data, sites of tryptic attack in the myosin molecule and the conformations around these sites were discussed.

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