Myosin from starfish egg: Properties and interaction with actin

Abstract

Egg myosin was purified from KCl extract of glycerinated starfish eggs. It was composed of three classes of polypeptides having chain weights of 210,000, 20,000 and 17,000 daltons. The molecular weight of egg myosin was estimated to be about 500,000 from the subunit chain weights or by combination of the Stokes' radius and the sedimentation coefficient. The ATPase activity of egg myosin was about 0·4 μ mol Pi/min per mg under conditions of 5 m m -Ca 2+ and 0·6 m -KCl at pH 6·5. ATP was hydrolyzed faster than other nucleoside triphosphates. K + + EDTA activation was observed at pH 8·0 to 8·5 at an EDTA concentration of 0·1 m m , although the activity was about 80% of the Ca 2+ -ATPase activity. SH-blocking reagents did not activate egg myosin ATPase activity but only inactivated. Egg myosin precipitated completely at an ionic strength of 0·2 forming aggregates of thick filaments. The thick filament formation began at an ionic strength of 0·4 to 0·45 and typical thick filaments were formed at an ionic strength of 0·35. The Mg 2+ -ATPase activity of egg myosin at low ionic strength was enhanced by rabbit skeletal muscle actin or by starfish egg actin up to sevenfold. The actin-egg myosin complex (acto-egg myosin) showed an arrowhead structure as viewed with an electron microscope. At an ionic strength of 0·1 or 0·2, Mg 2+ -ATPase activity of acto-egg myosin supplied with rabbit tropomyosin-troponin was higher in the presence of 0·05 m m -Ca 2+ than in the presence of 0·1 m m -ethylene glycol bis-( β -aminoethyl ether)- N,N′ -tetraacetic acid.