Electron microscopy revealed that bodywall muscle of the annelid
Urechis unicinctus consisted of B type smooth muscle cells containing parallel thick (40nm in diameter) and thin (7nm) filaments. Myosin was extracted from
Urechis bodywall muscle and purified to homogeneity.
Urechis myosin, two-headed myosin, consisted of 200kDa heavy chain and two species of light chains, 24 and 19kDa. The ATPase activity was approximately 0.1μmole/mg/min in the presence of Ca
2+ or EDTA. The Mg
2+-ATPase activity was remarkably enhanced from 0.005 to 0.2μmole/mg/min by rabbit skeletal muscle actin. EGTA inhibited this actin-activated ATPase activity suggesting that the regulation by calcium ion is of the scallop type.
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