<Ca^(2+)> regulatory mechanism of smooth muscle contraction

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Phosphorylation of myosin light chains by Ca-calmodulin dependent kinase is now widely believed to be the molecular mechanism of Ca-regulated activation of smooth muscle contraction. We have, however, proposed that the actin-binding protein, named 'leiotonin', is the real activator. In the present study, following results have been obtained, all of which demonstrate the discrepancy between 'kinase activity' and contraction-. stimulating activity.1) Digestion of 'activator fraction' by <V_8> protease, CANP or trypsin scarcely affected its kinase activity, but almost completely destroyed its contraction-stimulating activity.2) Chemical modification of Lys, Tyr or Cys residues of 'activator fraction' resulted in the loss of its contraction-stimulating activity, while sparing its kinase activity.3) Kinase activity was decreased with increase in pH from 6.8 to 8.0, while contraction-stimulating activity was almost the same in that range of pH.