Specific Cleavage of Calmodulin-binding Proteins by Low Ca2+-requiring Form of Ca2+-activated Neutral Protease in Human Platelets

Occurrence of Ca2+-dependent calmodulin-binding proteins in lysed human platelets and their cleavage by low Ca2+-requiring Ca2+-activated protease were investigated by a gel overlay technique using [125I]calmodulin. Calmodulin-binding polypeptides of Mr 100K, 90K, 60K, and 40K were detected in lysed platelets, of which 90K and 60K polypeptides were rapidly degraded to lower molecular weight products in the presence of micromolar concentrations of Ca2+. Then, we investigated cleavage of calmodulin-binding proteins by purified low Ca2+-requiring Ca2+-activated neutral protease from human platelets. As substrate, myosin light chain kinase and caldesmon purified from the chicken gizzard smooth muscle were used. In the presence of micromolar concentration of Ca2+, these two proteins were also rapidly degraded to lower molecular weight species, which were still capable of binding to calmodulin.