Membrane binding and autolytic activation of calpain-I in human platelets.

The binding of calpain-I (Ca2+ activated neutral protease with high Ca2+ sensitivity) to the membranes of human platelets and the subsequent autolytic activation of calpain-I were analyzed by an immunoblot technique. In A23187 stimulated platelets, cytosolic calpain-I translocated to the membranes with autolysis in a Ca2+ dependent manner and simultaneously underwent autolysis. Although calpeptin, a cell permeable calpain inhibitor, inhibited autolysis of calpain-I, it was unable to prevent the translocation of calpain-I. In a cell re-constituted system, the membrane binding of calpain-I was also Ca2+ dependent and was significantly inhibited by a substrate of calpain. It was suggested that the binding of calpain-I to the membranes required the substrate binding site of this enzyme.