Human erythrocytes contain a calpain activator protein with a molecular mass of approx. 40 kDa. The activator is present in association with the plasma membrane and promotes expression of calpain activity at a concentration of Ca2+ close to physiological values. The initial step of the activating mechanism involves association of the activator with calpain, followed by autoproteolytic activation of the proteinase in the presence of 1 microM Ca2+, at a rate identical to that induced by 1 mM Ca2+. In a reconstituted system, the activator binds to erythrocyte membranes, but not to phospholipid vesicles, suggesting the participation of an intrinsic membrane protein(s). In its membrane-associated form the activator selectively binds calpain, thus favouring interaction of the proteinase with the inner surface of plasma membranes. These results further confirm the importance of a natural activator protein in promoting intracellular activation of calpain under physiological conditions through a site-directed mechanism, which explains the high specificity of the proteinase for membrane of cytoskeletal proteins.

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