Calcium-binding properties of human erythrocyte calpain

The results presented provide more information on the sequential mechanism that promotes the Ca2+-induced activation of human erythrocyte mu-calpain under physiological conditions. The primary event in this process corresponds to the binding of Ca2+ to eight interacting sites, of which there are four in each of the two calpain subunits. Progressive binding of this metal ion is linearly correlated with the dissociation of the proteinase, which reaches completion when all eight binding sites are occupied. The affinity for Ca2+ in the native heterodimeric calpain is increased 2-fold in the isolated 80 kDa catalytic subunit, but it reaches a Kd consistent with the physiological concentration of Ca2+ only in the active autoproteolytically derived 75 kDa form. Binding of Ca2+ in physiological conditions, and thus the formation of the 75 kDa subunit, can occur only in the presence of positive modulators. These are represented by the natural activator protein, found to be a Ca2+-binding protein, and by highly digestible substrates. The former produces a very large increase in the affinity of calpain for Ca2+, and the latter a smaller but still consistent decrease in the Kd of the proteinase for the metal ion. As a result, both dissociation into the constituent subunits and the autoproteolytic conversion of the native 80 kDa subunit into the active 75 kDa form can occur within the physiological fluctuations in Ca2+ concentration. The delay in the expression of the proteolytic activity with respect to Ca2+ binding to native calpain, no longer detectable in the 75 kDa form, can be attributed to a Ca2+-induced functional conformational change, which is correlated with the accessibility of the active site of the enzyme.