Calpain I Activates Ca²⁺ Transport by the Human Erythrocyte Plasma Membrane Calcium Pump

Abstract

Many extracellular signals, including hormones, exert their effects on cells by elevating free Ca²⁺ concentration. For this reason, it is important for a resting cell to maintain a submicromolar concentration of free Ca²⁺ in the cytosol. In red cells, this function is largely provided by the plasma membrane-bound Ca²⁺- translocating ATPase. Both Ca²⁺ translocating and ATP-hydrolytic activities of this enzyme are stimulated by calmodulin (CaM) via reversible binding (see Al-Jobore et al., 1981). We recently reported an irreversible means of activating the ATP-hydrolytic activity of the enzyme, involving the cytosolic Ca²⁺ -dependent protease (calpain I) (Wang et al., 1988a; 1988b). However, it is not yet established whether the calcium translocating activity of the enzyme is also activated by calpain I. In this study, we further examine the effect of calpain I on the liposome-reconstituted calcium pump.