Interaction of the purified Ca2+, Mg2+-ATPase from human erythrocytes with phospholipids and calmodulin

The Ca2+-pumping ATPase has been purified in a functional form from human erythrocytes by calmodulin affinity chromatography. The purified enzyme has a specific activity at least 300-fold higher than the membrane bound enzyme. It consists of one major protein band of 140000 Dalton, and after reconstitution in liposomes it transports Ca2+ with an efficiency of at least 1 Ca2+/ATP. In the presence of calmodulin, the affinity of the enzyme for Ca2+, and its specific activity, are greatly increased. Acidic phospholipids have an unexpected effect on the isolated enzyme: ATPase isolated or reconstituted in acidic phospholipids behaves as if calmodulin were present. Acidic phospholipids mimic the effect of calmodulin.