Purification of the Ca2+-stimulated ATPase activator from human erythrocytes. Its membership in the class of Ca2+-binding modulator proteins

The activator of the Ca2+-stimulated ATPase of erythrocyte membranes was purified 13,000-fold to homogeneity from human erythrocytes. The protein gave a single band upon electrophoresis both with and without detergent, and upon isoelectric focusing. This protein was compared with Ca2+-binding modulator proteins from bovine brain and rat testis. All three proteins were homogeneous and co-migrated on electrophoresis both in the presence of detergent and without detergent at pH values on both sides of the isoelectric point of the protein. The amino acid compositions of the three proteins were nearly indistinguishable, and all three proteins contained 1 residue of the unusual amino acid, trimethyllysine. All three were also indistinguishable as measured by their ability to further stimulate the Ca2+-stimulated ATPase of human erythrocyte membranes. Thus, we conclude that they represent functionally the same protein. Upon storage of all three proteins, a second band was detectable by detergent gel electrophoresis; the biochemical activity and the behavior on nondetergent gels were not changed. The presence of this second band is probably responsible for previous reports of differences between the rat testis and bovine brain modulator protein. The possibility is discussed that this protein is a general intracellular Ca2+ receptor, which mediates the activities of Ca2+ as an intracellular messenger.