Interaction between cytoplasmic (Ca2+--Mg2+) ATPase activator and the erythrocyte membrane

Human red blood cells (RBC) contain a cytoplasmic, nonhemoglobin protein which activates the (Ca2+-Mg2+)ATPase of isolated RBC membranes. Results presented in this paper confirm that activation of (Ca2+-Mg2+)ATPase is associated with binding of the cytoplasmic activator to the membrane. Binding of the cytoplasmic activator is reversible and dependent on ionic strength and Ca2+. Cytoplasmic activator is sensitive to trypsin but is not degraded when intact RBC are exposed to trypsin. Cytoplasmic activator does not modify the (Ca2+-Mg2+)-ATPase of membranes from RBC exposed to activator prior to hemolysis. Thus, the activator is located in the cell and appears to act by binding to the inner membrane surface.