Ca has been found to increase the quantity of 32P incorporated into red cell ghosts from [γ-32P]ATP over the levels obtained by incubation with Mg alone or with Mg + Na, in correlation with the effect of Ca on the associated ATPase activities. When the 32P-labeled ghosts were solubilized in sodium dodecyl sulfate (SDS) and electrophoresed on acrylamide gels only two bands could be detected either by autoradiography or by counting the sliced gels. The faster moving band (P-2) had the same mobility and the same molecular weight (103,000) as the phosphoprotein found either with Mg alone or with Mg + Na. The slower moving band (P-1) was not found in extensively washed ghosts labeled in the absence of Ca. The molecular weight of P-1 is approximately 150,000. P-1 like P-2 was not affected by pretreatment of intact cells with Pronase before labeling indicating that neither the phosphorylating mechanism nor the phosphoprotein are accessible to externally applied Pronase. The demonstration that a Ca-phosphoprotein is separable from the Na-stimulated phosphoprotein suggests that the Ca-ATPase is distinct from and independent of the Na,K-ATPase. The fact that Ca blocks the dephosphorylation by K of the Na-phosphoprotein indicates that caution is required in interpreting results when the activities of the different phosphoproteins have not been separately determined.

Knauf, P. A., Proverbio, F., & Hoffman, J. F. (1974). Electrophoretic Separation of Different Phophosproteins Associated with Ca-ATPase and Na,K-ATPase in Human Red Cell Ghosts. The Journal of General Physiology, 63(3), 324–336.