Characterization of a Ca2+-stimulated Mg2+-dependent adenosine triphosphatase in Friend murine erythroleukemia cell plasma membranes.

Since agents which affect Ca2+ fluxes have been shown to affect the commitment to terminal differentiation of murine erythroleukemia cells treated with an inducer of differentiation (Bridges, K., Levenson, R., Housman, D., and Cantley, L. (1981) J. Cell Biol. 90, 542-544), we investigated the presence of Ca2+-transport systems in plasma membranes isolated from these cells. Plasma membranes from murine erythroleukemia cells exhibited a high affinity (Ca2+-Mg2+)-ATPase activity with a Vmax of 29 +/- 4 nmol/mg/min and an apparent K0.5 for free Ca2+ of 0.13 microM. This activity was strongly inhibited in a dose-dependent fashion by low concentrations of trifluoperazine, compound 48/80, lanthanum, and vanadate with I50 of 29 microM, 1.75 micrograms/ml, 1.5 microM, and 0.4 microM, respectively. The inhibitory effect of compound 48/80 was specifically reversed by exogenously added calmodulin. Phosphorylation of plasma membranes with [gamma-32P] ATP followed by sodium dodecyl sulfate electrophoretic analysis at low pH revealed a Ca2+-dependent phosphoprotein with an apparent molecular mass of 138,000 daltons which co-migrated with the Ca2+-dependent phosphoprotein from human erythrocytes and was separable from sarcoplasmic reticulum (Ca2+-Mg2+)-ATPase. The 32P incorporated into the phosphoprotein could be chased with unlabeled ATP and the protein-phosphate bond was unstable at alkaline pH suggesting an acylphosphate ATPase intermediate like that previously characterized in other (Ca2+-Mg2+)-ATPases.