Target sizes of human erythrocyte membrane Ca2+-ATPase and Mg2+-ATPase activities in the presence and absence of calmodulin

Abstract We have investigated the subunit structure of Ca 2+ -transport ATPase in human erythrocyte membranes using radiation inactivation analysis. All inactivation data were linear on a semilog plot down to at least 20% of the control activity. We found a target size for the calmodulin-dependent Ca 2+ -ATPase activity of 331 kDa, consistent with the presence of this enzyme as a dimer in calmodulin-depleted ghosts. Membranes which had been saturated with calmodulin before irradiation yielded a a similar size of 317 kDa, implying that activation of Ca 2+ -transport ATPase by calmodulin does not involve significant change in oligomeric structure. Basal (calmodulin-independent) Ca 2+ -ATPase activity corresponded to a size of 290 kDa, suggesting that this activity resides in the same, or similar-sized, complex as the calmodulin-dependent activity. Mg 2+ -ATPase activity, however, was found to reside in a smaller complex of 224 kDa, which proved to be statistically distinct from the target size of Ca 2+ -ATPase activity. It would appear that Mg 2+ -ATPase is a distinct entity whose function is likely unrelated to the Ca 2+ -transport ATPase.