title: 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 Ca2+-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 Ca2+-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 yield a a similar size of 317 kDa, implying that activation of Ca2+-transport ATPase by calmodulin does not involve significant change in oligomeric structure. Basal (calmodulin-independent) Ca2+-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. Mg2+-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 Ca2+-ATPase activity. It would appear that Mg2+-ATPase is a distinct entity whose function is likely unrelated to the Ca2+-transport ATPase.
authors.forename: L
authors.initials: L
authors.lastname: Hymel
authors.forename: M
authors.initials: M
authors.lastname: Nielsen
authors.forename: K
authors.initials: K
authors.lastname: Gietzen
mesh_terms: D000251:Adenosine Triphosphatases / Q000037:antagonists & inhibitors / Q000097:blood* / Q000528:radiation effects; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors / Q000097:blood* / Q000528:radiation effects; D002147:Calmodulin / Q000494:pharmacology*; D004307:Dose-Response Relationship, Radiation; D004910:Erythrocyte Membrane / Q000201:enzymology*; D006801:Humans; D007700:Kinetics; D046911:Macromolecular Substances; D008970:Molecular Weight
chemical_list: D002147:Calmodulin; D046911:Macromolecular Substances; D000251:Adenosine Triphosphatases; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases