The active species of plasma membrane Ca2+-ATPase are a dimer and a monomer-calmodulin complex.
The purified plasma membrane Ca2+-ATPase is fully activated through the enzyme concentration-dependent self-association at physiologically relevant Ca2+ concentrations (Kosk-Kosicka, D., and Bzdega, T. (1988) J. Biol. Chem. 263, 18184-18189; Kosk-Kosicka, D., Bzdega, T., and Wawrzynow, A. (1989) J. Biol. Chem. 264, 19495-19499). We have previously shown that the Ca2+-ATPase activity of the oligomeric enzyme is independent of calmodulin, in contrast to another active enzyme species, a presumable monomer, that is activated by calmodulin binding. Presently, we have succeeded in determining the molecular mass of the two active enzyme species by equilibrium ultracentrifugation. For the calmodulin-dependent species, the molecular mass is 170 +/- 30 kDa, which is consistent with predominantly monomeric Ca2+-ATPase with bound calmodulin. The molecular mass of calmodulin-independent oligomers is 260 +/- 34 kDa, indicating that they are dimers. Results of experiments performed under different calcium and potassium concentrations and in the presence of dextran that causes molecular crowding verify a strict Ca2+ requirement of the dimerization process. We conclude that the active species of the Ca2+-ATPase are a monomer-calmodulin complex and a dimer.
Sackett
D L
DL
Laboratory of Biochemical Pharmacology, NIDDK, National Institutes of Health, Bethesda, Maryland 20892, USA.
Kosk-Kosicka
D
D
D002118:Calcium / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000097:blood / Q000737:chemistry*; D002147:Calmodulin / Q000097:blood*; D002148:Calmodulin-Binding Proteins / Q000097:blood*; D004910:Erythrocyte Membrane / Q000201:enzymology*; D006801:Humans; D046911:Macromolecular Substances; D008565:Membrane Proteins / Q000737:chemistry; D008970:Molecular Weight; D011188:Potassium / Q000378:metabolism; D011485:Protein Binding
D002147:Calmodulin; D002148:Calmodulin-Binding Proteins; D046911:Macromolecular Substances; D008565:Membrane Proteins; D000252:Calcium-Transporting ATPases; D011188:Potassium; D002118:Calcium