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.