Self-association of plasma membrane Ca²⁺-ATPase by volume exclusion

At enzyme concentrations above 40 nM the configuration of the purified plasma membrane Ca²⁺-ATPase is that of calmodulin-insensitive dimers. Dilution of the enzyme generates progressively higher proportions of calmodulin-sensitive monomers with lower V_max and Ca²⁺ sensitivity than the dimeric enzyme. Dimerization from monomeric state had not been documented before. We investigated whether concentration by volume exclusion, obtained by addition of a large molecular weight dextran to a monomeric Ca²⁺-ATPase would elicit dimer-like behavior. Dextran induced self-association of monomers, as monitored by fluorescence energy transfer, but the Ca²⁺ sensitivity of the re-associated monomers was lower than that of the native dimers. These results suggest that the self-association reaction is structurally but not functionally reversible, and also document the existence of a hitherto unknown kinetic state of the oligomerized Ca²⁺-ATPase, with high V_max but low Ca²⁺-sensitivity.