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 aas 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.