Fluorescence studies on calmodulin binding to erythrocyte Ca2(+)-ATPase in different oligomerization states

The fluorescent spinach calmodulin derivative 2-(4-maleimidoanilino)naphthalene-6-sulfonic acid-calmodulin (MIANS-CaM) was used to investigate calmodulin interaction with the purified, detergent-solubilized erythrocyte Ca2(+)-ATPase. Previous studies have shown that the Ca2(+)-ATPase exists in equilibria between monomeric and oligomeric forms. We report here that MIANS-CaM binds to both enzyme forms in a Ca2(+)-dependent manner, with a approximately 50% fluorescence enhancement. These findings confirm our previous observation that enzyme oligomers retain their ability to bind calmodulin, even though they are fully activated in the absence of calmodulin. The Ca2+ dependence of MIANS-CaM binding to monomeric Ca2(+)-ATPase is of higher affinity (K 1/2 = 0.09 microM Ca2+) and less cooperative (nH = 1.1) than the Ca2+ dependence of enzyme activation by MIANS-CaM (K 1/2 = 0.26 microM Ca2+, nH = 2.8). These Ca2+ dependences and the order of events, in which calmodulin binding precedes enzyme activation, demonstrate that calmodulin indeed could be a physiological activator of the monomeric enzyme. The calcium dependence of calmodulin binding to oligomeric Ca2(+)-ATPase occurs at even lower levels of Ca2+ (K 1/2 = 0.04 microM Ca2+), in a highly cooperative fashion (nH = 2.3), and essentially in parallel with enzyme activation (K 1/2 = 0.05 microM Ca2+, nH = 2.9). The observed differences between monomers and oligomers suggest that the oligomerized Ca2(+)-ATPase is in a conformation necessary for efficient, cooperative calcium binding at nanomolar Ca2+, which the monomeric enzyme acquires only upon interaction with calmodulin.(ABSTRACT TRUNCATED AT 250 WORDS)