Comparison of calmodulin and troponin C with and without its amino-terminal helix (residues 1-11) in the activation of erythrocyte Ca(2+)-ATPase.

Troponin C can replace calmodulin in the activation of the Ca(2+)-ATPase of pig erythrocytes provided that the reaction medium contains relatively high free Ca2+ concentrations (> 0.5 microM). In the presence of 10 microM free Ca2+, the troponin C-activated ATPase reaches a maximal velocity of approximately 70% of that attained with calmodulin. The half-maximal concentration for troponin C activation is about 200 times greater than for calmodulin. Troponin C displaces the half-maximal concentration for activation by Ca2+ to pCa 5.46 and the cooperativity between the Ca2+ binding sites to nH 1.1, compared with pCa 6.14 and nH 1.72 when calmodulin is used. Both EF-hand proteins also elicit activation by ATP at a nucleotide regulatory site, as well as a Ca(2+)-dependent p-nitrophenyl phosphatase activity. Troponin I prevents activation of the enzyme by troponin C. A mutant of troponin C with the amino-terminal helix deleted (NHdel) activates the Ca(2+)-ATPase to the same extent and with the same Ca2+ dependence as wild-type troponin C (rTnC); the half-maximal concentration for activation by NHdel is 2.5 times smaller than that for rTnC. We conclude that the structural features that distinguish the two EF-hand proteins affect their binding to the target enzyme more than their ability to transform the enzyme's response to Ca2+ or ATP. The differences in the amino-terminal domains of troponin C and calmodulin cannot account for the differences in ability of these proteins to activate the target system used as a model.