COMPARISON OF CALMODULIN AND TROPONIN-C WITH AND WITHOUT ITS AMINO-TERMINAL HELIX (RESIDUES-1-11) IN THE ACTIVATION OF ERYTHROCYTE CA2-ATPASE()

Troponin C can replace calmodulin in the activation of the Ca2+-ATPase of pig erythrocytes provided that the reaction medium contains relati vely high free Ca2+ concentrations (>0.5 muM). In the presence of 10 m uM free Ca2+, the troponin C-activated ATPase reaches a maximal veloci ty of approximately 70% of that attained with calmodulin. The half-max imal concentration for troponin C activation is about 200 times greate r than for calmodulin. Troponin C displaces the half-maximal concentra tion for activation by Ca2+ to pCa 5.46 and the cooperativity between the Ca2+ binding sites to n(H) 1.1, compared with pCa 6.14 and n(H) 1. 72 when calmodulin is used. Both EF-hand proteins also elicit activati on by ATP at a nucleotide regulatory site, as well as a Ca2+-dependent p-nitrophenyl phosphatase activity. Troponin I prevents activation of the enzyme by troponin C. A mutant of troponin C with the amino-termi nal helix deleted (NHdel) activates the Ca2+-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 small er than that for rTnC. We conclude that the structural features that d istinguish 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 the se proteins to activate the target system used as a model.