TROPONIN-C MUTANTS WITH INCREASED CALCIUM AFFINITY

Binding of two Ca2+ to the regulatory sites I and II of troponin C (TnC) induces a conformational transition believed to be responsible for the activation of muscle contraction. Based on the known crystal structure (2Ca2+ state), a model for the transition to the 4Ca2+ state has been proposed [Herzberg, O., Moult, J. & James, M. N. G. (1986) J. BioL Chem. 261, 2638-2644]. The proposed conformational transition predicts that during Ca2+ binding a number of nonpolar residues become exposed to the solvent, creating a hydrophobic patch. Such a model implies that mutation of the hydrophobic to polar residues should increase the Ca2+ affinity at the regulatory sites and reduce the Ca2+ concentration necessary for muscle activation. To test this prediction, we have constructed and functionally characterized two troponin-C mutants (V45T and M48A mutations). Direct calcium-binding measurements in the mutants demonstrate an increase in the Ca2+ affinity for two low-affinity sites. Replacement of endogenous troponin C in skinned muscle fibers by TnC with mutations V45T or M48A increased the Ca2+ sensitivity of their tension development. These results show that the model can be used to construct mutants that regulate muscle contraction at lower Ca2+ concentrations. They provide further experimental support for the proposed calcium-induced conformational change of troponin C and suggest that the predicted transition plays a central role in the activation of the thin filament.