The Molecular Switch in Troponin C

Abstract

Conformational changes in troponin C (TnC) associated with Ca²⁺-induced triggering of muscle contraction are discussed in light of the model proposed by Herzberg, Moult and James (J. Biol. Chem. 261, , 2638, 1986) and of our recent work on mutants of troponin C. The model involves a Ca²⁺-induced angular movement of one pair of α-helical segments relative to another pair of helices in the N-terminal domain. A disulfide bridge introduced into the N-terminal domain reversibly blocks the key conformational transition and the Ca²⁺-regulatory activity. Binding of troponin I (TnI) to the disulfide form of TnC is weakened owing to the blocking of its interaction with the N-terminal domain; however incorporation of the mutant into TnC-extracted myofibrils is not abolished. Introduction of a Cys residue in the C-terminal domain of TnC leads to disulfide formation between it and the indigenous Cys-98, with accompanying inhibition of regulatory activity attributable to interference with binding to TnI and, consequently, incorporation into the thin filaments. Evidence for movement of helical segments upon Ca²⁺-binding to TnC was obtained by measurements of excimer fluorescence and of resonance energy transfer with probes attached to Cys residues introduced by site-directed mutagenesis at suitable locations. Introduction of a disulfide bridge into calmodulin, another member of the super-family of Ca²⁺-binding proteins to which TnC belongs, abolishes its interaction with target enzymes. This suggests that the type of conformational change involving angular movement of helical segments that takes place in TnC is also involved in signal transmission in other Ca²⁺-dependent regulatory proteins.