ABSTRACT
The global conformation of cardiac muscle troponin I (cTnI) was investigated with single-cysteine mutants by using a combination of sulfhydryl reactivity and fluorescence resonance energy transfer (FRET) to determine cysteine accessibility and intersite distances. The reactivity was determined with a fluorescent reagent for its reaction with cysteine residues singly located at positions 5, 40, 81, 98, 115, 133, 150, 167, and 192. FRET measurements were made by using the endogenous single Trp-192 as the energy donor and an acceptor probe covalently attached to the cysteines as energy acceptor. The results suggest an open and extended conformation of cTnI with a large curvature in which the cysteines are highly exposed to the solvent. These conformational features are largely retained in the segment between residues 40 and 192 upon phosphorylation at Ser-23 and Ser-24. The sulfhydryl groups of the Cys-133 and Cys-150 of the cTnI incorporated into the binary cTnC-cTnI and fully reconstituted troponin complexes experience large reduced exposure resulting from the binding of Ca(2+) to the regulatory site of cTnC, suggesting that key regions of cTnI involved in activation become highly shielded upon activation. In the cTnC-cTnI complex, every intramolecular distance in the cTnI is lengthened and the overall conformation of the bound cTnI remains elongated with reduced exposure for the cysteines. The global conformation of the troponin C-troponin I complex from cardiac muscle has an elongated shape with constrained flexibility. The highly flexible nature of the N-terminal extension of cTnI is preserved in the complex, suggesting that this segment of cTnI is either not bound or only loosely bound to the C-domain of cTnC.
