Calcium binding to skeletal muscle troponin C and the regulation of muscle contraction.

Skeletal muscle contraction is initiated by Ca2+ ion binding to troponin C (TnC), a protein of the thin filament. Our three-dimensional structure determination of turkey skeletal TnC at 2.8 A resolution revealed an extended molecule consisting of two domains connected through a long nine-turn alpha-helix. The C-terminal domain has two Ca2+ ions bound in the expected manner of EF hands, whereas the N-terminal regulatory domain is Ca2+-free with a helix-loop-helix conformation different from that of an EF hand. The refinement of TnC at 2.2 A resolution highlights the intricate hydrogen-bonded network common to the Ca2+-bound loops and provides an explanation for the presence of a water molecule in the 5th coordination position of the Ca2+ ion. We propose that Ca2+ binding to the regulatory domain is accompanied by a conformational transition by which its structure becomes similar to that of the C-terminal domain. Dramatic movements of residues in the B and C helices and the connecting peptide of up to 14 A constitute the bulk of this change. A hydrophobic site that could be the site of interaction with troponin I is thereby exposed. We have also demonstrated that this model of the Ca2+-bound conformation can be reached from the Ca2+-free state without having to surmount large energy barriers.