STRUCTURAL DETAILS OF A CALCIUM-INDUCED MOLECULAR SWITCH - X-RAY CRYSTALLOGRAPHIC ANALYSIS OF THE CALCIUM-SATURATED N-TERMINAL DOMAIN OF TROPONIN-C AT 1.75 ANGSTROM RESOLUTION

We have solved and refined the crystal and molecular structures of the calcium-saturated N-terminal domain of troponin C (TnC) to 1.75 Angst rom resolution. This has allowed for the first detailed analysis of th e calcium binding sites of this molecular switch in the calcium-loaded state. The results provide support for the proposed binding order and qualitatively, for the affinity of calcium in the two regulatory calc ium binding sites. Based on a comparison with the high-resolution ape- form of TnC we propose a possible mechanism for the calcium-mediated e xposure of a large hydrophobic surface that is central to the initiati on of muscle contraction within the cell. (C) 1997 Academic Press Limi ted.