TROPONIN-I ENCOMPASSES AN EXTENDED TROPONIN-C IN THE CA2+-BOUND COMPLEX - A SMALL-ANGLE X-RAY AND NEUTRON-SCATTERING STUDY

We have studied the solution structure of skeletal muscle troponin C complexed with troponin I in the presence of calcium using small-angle X-ray and neutron scattering. 4Ca(2+).troponin C in the complex has an extended dumbbell shape with a radius of gyration of 23.9 +/- 0.5 Angstrom and a maximum linear dimension of approximate to 72 Angstrom, similar to the values obtained from the crystal structure coordinates of troponin C (Herzberg and James, 1985). Troponin I is even more extended than troponin C with a radius of gyration of 41 +/- 2 Angstrom and a maximum linear dimension of approximate to 118 Angstrom. The centers-of-mass for each component of the complex are approximately coincident (<10-Angstrom separation) as are their long axes, and the troponin I component encompasses the 4Ca(2+) troponin C. These data provide new insights into the nature of the conformational arrangement of this important Ca2+-sensitive molecular switch.