Solution structure of calcium-free calmodulin

The three-dimensional structure of calmodulin in the absence of Ca 2+ has been determined by three- and four-dimensional heteronuclear NMR experiments, including ROE, isotope-filtering combined with reverse labelling, and measurement of more than 700 three-bond J -couplings. In analogy with the Ca 2+ -ligated state of this protein, it consists of two small globular domains separated by a flexible linker, with no stable, direct contacts between the two domains. In the absence of Ca 2+ , the four helices in each of the two globular domains form a highly twisted bundle, capped by a short anti-parallel β-sheet. This arrangement is qualitatively similar to that observed in the crystal structure of the Ca 2+ -free N-terminal domain of troponin C.