SOLUTION STRUCTURE OF CALCIUM-FREE CALMODULIN

The three-dimensional structure of calmodulin in the absence of Ca2+ h as been determined by three- and four-dimensional heteronuclear NMR ex periments, including ROE, isotope-filtering combined with reverse labe lling, and measurement of more than 700 three-bond I-couplings. In ana logy with the Ca2+-ligated state of this protein, it consists of two s mall globular domains separated by a flexible linker, with no stable, direct contacts between the two domains. In the absence of Ca2+, the f our helices in each of the two globular domains form a highly twisted bundle, capped by a short anti-parallel beta-sheet. This arrangement i s qualitatively similar to that observed in the crystal structure of t he Ca2+-free N-terminal domain of troponin C.