Influence of Ca2+ and trifluoperazine on the structure of calmodulin. A 1H-nuclear magnetic resonance study.

Ca2+-induced conformational changes of calmodulin under a variety of different experimental conditions have been studied by 1H-nuclear magnetic resonance techniques. The assignment for Tyr-99 has been corrected. Ca2+ titration performed at pH 7.5 and greater than 9.5 apparently induces a different sequence of the protein folding process as can be monitored by the resonances of His-107. These two structural forms cannot be interconverted. The phenylalanine residue(s) responsible for the resonances at 6.47 ppm (Ca2+-free form) and 6.64 ppm (Ca2+-saturated form) respectively, are apparently located close to Ca2+-binding sites III and IV. This can be recognized from nuclear Overhauser enhancement and Gd3+-broadening techniques. Gd3+-broadening experiments classify Ca2+-binding site IV as the site with the highest Gd3+/ca2+-exchange rate. The antipsychotic drug trifluoperazine, which is known to bind to calmodulin in a calcium-dependent way [Levin, R. M. and Weiss, B. (1977) Mol. Pharmacol. 13, 690-697], has been found to induce a conformational change of the Ca2+-saturated form of calmodulin. The methionine and phenylalanine residues were especially affected. Possible binding site(s) for trifluoperazine are discussed.