The three-dimensional structure of the complex between calmodulin (CaM) and a peptide corresponding to the N-terminal portion of the CaM-binding domain of the plasma membrane calcium pump, the peptide C20W, has been solved by heteronuclear three-dimensional nuclear magnetic resonance (NMR) spectroscopy. The structure calculation is based on a total of 1808 intramolecular NOEs and 49 intermolecular NOEs between the peptide C20W and calmodulin from heteronuclear-filtered NOESY spectra and a half-filtered experiment, respectively. Chemical shift differences between free Ca(2+)-saturated CaM and its complex with C20W as well as the structure calculation reveal that C20W binds solely to the C-terminal half of CaM. In addition, comparison of the methyl resonances of the nine assigned methionine residues of free Ca(2+)-saturated CaM with those of the CaM/C20W complex revealed a significant difference between the N-terminal and the C-terminal domain; i.e., resonances in the N-terminal domain of the complex were much more similar to those reported for free CaM in contrast to those in the C-terminal half which were significantly different not only from the resonances of free CaM but also from those reported for the CaM/M13 complex. As a consequence, the global structure of the CaM/C20W complex is unusual, i.e., different from other peptide calmodulin complexes, since we find no indication for a collapsed structure. The fine modulation in the peptide protein interface shows a number of differences to the CaM/M13 complex studied by Ikura et al. [Ikura, M., Clore, G. M., Gronenborn, A. M., Zhu, G., Klee, C. B., and Bax, A. (1992) Science 256, 632-638]. The unusual binding mode to only the C-terminal half of CaM is in agreement with the biochemical observation that the calcium pump can be activated by the C-terminal half of CaM alone [Guerini, D., Krebs, J., and Carafoli, E. (1984) J. Biol. Chem. 259, 15172-15177].

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The three-dimensional structures of the isolated Cd(3)beta domains from Homarus americanus metallothionein have been determined by NMR methods in order to establish a set of beta-domain structures for comparative analysis. First, it was determined that the Cd-cysteine connectivities forming the Cd(3)S(9) metal center were identical to those observed for the beta(N) domain in the native holoprotein. Time- and temperature-dependence studies of the (113)Cd and (1)H 1D-NMR spectra indicated that the beta(N) domain undergoes slow conformational changes before reaching an equilibrium structure. In addition to structural information provided by the metal-to-cysteine connectivities, Phi, chi(1) and chi(2) angle constraints, three H(N...)S hydrogen bond interactions were also determined from a long-range optimized (1)H(N)-(113)Cd HMQC experiment. A simulated annealing protocol was applied to the distance and angle constraints obtained from the 2D-NMR experiments to calculate the three-dimensional structure of the synthetic Cd(3)beta(N) domain of lobster metallothionein. Structure-reactivity relationships are proposed for the reactions of Cd(3)beta domains with 5,5'-dithiobis(2-nitrobenzoate), based on comparisons of surface exposure of sulfur atoms of the lobster and rabbit Cd(3)beta domain structures. Finally, the surface exposure of the beta domains of lobster is compared with beta domains from mammalian metallothioneins.