ABSTRACT
The structure of the purple sea urchin Strongylocentrotus purpuratus recombinant Cd7-MTA has been determined by homonuclear *H NMR and heteronuclear ^H, 113Cd] correlation spectroscopy [35]. This MT is a 64-residue protein containing essentially the same number of metal-chelating Cys-Cys and Cys-X-Cys motifs as the mammalian MTs and forms a two-domain structure with one domain containing a three-metal cluster and the other a four-metal cluster. However, the sequential order of the a and (3 domains, and hence of the three-and four-metal clusters, is inverted between the two MT types. In mammalian MTs, the three-metal-containing (3 domain corresponds to the N-terminal half of the sequence and the a domain is located in the C-terminal part, whereas in sea urchin MT the N-terminal a domain contains four metal ions and the C-terminal P domain three metal ions [35], Despite this domain inversion, the Cd binding affinity and the metal binding properties of the two MT types are similar. Substantial differences exist in the structural roles of the corresponding cysteine residues, i.e., in their serving as singly bound or bridging ligands. In particular, the locations of the bridging Cys along the sequences are clearly different. As a consequence, the geometries of the a-domain metal-sulfur clusters in the two species are different [35]. Besides, in contrast to all other presently known three-metal cluster-containing MT domains, the chirality of the sea urchin MTA p domain is left-handed [35].
