NMR structure of the sea urchin (Strongylocentrotus purpuratus) metallothionein MTA.

The three-dimensional structure of [(113)Cd7]-metallothionein-A (MTA) of the sea urchin Strongylocentrotus purpuratus was determined by homonuclear(1)H NMR experiments and heteronuclear [(1)H, (113)Cd]-correlation spectroscopy. MTA is composed of two globular domains, an N-terminal four-metal domain of the amino acid residues 1 to 36 and a Cd4Cys11cluster, and a C-terminal three-metal domain including the amino acid residues 37 to 65 and a Cd3Cys9cluster. The structure resembles the known mammalian and crustacean metallothioneins, but has a significantly different connectivity pattern of the Cys-metal co-ordination bonds and concomitantly contains novel local folds of some polypeptide backbone segments. These differences can be related to variations of the Cys sequence positions and thus emphasize the special role of the cysteine residues in defining the structure of metallothioneins, both on the level of the domain architecture and the topology of the metal-thiolate clusters.