In vivo copper- and cadmium-binding ability of mammalian metallothionein β domain

The β domain of mouse metallothionein 1 (βMT) was synthesized in Escherichia coli cells grown in the presence of copper or cadmium. Homogenous preparations of Cu-βMT and Cd-βMT were used to characterize the corresponding in vivo-conformed metal-clusters, and to compare them with the species obtained in vitro by metal replacement to a canonical Zn3-βMT structure. The copper-containing βMT clusters formed inside the cells were very stable. In contrast, the nascent β peptide, although it showed cadmium binding ability, produced a highly unstable species, whose stoichiometry depended upon culture conditions. The absence of βMT protein in E. coli protease-proficient hosts grown in cadmium-supplemented medium pointed to drastic proteolysis of a poorly folded β peptide, somehow enhanced by the presence of cadmium. Possible functional and evolutionary implications of the bioactivity of mammalian βMT in the presence of monovalent and divalent metal ions are discussed.