Substitution of Glutamic Acids for the Conserved Lysines in the α Domain Affects Metal-Binding in Both the α Domain and β Domain of Mammalian Metallothionein

Lysine residues are highly conserved in mammalian metallothioneins(MTs). Recombinant mutant Chinese hamster MT2 in which all of the lysines (K) in the α-domain were substituted by glutamic acids (E) was assayed with, expressed in and purified from a cadmium sensitive strain of yeast Saccharomyces cerevisiae. Circular dichroism analyses of the mutated protein, mutein K43,51,56E, revealed that the overall structure remained unchanged. However, a 1-D ¹¹³Cd NMR study detected significant differences in the chemical shifts of the corresponding resonances between wild type protein and the recombinant mutein. Reduction of integrated intensity in the NMR spectra was also observed for resonances from the four-metal cluster (I and V-VII) in the α-domain of the mutein. At various temperatures, facile intermolecular exchange of metals in the β-domain of the mutein was also observed, which was unexpected and was different from wild type. Our results thus demonstrate that replacing all three lysines by glutamic acids in the α-domain changed metal-thiolate interactions in both domains of the recombinant mutein. This may explain the reduced ability of the mutein to convey cadmium resistance. We propose that while the lysine residues in the α-domain of wild type MT are not critical for maintaining protein structure, they play a role in regulating the microenvironment and stability of both metal-binding clusters, a feature critical to metal detoxification.