Mutation of invariant cysteines of mammalian metallothionein alters metal binding capacity, cadmium resistance, and ¹¹³Cd NMR spectrum

Using a yeast expression vector system, we have expressed both wild type and six mutated Chinese hamster metallothionein coding sequences in a metal-sensitive yeast strain in which the endogenous metallothionein gene has been deleted. The mutant proteins have single or double cysteine to tyrosine replacements (C13Y, C50Y, and C13,50Y), single cysteine to serine replacements (C13S and C50S), or a single cysteine to alanine replacement (C50A). These proteins function in their yeast host in cadmium detoxification to differing extents. Metallothioneins which contain a cysteine mutation at position 50 (C50Y, C50S, C50A, and C13,50Y) conferred markedly less cadmium resistance than wild type metallothionein, or metallothionein with a single cysteine mutation at position 13 (C13Y and C16S). Wild type and three of the mutant Chinese hamster metallothioneins (C13Y, C50Y, and C13,50Y) were purified from yeast grown in subtoxic levels of either CdCl₂ or ¹¹³CdCl₂. All three of the mutant proteins bound less cadmium than the wild type protein when metal-binding stoichiometries were determined. The one-dimensional ¹¹³Cd NMR spectrum of the recombinant wild type Chinese hamster metallothionein was compared to the spectra of native rat and rabbit liver metallothioneins. The close correspondence between the ¹¹³Cd chemical shifts in these metallothioneins is consistent with the presence of two separate metal clusters, A and B, corresponding, respectively, to the α- and β-domains, in the recombinant metallothionein. The one-dimensional ¹¹³Cd NMR spectra recorded on each of the three mutant metallothioneins, on the other hand, provide some indication as to the structural basis for the reduced, by one, metal stoichiometry of each of the mutant metallothioneins. For the C13Y mutant, it appears that the β-domain now binds a total of two metal ions whereas with the C50Y mutant, the α-domain appears metal-deficient. For the double mutant, C13,50Y, the ¹¹³Cd resonances are indicative of major structural reorganizations in both domains.