A mutant metallothionein which has inverse fragment composition exhibits high cadmium-binding ability

In order to investigate the role of the alpha-fragment of metallothionein (MT) in metal-binding, two mutant MTs, beta Ala alpha Cys- and alpha Cys beta Cys-mutant MTs, expressed in Escherichia coli were analyzed for their metal-binding ability. A mutant MT where all of the cysteine residues in the beta-fragment of MT were substituted by alanine residues and another mutant MT that had the inverse fragment composition (alpha-beta, i.e., beta-alpha in wild-type MT) were designated as the beta Ala alpha Cys and the alpha Cys beta Cys-mutant MT's, respectively. Both expressed Cd-binding mutant MTs were identified by amino acid analyses. From their metal-binding capacities, the two mutant MTs exhibited higher Cd-binding abilities than wild-type MT. The results suggested that the alpha-fragment plays a key role in the Cd-binding of MT, and that the metal-binding tightness of MT is dependent on the metal-binding ability of a prior fragment in MT.