Independent self-assembly of cadmium-binding α-fragment of metallothionein in Escherichia coli without participation of β-fragment

We examined the independent self-assembly of the α- and β-fragments of human metallothionein (MT) into cadmiumbinding conformation in an Escherichia coli expression system, in addition to wild-type MT expression. The expressed α-fragment formed independently the structure of a metal-binding cluster without the aid of the β-fragment. The α-fragment and wild-type MT expressed in E.coli were purified and analyzed for their biochemical and spectroscopic properties. The apparent cadmium binding of the α-fragment was approximately 12-fold greater than that for the wild-type MT, whereas in other respects the studied biochemical properties were similar. In contrast, we were unable to obtain any independently expressed β-fragment as the cadmium-binding form in this study. Possible explanations for this phenomenon are discussed.