The type 4 metallothionein from Brassica napus seeds folds in a metal-dependent fashion and favours zinc over other metals

The problem of handling zinc in the cell is of great importance because zinc is an indispensable micronutrient involved in most physiological processes in all living organisms. Moreover, our understanding of mechanisms governing the discrimination between micronutrients and toxic metals on the level of individual proteins to the whole-organism level is incomplete. Metallothioneins are able to bind heavy metal ions, and roles in zinc homeostasis have been proposed. Here, we have studied the in vitro and in vivo metal-binding abilities of Brassica napus type 4 metallothionein (BnMT4) and its expression in germinating seeds in response to metal treatment. Our studies on the regulation of MT4 expression by metals at early stages of ontogenic development have revealed for the first time that the mRNA levels of BnMT4 were elevated in response to cadmium and zinc. Given this unexpected metalloregulation, and the dramatic differences in protein folding as detected by H-1 NMR spectroscopy, we suggest that the BnMT4 protein may not only have a role in zinc homeostasis in early ontogenesis, but also the potential to discriminate between zinc and cadmium, perhaps via differential recognition of Cd- and Zn-complexes by cellular components involved in protein turnover.