Toward a property/function relationship for metallothioneins: histidine coordination and unusual cluster composition in a zinc-metallothionein from plants.

Early cysteine labeled (E(C)) proteins are plant metallothioneins, which were first identified in wheat embryos and are thought to be seed-specific. An exhaustive analysis of expressed sequence tag (EST) entries reveals that homologs are expressed in embryos of both classes of flowering plants (monocotyledons and dicotyledons), but also occur in conifers (gymnosperms) and seed-free spike moss (lycophyta). Mass spectrometric and elemental analysis results indicate that, contrary to the widely propagated number of five, E(C) binds predominantly six zinc ions in at least two zinc-thiolate clusters. 1H and 111Cd NMR experiments suggest that, in contrast to the majority of previously characterized metallothioneins, two conserved histidine residues participate in metal binding. The collected data is consistent with the presence of clusters unprecedented in metallothioneins so far. This novel cluster composition is accompanied by metal-binding properties that are substantially different from other metallothioneins; thus wheat E(C) binds zinc less strongly than either mammalian or cyanobacterial MTs. The unique biochemical properties of wheat E(C) render it ideally suited for a role in zinc donation to nascent proteins during seed development, a role that has been suggested based on the fact that E(C) is induced by the plant hormone abscisic acid, but not by heavy metals. Our results provide a step further toward developing a property/function relationship for metallothioneins.