| Summary: | Metallothioneins are a superfamily of small, cysteine-rich, metal-binding proteins, which have been the subject of intense scientific interest since their discovery in the late 1950s. Over the last decade, the advancement of genome sequencing, microarray and high-throughput protein identification techniques has resulted in an exponential increase in the number of plant MT sequences in protein and translated nucleotide databases. These studies show that plant metallothioneins display significant variation in sequence and are discretely expressed during various developmental stages and in different organs. Therefore, it is likely that plant metallothioneins carry out compartmentalised functions for which they possess specific properties. However, there is a relative paucity of academic literature on the structure and biochemical properties of plant MTs and this is a void that needs to be filled before we can establish structure/property and structure/function relationships for these proteins. This research has focussed on a comprehensive characterisation of the solution structure and metal binding dynamics of the wheat Eel/II metallothionein using a range of analytical and biophysical techniques. Mass spectrometry and nuclear magnetic resonance studies have identified six zinc ions in two distinct domains with hitherto unprecedented stoichiometries for MTs: Zn2Cys6 and Zn4CysllHis2. Structure calculations revealed that in addition to these unprecedented stoichiometries, the individual Ee domains possess unique structural features not previously reported in MT literature. In Domain 1, the binding of two zinc ions in a binuclear cluster was observed. When compared to Domain 2, the binuclear cluster displayed remarkable stability towards Wand EDTA attack. In Domain 2 an isolated binding site with stoichiometry ZnCys2His2 was observed, which is proposed to confer a well-defined structure for this domain. However, disruption of this site through mutation or cadmium binding abolishes ordered structure along with the defined binding of six zinc ions. These novel structural features confer distinct backbone and metal dynamic properties in each domain, and are most likely to have a functional significance. A separate avenue of investigation identified a potential role for Ee in zinc donation. A re-examination of previously published literature on the expression levels of Ee mRNA and protein demonstrated that it is conspicuously accumulated during desiccation, which suggests that it is perfectly poised to satisfy the microquota of zinc-dependent sites during rehydration processes. In conclusion, this research has made significant advances in the understanding of the structure/function relationship of the wheat Eel/II metallothionein.
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