Like other eukaryotic organisms, plants produce several metallothioneins (see our review in Metallomics). Intriguingly, the variety of MT protein sequences found in a single plant species is greater than that found in other organisms, raising questions about relationships between structures and functions - in particular regarding the metal-specificity of the proteins.
Plant MTs have been classified into four sub-groups, termed MT-1 through to MT-4. They display distinct differences in the times and locations where they are synthesised. For example, certain types of MT-3s are expressed at very high levels in ripening fruit: the transcriptome of ripening pineapple consists to 50% of MT transcripts (Moyle et al. J. Exp. Botany 2005). The significance of this finding has remained unclear, but might be connected to protection from oxidative stress. Other MTs play clear roles in protection against heavy metal toxicity, with obvious implications for bio-remediation.
Perhaps one of the most intriguing MTs of all are the type 4 MTs. They are expressed in maturing seeds, and in the case of wheat, are the major destination of cysteine in maturing plant embryos. This has allowed the isolation of wheat ECI/II directly from wheat germs and established its association with zinc (Lane et al., Biochem Cell Biol 1987).
We hypothesise that the tiny zinc parcels stored by EC in the plant embryo are essential for germination, as several of the processes required for the successful generation of a seedling are zinc-dependent. This has obvious implications for food security and needs further work.
The fact that EC and its MT-4 relatives seem to have evolved to "select" zinc is exciting in terms of the importance for biology as well as for of underlying structural inorganic chemistry (see our papers: Toward a property/function relationship for metallothioneins: Histidine coordination and unusual cluster composition in a zinc-metallothionein from plants, Zinc transfer from the embryo-specific metallothionein EC from wheat), but has hampered a definitive structure elucidation for these proteins, as the NMR-spectroscopic probe of choice, cadmium, cannot be incorporated into EC with out severely disturbing the 3D structure of domain II. The "culprits" for this unprecedented behaviour are two conserved histidine (see also histidines in MTs) residues, which mediate an ordered structure in the presence of Zn2+, but not Cd2+, as the latter ion is much softer and prefers to bind exclusively to sulfur-containing cysteine residues (see: The isolated Cys2His2 site in EC metallothionein mediates metal-specific protein folding).