In collaboration with Dr. Stephen Stuerzenbaum, we have currently several metallothioneins from the nematode Caenorhabditis elegans and the annelid Lumbricus rubellus under study.
C. elegans has exactly two MTs, which share a high degree of sequence similarity, but MT-1 has a C-terminal extension and overall four extra potential metal-binding residues: three histidines and one cysteine:
Significantly, although the synthesis of both MTs can be induced by exposing the worms to excess Cd, CeMT-1 is also always expressed in the throat of the worms (see Stephen Stuerzenbaum's Toxicogenomics website).
Our collaborators recently found that in vivo, the two MTs have different effects on the accumulation of zinc and cadmium - with the effect of MT-1 more pronounced for zinc, and that of MT-2 more pronounced for cadmium. Intriguingly, our studies on the protein level demonstrate that the distinction in vivo is reflected in vitro (or vice versa...).
The most important "chemical" aspect in this context are the relative affinities of the two proteins for the two metal ions. See our papers: The two Caenorhabditis elegans metallothioneins discriminate between essential zinc and toxic cadmium and Tools for metal ion sorting: in vitro evidence for partitioning of zinc and cadmium in C. elegans metallothionein isoforms.
The two metallothioneins also play a role in resistance against reactive oxygen species; this is reported here: C. elegans metallothioneins: response to and defence against ROS toxicity