Metal ion trafficking in earthworms. Identification of a cadmium-specific metallothionein

Exposure to cadmium poses a considerable risk to human health and environmental safety. Earthworms reside in the most contaminated sites on earth, displaying a phenomenal tolerance to toxic heavy metals. They exhibit a distinct metabolic pathway that allows the bio-accumulation of cadmium to yield body burdens in excess of 1/1000th of total dry body weight, a most impressive figure by any standard. However, the precise molecular mechanism underlying this phenomenon remains to be unraveled. This study meets this challenge by fully characterizing the major metal-binding protein in earthworms, namely the two isoforms of metallothionein. Chemical analysis of recombinant protein showed that although both isoforms bind equimolar amounts of cadmium (6 mol), wMT-2 is more stable during proton competition. Furthermore, isoform-specific transcript analysis demonstrated that only wMT-2 is responsive to cadmium in a dose and temporal manner. The specific sequestration of cadmium to wMT-2 protein was confirmed in situ using polyclonal antisera. The latter also provided the means for mapping the cellular and intracellular distribution of metallothionein, thus yielding a holistic insight into its involvement in cadmium transit during absorption, storage, and excretion. The structure-function relationship of wMT-2 and its role in cadmium detoxification through sequestration and compartmentalization is discussed.