Metal ion trafficking in earthworms - Identification of a cadmium-specificmetallothionein

Exposure to cadmium poses a considerable risk to human health and environme ntal safety. Earthworms reside in the most contaminated sites on earth, dis playing a phenomenal tolerance to toxic heavy metals. They exhibit a distin ct metabolic pathway that allows the bio-accumulation of cadmium to yield b ody burdens in excess of 1/1000th of total dry body weight, a most impressi ve figure by any standard. However, the precise molecular mechanism underly ing this phenomenon remains to be unraveled. This study meets this challeng e by fully characterizing the major metal-binding protein in earthworms, na mely the two isoforms of metallothionein. Chemical analysis of recombinant protein showed that although both isoforms bind equimolar amounts of cadmiu m (6 mol), wMT-2 is more stable during proton competition. Furthermore, iso form-specific transcript analysis demonstrated that only wMT-2 is responsiv e to cadmium in a dose and temporal manner. The specific sequestration of c admium to wMT-2 protein was confirmed in situ using polyclonal antisera. Th e 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 detoxi fication through sequestration and compartmentalization is discussed.