Isolation and characterization of a self-sufficient one-domain protein - (Cd)-metallothionein from Eisenia foetida

Earthworms have been shown to accumulate trace elements in general, and par ticularly high amounts of metal ions such as cadmium, copper and zinc. The earthworm's response to metal contamination has been linked to the inductio n and expression of metallothionein (MT) proteins, a detoxification strateg y analogous to that found in other biological systems. The present study fo cuses on an inducible Cd-MT isolated from the compost-dwelling brandling wo rm Eisenia foetida (Savigny). A full characterization of the protein (inclu ding protein induction, MT cDNA, amino-acid sequence and metal stoichiometr y) revealed a new dimension of knowledge to the molecular genetic informati on available to date. Whereas the elucidated cDNA codes for a putative prot ein which possesses 80 amino-acid residues, the characterized protein bears only 41 amino acids. The isolated product has evidently attained its size and shape by cleavage near the N-terminal site and at the linker region bet ween the two putative metal-binding domains of the translated product, yiel ding a small MT moiety which contains 12 Cys residues (including one triple Cys-motif) binding four cadmium ions. It can be shown that the isolated MT molecule represents a self-sufficient one-domain MT which is stable in vit ro. The isolation of the single-domain MT peptide raises the question about the method of formation and significance in vivo of such small MT moieties from tissues of E. foetida and possibly other terrestrial invertebrates. I n this respect, two hypotheses are discussed: firstly, the possibility of f ormation of small MT peptides due to enzymatic cleavage of the intact prote in during the process of preparation and isolation; and secondly, the possi bility of deliberate posttranslational processing of the translated gene pr oduct to yield functional one-domain MT moieties.