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.
Stürzenbaum
S R
SR
School of Biosciences, P. O. Box 911, Cardiff University, Cardiff CF1 3US, Wales, United Kingdom. SturzenbaumSR@Cardiff.ac.uk
Winters
C
C
Galay
M
M
Morgan
A J
AJ
Kille
P
P
D000595:Amino Acid Sequence; D000818:Animals; D001667:Binding, Competitive; D001692:Biological Transport; D001835:Body Weight; D002104:Cadmium / Q000378:metabolism*; D003001:Cloning, Molecular; D004305:Dose-Response Relationship, Drug; D006863:Hydrogen-Ion Concentration; D007150:Immunohistochemistry; D007477:Ions; D008668:Metallothionein / Q000737:chemistry* / Q000378:metabolism*; D008969:Molecular Sequence Data; D009835:Oligochaeta; D011485:Protein Binding; D020033:Protein Isoforms; D012333:RNA, Messenger / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D013997:Time Factors
D007477:Ions; D020033:Protein Isoforms; D012333:RNA, Messenger; D011993:Recombinant Fusion Proteins; D011994:Recombinant Proteins; D002104:Cadmium; D008668:Metallothionein