Metallothioneins in insect and their role in tolerance to heavy metals

Abstract

Metallothionein is a ubiquitous cysteine-rich protein that binds heavy metals (e.g. cadmium, zinc, and copper). It has a small molecular mass, and its production is inducible by heavy metals, heat shock, and other environmental stresses. As heavy metals are strongly sequestered by metallothionein, de novo synthesis of this protein in the insect midgut may protect against the damage caused by heavy metals that enter the body. Metallothonein levels are greatly elevated in the larvae of heavy metal-tolerant fruit fly (Drosophila melanogaster) lines, and duplication of metallothionein genes plays an important role in this elevated production. In contrast, duplication of metallothionein genes has not been demonstrated in the springtail (Orchesella cincta) ; instead, high levels of expression of these genes in heavy metal-tolerant springtail lines may be caused by selective expression of genes that have highly efficient promoter regions upstream. Insects inhabiting cadmiumcontaminated lakes and rivers have been observed to accumulate heavy metals, and the levels of metallothionein induced in such insects are correlated with the concentrations of cadmium in the ambient water. Metallothionein is likely to be a good marker for use in the biomonitoring of heavy metals in aquatic environments