Functional characterization of a heavy metal binding protein CdI19 from Arabidopsis

Heavy metals are potentially highly toxic for organisms. Plants possess the ability to minimize damage but the underlying molecular mechanisms have yet to be detailed. Screening Cd-responsive genes in Arabidopsis, we previously identified a gene encoding a putative metal binding protein CdI19, which, upon introduction into yeast cells, conferred marked toleration of Cd exposure. Here we describe that bacterially expressed CdI19 directly interacts with Cd at its CXXC motif, as revealed by circular dichroism analysis, and that it is exclusively localized at plasma membranes, as revealed by heterologous expression of fusion product with a green fluorescent protein in BY2 cells. Northern blot analyses indicated that CdI19 transcripts were induced not only by Cd, but also by dicationic forms of Hg, Fe and Cu. Histochemical assays using transgenic Arabidopsis expressing the CdI19 promoter::GUS showed CdI19 to be expressed in petiole, hypocotyl, peduncle and vascular bundles in root tissues. Overexpression of the CdI19 cDNA conferred Cd tolerance in transgenic Arabidopsis. These results suggest that CdI19 plays an important role in the maintenance of heavy metal homeostasis and/or in detoxification by endowing plasma membranes with the capacity to serve as an initial barrier against inflow of free heavy metal ions into cells.