MOLECULAR CHARACTERIZATION OF A PUTATIVE ARABIDOPSIS-THALIANA COPPER TRANSPORTER AND ITS YEAST HOMOLOG

At the molecular level, little is known about the transport of copper across plant membranes. We have isolated an Arabidopsis thaliana cDNA by complementation of a mutant (ctr1-3) of Saccharomyces cerevisiae de fective in high affinity copper uptake. This cDNA codes for a highly h ydrophobic protein (COPT1) of 169 amino acid residues and with three p utative transmembrane domains. Most noteworthy, the first 44 residues display significant homology to the methionine- and histidine-rich cop per binding domain of three bacterial copper binding proteins, among t hese a copper transporting ATPase. Mutant yeast cells expressing COPT1 exhibit nearly wild type behavior with regard to growth on a nonferme ntable carbon source and resistance to copper and iron starvation. Exp ression of COPT1 is also associated with an increased sensitivity to c opper toxicity. Additionally, COPT1 shows significant homology to an o pen reading frame of 189 amino acid residues on yeast chromosome VIII. This gene (CTR2) may encode an additional yeast metal transporter abl e to mediate the uptake of copper. A mutation in CTR2 displays a highe r level of resistance to toxic copper concentrations. Overexpression o f CTR2 provides increased resistance to copper starvation and is also associated with an increased sensitivity to copper toxicity. The amino acid sequence of CTR2, like Arabidopsis COPT1, contains three potenti al transmembrane domains. Taken together, the data suggest that a plan t metal transporter, which is most likely involved in the transport of copper, has been identified.