Cucumber Metallothionein-Like 2 (CsMTL2) Exhibits Metal-Binding Properties.

We identified a novel member of the metallothionein (MT) family, metallothionein-like 2 (CsMTL2), by screening a young cucumber fruit complementary DNA (cDNA) library. The encodes a putative 77-amino acid Class II MT protein that contains two cysteine (Cys)-rich domains separated by a Cys-free spacer region. We found that expression was regulated by metal stress and was specifically induced by Cd treatment. We investigated the metal-binding characteristics of CsMTL2 and its possible role in the homeostasis and/or detoxification of metals by heterologous overexpression in cells. Furthermore, we produced a deletion mutant form of the protein, CsMTL2m, that contained the two Cys-rich clusters but lacked the spacer region, in . We compared the metal-binding properties of CsMTL2 with those of CsMTL2m, the β domain of human metallothionein-like protein 1 (HsMTXb), and phytochelatin-like (PCL) heterologously expressed in using metal-binding assays. We found that cells expressing accumulated the highest levels of Zn and Cd of the four transformed cell types, with levels being significantly higher than those of control cells containing empty vector. cells expressing CsMTL2 had a higher tolerance for cadmium than for zinc ions. These findings show that CsMTL2 improves metal tolerance when heterologously expressed in . Future studies should examine whether CsMTL2 improves metal tolerance in planta.