9.A.43 The Cadmium Tolerance Efflux Pump (CTEP) Family
Cadmium (Cd) causes the generation of reactive oxygen species (ROS), which in turn causes cell damage. Kim et al (2008) isolated a novel gene from a wheat root cDNA library, which conferred Cd2+-specific tolerance when expressed in Saccharomyces cerevisiae. The gene, called TaTM20, for Triticum aestivum transmembrane 20, encodes a hydrophobic polypeptide of 889 amino acids, containing 20 transmembrane domains arranged as a 5-fold internal repeating unit of 4 transmembrane domains each. Expression of TaTM20 in yeast cells stimulated Cd2+ efflux resulting in a decrease in the content of yeast intracellular Cd2+. TaTM20-induced Cd2+ tolerance was maintained in yeast even under conditions of reduced glutathione (GSH). Thus, TaTM20 enhances Cd2+ tolerance in yeast through the stimulation of Cd2+ efflux from the cell, partially independent of GSH. Treatment of wheat seedlings with Cd2+ induced their expression of TaTM20, decreasing subsequent root Cd2+ accumulation and suggesting a possible role for TaTM20 in Cd2+ tolerance in wheat (Kim et al. 2008).
The proteins of this family seem to be exclusively from plants, but many of them do not have the 4 TMS repeats. They instead each have a single 4 TMS unit, usually at their C-termini. It is possible that these proteins are related to the bacterial protein in TC family 9.B.257, but this has not been examined carefully.