ABC Transporters and Heavy Metals

The first evidence showing that ABC transporters are involved in heavy metal resistance in eukaryotic cells has been obtained from experiments in Schizosaccharomyces pombe and Saccharomyces cerevisae, where a half-size transporter of the ABCB subclass and an ABCC-type transporter, respectively, have been shown to confer heavy metal tolerance. Biochemical studies have indicated that vacuolar ABC transporters should also play an important role in heavy metal detoxification in plants. But it was only recently that two ABCC-type transporters, AtABCC1 and AtABCC2, have been identified as major apo-phytochelatin and phytochelatin-heavy metal(oid) complex transporters. Several plasma membrane transporters have also been shown to confer heavy metal resistance. However, with the exception of STAR1, an UDP glucose exporter, which—by altering cell wall composition—confers aluminum tolerance, the substrates required to be transported to confer heavy metal resistance by these plasma membrane-localized ABC proteins are still not elucidated. A mitochondrial ABC transporter AtATM3 was shown to be required for plant growth and development. The different studies indicate that this transporter is important for the production of cytosolic iron sulfur complexes and molybdenum cofactors, prosthetic groups required for several enzymes. However, the final proof as to which substrate is transported by AtATM3 is still missing. Several laboratories took advantage of the fact that ABC transporters are involved in heavy metal tolerance to generate transgenic plants suitable for phytoremediation. The results show that overexpression of ABC proteins alone is not sufficient to produce plants that can efficiently decontaminate soils, but they indicate that this class of transporters, when combined with other transporters and enzymes involved in heavy metal transport and detoxification, may prove a good solution to produce plants that can stabilize, and in the long term clean up, soils contaminated with heavy metals.