A Common Highly Conserved Cadmium Detoxification Mechanism from Bacteria to Humans

Abstract

Cadmium poses a significant threat to human health due to its toxicity. In mammals and in bakers' yeast, cadmium is detoxified by ATP-binding cassette transporters after conjugation to glutathione. In fission yeast, phytochelatins constitute the cosubstrate with cadmium for the transporter SpHMT1. In plants, a detoxification mechanism similar to the one in fission yeast is supposed, but the molecular nature of the transporter is still lacking. To investigate further the relationship between SpHMT1 and its co-substrate, we overexpressed the transporter in a Schizosaccharomyces pombe strain deleted for the phytochelatin synthase gene and heterologously in Saccharomyces cerevisiae and in Escherichia coli. In all organisms, overexpression of SpHMT1 conferred a markedly enhanced tolerance to cadmium but not to Sb(III), AgNO 3 , As(III), As(V), CuSO 4 , or HgCl 2 . Abolishment of the catalytic activity by expression of SpHMT1 K623M mutant suppressed the cadmium tolerance phenotype independently of the presence of phytochelatins. Depletion of the glutathione pool inhibited the SpHMT1 activity but not that of AtHMA4, a P-type ATPase, indicating that GSH is necessary for the SpHMT1-mediated cadmium resistance. In E. coli, SpHMT1 was targeted to the periplasmic membrane and led to an increased amount of cadmium in the periplasm. These results demonstrate that SpHMT1 confers cadmium tolerance in the absence of phytochelatins but depending on the presence of GSH and ATP. Our results challenge the dogma of the two separate cadmium detoxification pathways and demonstrate that a common highly conserved mechanism has been selected during the evolution from bacteria to humans.Cadmium is a trace element, the presence of which in the environment is essentially due to human activities. It is a highly toxic non-biological heavy metal able to enter living cells via transporters usually used for the uptake of essential cations such as calcium, iron, zinc, and so forth (1). The reactivity of cadmium with thiol groups and its ability to displace essential biological metals result in oxidative stress and eventually cell death (2). To cope with cadmium toxicity, living organisms have developed different strategies.In animals, as in the bakers' yeast cytoplasmic cadmium is complexed with the thiol tripeptide glutathione, a general redox regulator (3, 4). Bis(glutathionato)-cadmium complexes (Cd-GS 2 ) 4 are then driven from the cytoplasm to lesser sensitive cellular compartments by dedicated transporters. The prototypical transporter of Cd-GS 2 is the GS-X pump, ScYCF1, in Saccharomyces cerevisiae (5) and, even if still controversial, to a lesser extent HsMRP1 in humans (6). HsMRP1 probably acts as an efflux pump at the plasma membrane, delivering cadmium in the extracellular medium, whereas ScYCF1 allows sequestration of cadmium into the vacuole (5). A study of a deficient Scycf1 strain has shown that it was extremely cadmium-sensitive, pointing to a major role of ScYCF1 in cadmium tolerance and detoxification (5). Additionally, ScYCF1 wa...

Tonoplast-localized Abc2 Transporter Mediates Phytochelatin Accumulation in Vacuoles and Confers Cadmium Tolerance

“…Hmt1 requires GSH to confer Cd tolerance (40), but analyses of the peptide content of vacuolar HMWCs in S. pombe have failed to detect significant amounts of GSH inside vacuoles (24,25,56). Moreover, vacuolar vesicles isolated from S. pombe overexpressing Hmt1 did not show significant uptake rates of GSH or GS 2 -Cd (19), suggesting that the GSH requirement of Hmt1 may not be related to GS 2 -Cd transport.…”

“…1, 2, 3E, and 4E). These results have encouraged several groups to re-assess the function of Hmt1 (33,40) (Fig. 1).…”

“…In addition, an HMT1 protein from the non-PC producing Drosophila melanogaster was able to rescue the Cd sensitivity of the S. pombe hmt1 mutant (33). Conversely, S. pombe Hmt1 conferred tolerance to Cd in Escherichia coli and Saccharomyces cerevisiae, organisms devoid of phytochelatins (40). Furthermore, recent data showed that the S. pombe hmt1 mutant accumulates significant levels of PCs in vacuoles (33), suggesting that other, yet unidentified proteins mediate PC transport into vacuoles.…”

“…ϩ or pcs ϩ cDNAs in S. cerevisiae dramatically increases Cd tolerance (15,16,40). To test whether Hmt1 and PC synthase have an additive effect on Cd tolerance in yeast, we expressed S. pombe hmt1 ϩ in a S. cerevisiae strain engineered to synthesize phytochelatins (strain Sm15).…”

Phytochelatin–metal(loid) transport into vacuoles shows different substrate preferences in barley and Arabidopsis

“…Moreover, SpHMT1 enhanced the Cd resistance of Sa. cerevisiae and Escherichia coli , which do not produce PCs (Preveral et al 2009). These results suggest that SpHMT1 may transport Cd bound to different ligands, including but not limited to PCs.…”