CeHMT-1, a putative phytochelatin transporter, is required for cadmium tolerance in Caenorhabditis elegans.

Phytochelatins (PCs), (gamma-Glu-Cys)n Gly polymers that were formerly considered to be restricted to plants and some fungal systems, are now known to play a critical role in heavy metal (notably Cd2+) detoxification in Caenorhabditis elegans. In view of the functional equivalence of the gene encoding C. elegans PC synthase 1, ce-pcs-1, to its homologs from plant and fungal sources, we have gone on to explore processes downstream of PC fabrication in this organism. Here we describe the identification of a half-molecule ATP-binding cassette transporter, CeHMT-1, from C. elegans with an equivalent topology to that of the putative PC transporter SpHMT-1 from Schizosaccharomyces pombe. At one level, CeHMT-1 satisfies the requirements of a Cd2+ tolerance factor involved in the sequestration and/or elimination of Cd x PC complexes. Heterologous expression of cehmt-1 in S. pombe alleviates the Cd2+-hypersensitivity of hmt- mutants concomitant with the localization of CeHMT-1 to the vacuolar membrane. Suppression of the expression of ce-hmt-1 in intact worms by RNA interference (RNAi) confers a Cd2+-hypersensitive phenotype similar to but more pronounced than that exhibited by ce-pcs-1 RNAi worms. At another level, it is evident from comparisons of the cell morphology of ce-hmt-1 and cepcs-1 single and double RNAi mutants that CeHMT-1 also contributes to Cd2+ tolerance in other ways. Whereas the intestinal epithelial cells of ce-pcs-1 RNAi worms undergo necrosis upon exposure to toxic levels of Cd2+, the corresponding cells of ce-hmt-1 RNAi worms instead elaborate punctate refractive inclusions within the vicinity of the nucleus. Moreover, a deficiency in CeHMT-1 does not interfere with the phenotype associated with CePCS-1 deficiency and vice versa. Double ce-hmt-1; ce-pcs-1 RNAi mutants exhibit both cell morphologies when exposed to Cd2+. These results and those from our previous investigations of the requirement for PC synthase for heavy metal tolerance in C. elegans demonstrate PC-dependent, HMT-1-mediated heavy metal detoxification not only in S. pombe but also in some invertebrates while at the same time indicating that the action of CeHMT-1 does not depend exclusively on PC synthesis.

Worms take the 'phyto' out of 'phytochelatins'.

Phytochelatin synthase is the enzyme responsible for the synthesis of heavy-metal-binding peptides (phytochelatins) from glutathione and related thiols. It has recently been determined that it is not only restricted to plants and some fungi, as was once thought, but also has an essential role in heavy-metal detoxification in the model nematode Caenorhabditis elegans. These findings and others that demonstrate phytochelatin synthase-coding sequences in the genomes of several other invertebrates, including pathogenic nematodes, schistosomes and roundworms, herald a new era in phytochelatin research, in which these novel post-translationally synthesized peptides will not only be investigated in the context of phytoremediation but also from a clinical parasitological standpoint.