CeHMT-1, a Putative Phytochelatin Transporter, Is Required for Cadmium Tolerance in Caenorhabditis elegans

Abstract

Phytochelatins (PCs), (␥-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 Cd 2؉ ) 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, Ce-HMT-1 satisfies the requirements of a Cd . 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 Ce-HMT-1 does not depend exclusively on PC synthesis.The toxicity of supraoptimal levels of the essential heavy metals copper and zinc and of trace or higher levels of the nonessential heavy metals cadmium, arsenic, mercury, and lead is thought to result from the displacement of endogenous co-factors from their cellular binding sites, thiol-capping of essential proteins and peptides, and promotion of the formation of reactive oxygen species (1). In humans, the repercussions of heavy metal exposure can range from acute poisoning to progressive kidney, liver, and lung dysfunction and, in some instances, cancer. Although its true clinical and epidemiological significance remains to be determined, chronic exposure to heavy metals is often associated with muscular and neurological degenerative conditions reminiscent of muscular dystrophy, multiple sclerosis, Alzheimer disease, and Parkinson disease (2-4).Three classes of heavy metal-binding peptides are known to participate in the homeostasis and detoxification of heavy metals in most animals. These are the ubiquitous thiol tripeptide glutathione (GSH), a family of small (4 -8 kDa) cysteine-rich proteins termed metallothioneins, and several higher molecular mass albeit sequence-unrelated metal-binding proteins. In all of the plants studied and in some fungi (as exemplified by Schizosaccharomyces pombe and Candida glabrata) and some animals (as exemplified by the model nematode Caenorhabditis elegans), a third class of cysteine-rich peptides termed phytochelatins (PCs) 1 has also been shown to be involved in heavy metal detoxification.PCs, which constitute a family of short-chain heavy metalbinding peptides with the general structure (␥-EC) n Xaa, where n ϭ 2-11, are derived from GSH and related thiols in a ␥-glutamylcysteinyl transpeptidation reaction catalyzed by phytochelatin synthases (EC 2.3.2.15) (1,5,6). It is now almost 15 years since the partial purification of the enzyme capable of catalyzing PC synthesis (5), yet it is onl...

Identification of Mutations in Caenorhabditis elegans That Cause Resistance to High Levels of Dietary Zinc and Analysis Using a Genomewide Map of Single Nucleotide Polymorphisms Scored by Pyrosequencing

“…Phytochelatins are peptides that can bind metals and contribute to cadmium tolerance. C. elegans pcs-1 mutants (defective in phytochelatin synthesis) and hmt-1 mutants (defective in phytochelatin transport) display enhanced cadmium toxicity (Vatamaniuk et al 2001(Vatamaniuk et al , 2005. These results indicate that metallothioneins and phytochelatins are involved in cadmium detoxification.…”

Metalloproteins and phytochelatin synthase may confer protection against zinc oxide nanoparticle induced toxicity in Caenorhabditis elegans

“…CeMT-1 and CeMT-2, respectively), are believed to be key players in the protection against metal and ROS induced toxicity (Swain et al, 2004;Zeitoun-Ghandour et al, 2011). In addition, phytochelatin synthase, pcs-1, encodes a metal-dependent antioxidant enzyme (Vatamaniuk et al, 2001(Vatamaniuk et al, , 2005Schwartz et al, 2010) which, along with glutathione (GSH), synthesizes phytochelatins (PCs).…”

“…The nematode Caenorhabditis elegans, a powerful model organism due to the availability of a completely sequenced genome (Hillier et al, 2005) and many molecular genetics tools has been used in ecotoxicological research to study the molecular to organismal level responses to ROS and heavy metal challenges (Roh et al, 2006;Hughes and Sturzenbaum 2007;Swain et al, 2004Swain et al, , 2010Zeitoun-Ghandour et al, 2010; The roles of the metalloproteins metallothionein (MT) and phytochelatin (PC) are Furthermore, ZnONP mediated toxicity may result from the release of free ionic zinc (George et al 2010;Li et al, 2012;Poynton et al, 2011;Wang et al, 2009), which induces cellular damage via the generation of free reactive oxygen species (ROS), which in turn can promote pro-inflammatory effects Mocchegiani et al, 2011). assumed to be multi-functional, including metal sequestration, transportation, detoxification, protection against antioxidants (Swain et al, 2004;Margoshes and Valee, 1957;Sato and Bremner, 1993;Cobbett, 2000;Vatamaniuk 2001Vatamaniuk , 2005Freedman et al, 1993). When nematodes are exposed to excess metal ions, such as Cd 2+ or Zn 2+…”

A Common Highly Conserved Cadmium Detoxification Mechanism from Bacteria to Humans

“…This scheme, discriminating different strategies engaged by plants and animals in cadmium detoxification, has recently been ruled out by the discovery of a functional phytochelatin synthase enzyme in the worm Caenorhabditis elegans that is able to complement the cadmium sensitivity of an S. pombe PCS knock-out strain (19). CePCS1-deficient worms were found markedly more sensitive to cadmium intoxication, leading to the first demonstration of the PCmediated detoxification of cadmium in an animal (19).…”