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Page 1 of 7 Metallomics Metallomics Accepted Manuscript This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. www.rsc.org/metallomics Page 1 of 7 Metallomics 1 2 Metallomics RSCPublishing 3 4 5 MINI-REVEW 6 7 8 9 Metabolites and metals in Metazoa – what role do 10 phytochelatins play in animals? 11 Cite this: DOI: 10.1039/x0xx00000x 12 13 J. G. Bundy,a P. Killeb 14 15 Received 00th January 2012, Phytochelatins are sulfur-rich metal-binding peptides, and phytochelatin synthesis is one of the 16 Accepted 00th January 2012 key mechanisms by which plants protect themselves against toxic soft metal ions such as 17 cadmium. It has been known for a while now that some invertebrates also possess functional DOI: 10.1039/x0xx00000x 18 phytochelatin synthase (PCS) enzymes, and that at least one species, the nematode 19 www.rsc.org/ Caenorhabditis elegans, produces phytochelatins to help detoxify cadmium, and probably also 20 other metal and metalloid ions including arsenic, zinc, selenium, silver, and copper. Here, we 21 review recent studies on the occurrence, utilization, and regulation of phytochelatin synthesis 22 in invertebrates. The phytochelatin synthase gene has a wide phylogenetic distribution, and can 23 be found in species that cover almost all of the animal tree of life. The evidence to date, 24 though, suggests that the occurrence is patchy, and even though some members of particular 25 26 taxonomic groups may contain PCS genes, there are also many species without these genes. 27 For animal species that do possess PCS genes, some of them (e.g. earthworms) do synthesize Manuscript 28 phytochelatins in response to potentially toxic elements, whereas others (e.g. Schistosoma 29 mansoni, a parasitic helminth) do not appear to do so. Just how (and if) phytochelatins in 30 invertebrates complement the function of metallothioneins remains to be elucidated, and the 31 temporal, spatial, and metal specificity of the two systems is still unknown. 32 33 34 35 36 37 Accepted 38 39 40 What are phytochelatins? Because glutathione is usually present at high cytosolic concentrations in cells, any influx of metal ions will lead to 41 Phytochelatins (PCs) are cysteine-rich non-ribosomal peptides, GSH-complexed species, which are then rapidly converted into 42 typically with the structure (γ-GluCys) Gly where n is between n PCs by constitutively-expressed PCS. The multidentate 43 2 to 4, although some species also have a different terminal phytochelatins bind metal ions such as cadmium much more amino acid than glycine.1 They are widely found in plants, 44 strongly than the unidentate GSH molecules.4 where they play a key role in metal ion detoxification. PCs 45 The other key metal-handling and -detoxification system in chelate metals through the free thiols on the cysteine residues, 46 animals is metallothioneins (MTs). These, like PCs, are and hence bind to soft metal ions; prototypically cadmium, but 47 cysteine-rich peptides; the key difference is that the sequence of also to a range of other elements, including As, Hg, Ag, Zn, and MTs is genetically encoded. This has a number of 48 Cu.1 49 consequences: one is that, because there can be differences in Phytochelatins are synthesized from glutathione by the the peptide chain, animals can (and generally do) have more Metallomics 50 enzyme phytochelatin synthase (PCS). The PCS enzymes than one MT isoform. This also means that different MT belong to the papain-like cysteine protease superfamily; they 51 isoforms can have preferences for different metal ions, and that comprise a conserved N-terminal domain, which contains the 52 these can differ between species. In contrast, PCs have the same active catalytic site (including a conserved 53 binding affinities for metals no matter what species they are cysteine/histidine/aspartate triad), as well as a more variable C- 54 found in – clearly the situation is more complex for MTs. terminal domain, which contributes to the regulation of the 55 2,3 However, excellent and exhaustive reviews of MTs already enzymatic activity. In general, the substrate for PCS is the 1,5,6 7,8 exist, including reviews of MTs in invertebrates (most 56 glutathione-chelated metal ion complex, and so PCS does not relevant to our current work) as well as mammals,9,10 plus more 57 need external regulatory mechanisms to induce its activity – an specific reviews focussing on aspects such as the biochemistry 58 increase in cytosolic metal ion concentrations is sufficient.2 59 60 This journal is © The Royal Society of Chemistry 2013 J. Name., 2013, 00, 1-3 | 1 Metallomics Page 2 of 7 MINI-REVIEW Metallomics 1 11 of metal ion binding. There are also excellent and in animals would therefore be more of a curiosity than of 2 comprehensive reviews of PCs that deal largely with their general importance, but this is belied by the very wide 3 occurrence in plants.1,2,12,13 Because of this, we have here taxonomic distribution of the species with PCS homologues 4 decided to focus solely on PCs in invertebrates. described so far. In fact, PCS-containing species are found 5 across the deepest divisions of the Metazoa. Clemens and 6 Persoh12 listed putative PCS genes in species from seven 7 Phytochelatins are found in animals metazoan phyla: Nematoda, Platyhelminthes, Annelida, 8 A few years after the discovery of PCs, homologues of the PCS Mollusca, Chordata, Cnidaria, and Echinodermata. In addition, the NCBI protein database currently (February 2014) also lists 9 gene were identified in metazoans, including the nematode Caenorhabditis elegans. In 2001, two independent studies two sequences from the acorn worm Saccoglossus kowalevskii 10 (phylum Hemichordata), although these may not represent 11 showed that the Caenorhabditis elegans PCS (CePCS; we will throughout refer to ‘XyPCS’ for different species, where X and ‘standard’ animal PCS enzymes, as they are shorter than most 12 y are the first letters of the genus and species, respectively) PCS proteins, being restricted to the more conserved N- 13 could synthesize PCs when recombinantly expressed in an terminal region (accession numbers XP_002730374.1 and 14 appropriate host (either Saccharomyces cerevisiae, which does XP_002730372.1). The exact organization of the animal tree of life at the phylum level is still controversial;27 here, we follow 15 not possess a PCS gene, or Schizosaccharomyces pombe, with 28 16 its own PCS gene deleted).14,15 What’s more, knocking down Jones and Blaxter in assuming a superphyletic organization the CePCS gene made the nematodes more sensitive to the into three major groups, Lophotrochozoa, Ecdysozoa, and 17 Deuterostomia, with additional outgroups including Ctenophora 18 canonical PC-binding metal cadmium – the first direct evidence that PCs have a functional role in response to metals in a and Cnidaria. The phyla listed by Clemens and Persoh contain 19 15 metazoan. Several studies have since measured PCs by direct examples from all three of these superphyletic groups, as well 20 biochemical analysis of C. elegans tissue extracts, and found as the outlying phylum Cnidaria – although PCS is not present 21 in the sole ctenophoran sequenced to date, Mnemniopsis that cadmium exposure did indeed increase PC levels in C. 29 16-18 leidyi. Interestingly, Tetrahymena thermophila (a single- 22 elegans: PC2, PC3, and PC4 have all been found, with PC2 23 the highest concentration. Subsequent studies have shown that celled eukaryote, but not an animal) contains a PCS-like 24 PCs are presumably protective against more elements than just protein, or “pseudo-phytochelatin synthase”. This TtPCS is transcriptionally upregulated in response to cadmium, but does 25 cadmium in C. elegans, as the CePCS knockout strain also has increased sensitivity to arsenic, zinc, selenium, silver, and not appear to make full-length phytochelatins: rather, it 26 possesses the PCS hydrolase activity by converting glutathione copper – although PC concentrations have not yet been Manuscript 27 18-21 measured directly in response to these. to γ-glutamylcysteine, but is unable to then combine this with a 30 28 Phytochelatin production has been studied in a number of second glutathione molecule. The authors pointed out that this 29 other animal species. Still within the phylum Nematoda, makes the TtPCS more similar to bacterial PCS genes (or 30 Rigouin et al. looked at the parasitic species Ancylostoma pseudo-PCS) than to higher eukaryote PCS, and suggested that 31 ceylanicum, and showed that the AcPCS also produced PCs it might be an example of an intermediate evolutionary form. 32 when expressed in S. cerevisiae.22 Furthermore, they reported We present a phylogenetic tree of selected PCS sequences from close homologues of PCS in other parasitic nematode species, both metazoans and single-celled eukaryotes (Figure 1). There 33 is some differentiation of the sequences according to the 34 including Brugia malayi, Loa loa, and Ascaris suum. Ray et al.
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