The Saccharomyces cerevisiae Crs5 Metallothionein metal-binding abilities and its role in the response to zinc overload

Intracellular sequestration of cadmium and zinc in ectomycorrhizal fungus Amanita muscaria (Agaricales, Amanitaceae) and characterization of its metallothionein gene

Amanita muscaria is an ectomycorrhizal mushroom that commonly grows at metal-polluted sites. Sporocarps from the lead smelter-polluted area near Příbram (Central Bohemia, Czech Republic) showed elevated concentrations of Cd and Zn. Size exclusion chromatography of the cell extracts of the sporocarps from both polluted and unpolluted sites indicated that substantial part of intracellular Cd and Zn was sequestered in 6-kDa complexes, presumably with metallothionein(s) (MT). When the cultured mycelial isolates were compared, those from Příbram were more Cd-tolerant and accumulated slightly less Cd and Zn than those from the unpolluted site. The analysis of the available A.muscaria sequence data returned a 67-amino acid (AA) MT encoded by the AmMT1 gene. Weak Cd and Zn responsiveness of AmMT1 in the mycelia suggested its metal homeostasis function in A.muscaria, rather than a major role in detoxification. The AmMT1 belongs to a ubiquitous peptide group in the Agaricomycetes consisting of 60-70-AA MTs containing seven cysteinyl domains and a conserved histidyl, features observed also in a newly predicted, atypical 45-AA RaMT1 of the Zn-accumulator Russula bresadolae in which the C-terminal cysteinyl domains VI and VII are missing. Heterologous expression in metal-sensitive yeast mutants indicated that AmMT1 and RaMT1 encode functional peptides that can protect cells against Cd, Zn, and Cu toxicity. The metal protection phenotype observed in yeasts with mutant variants of AmMT1 and RaMT1 further indicated that the conserved histidyl seems to play a structural, not metal binding role, and the cysteinyls of the C-terminal domains VI and VII are important for Cu binding. The data provide an important insight into the metal handling of site-associated ectomycorrhizal species disturbed by excess metals and the properties of MTs common in Agaricomycetes.

Identification and characterization of eight metallothionein genes involved in heavy metal tolerance from the ectomycorrhizal fungus Laccaria bicolor

Metallothioneins (MTs) are small, cysteine-rich, heavy metal-binding proteins involved in metal homeostasis and detoxification. The increasing numbers of available genomic sequences of ectomycorrhizal (ECM) fungi enable deeper insights into the characteristics of MT genes in these fungi that form the most important symbiosis with the host trees in forest ecosystems. The aim of this study was to establish a comprehensive, genome-wide inventory of MT genes from the ECM fungus Laccaria bicolor. Eight MT genes in L. bicolor were cloned, and the expression patterns of their transcripts at various developmental stages based on expressed sequence tag (EST) counts were analyzed. The expression levels of four MTs were significantly increased during symbiosis stages. Quantitative real-time PCR (qRT-PCR) analysis revealed that transcripts of LbMT1 were dominant in free-living mycelia and strongly induced by excessive copper (Cu), cadmium (Cd), and hydrogen peroxide (H2O2). To determine whether these eight MTs functioned as metal chelators, we expressed them in the Cu- and Cd-sensitive yeast mutants, cup1∆ and yap1∆, respectively. All LbMT proteins provided similar levels of Cu(II) or Cd(II) tolerance, but did not affect by H2O2. Our findings provide novel data on the evolution and diversification of fungal MT gene duplicates, a valuable resource for understanding the vast array of biological processes in which these proteins are involved.

Genome-Scale Screening and Combinatorial Optimization of Gene Overexpression Targets to Improve Cadmium Tolerance in Saccharomyces cerevisiae

Heavy metal contamination is an environmental issue on a global scale. Particularly, cadmium poses substantial threats to crop and human health. Saccharomyces cerevisiae is one of the model organisms to study cadmium toxicity and was recently engineered as a cadmium hyperaccumulator. Therefore, it is desirable to overcome the cadmium sensitivity of S. cerevisiae via genetic engineering for bioremediation applications. Here we performed genome-scale overexpression screening for gene targets conferring cadmium resistance in CEN.PK2-1c, an industrial S. cerevisiae strain. Seven targets were identified, including CAD1 and CUP1 that are known to improve cadmium tolerance, as well as CRS5, NRG1, PPH21, BMH1, and QCR6 that are less studied. In the wild-type strain, cadmium exposure activated gene transcription of CAD1, CRS5, CUP1, and NRG1 and repressed PPH21, as revealed by real-time quantitative PCR analyses. Furthermore, yeast strains that contained two overexpression mutations out of the seven gene targets were constructed. Synergistic improvement in cadmium tolerance was observed with episomal co-expression of CRS5 and CUP1. In the presence of 200 μM cadmium, the most resistant strain overexpressing both CAD1 and NRG1 exhibited a 3.6-fold improvement in biomass accumulation relative to wild type. This work provided a new approach to discover and optimize genetic engineering targets for increasing cadmium resistance in yeast.

Analysis of spectrometry and thermodynamics of the metallothionein in freshwater crab Sinopotamon henanense for its binding ability with different metals

The metal binding nature of heterologously expressed metallothionein of Sinopotamon henanense (ShMT) had been demonstrated previously. In this study, we analysed the stoichiometry of ShMT yielded in vivo and exchange reactions of the Zn-ShMT with Cd²⁺, Pb²⁺ and Cu²⁺in vitro via electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS), circular dichroism (CD) spectroscopy, inductively coupled plasma mass spectrometry (ICP-MS), and isothermal titration calorimetry (ITC). The results of ESI-TOF-MS analyses showed that metal-ShMT synthesized in vivo had three major forms, namely Zn₁₅-, Cd₉-, and Pb₅-ShMT. The ITC analyses of exchange reactions demonstrated that Zn-ShMT exhibited up to 6, 6, and 7 binding sites for Cd²⁺, Pb²⁺ and Cu²⁺. By the analyses of the UV and CD spectra in the substitution experiments showed that the geometric structural stability of metal-ShMT could be influenced when excess of over 6, 6, or 7 equivalents of Cd²⁺, Pb²⁺, or Cu²⁺ were added into Zn-ShMT. Although both the reconstructed apo-ShMT and substituted Zn-ShMT with three metal ions fitted the same M₆Ⅱ- and M₇Ⅰ-ShMT binding models for divalent and monovalent metals, the differences in their thermodynamic data suggested that discrepancies exit in their physiological functions. These results suggested that ShMT yielded in vivo had a higher storage capability for Zn²⁺ and a uptake ability for Cd²⁺, and Zn-ShMT was more easy to release Zn²⁺ as well as to uptake Cd²⁺, Cu²⁺, or Pb²⁺. The results presented in this work will be very valuable to understand the function(s) of ShMT not only in a normal physiological condition but also in the presence of non-essential metals in crabs.

Chromatographic detection of low-molecular-mass metal complexes in the cytosol of Saccharomyces cerevisiae

Fluorescence-based chelators are commonly used to probe labile low-molecular-mass (LMM) metal pools in the cytosol of eukaryotic cells, but such chelators destroy the complexes of interest during detection. The objective of this study was to use chromatography to directly detect such complexes. Towards this end, 47 batches of cytosol were isolated from fermenting S. cerevisiae yeast cells and passed through a 10 kDa cut-off membrane. The metal contents of the cytosol and resulting flow-through solution (FTS) were determined. FTSs were applied to a size-exclusion LC column located in an anaerobic refrigerated glove box. The LC system was coupled to an online inductively-coupled-plasma mass spectrometer (ICP-MS) for detection of individual metals. Iron-detected chromatograms of cytosolic FTSs from WT cells exhibited 2-4 major species with apparent masses between 500-1300 Da. Increasing the iron concentration in the growth medium 40-fold increased the overall intensity of these peaks. Approximately 3 LMM cytosolic copper complexes with apparent masses between 300-1300 Da were also detected; their LC intensities were weak, but these increased with increasing concentrations of copper in the growth medium. Observed higher-mass copper-detected peaks were tentatively assigned to copper-bound metallothioneins Cup1 and Crs5. FTSs from strains in which Cup1 or the Cox17 copper chaperone were deleted altered the distribution of LMM copper complexes. LMM zinc- and manganese-detected species were also present in cytosol, albeit at low concentrations. Supplementing the growth medium with zinc increased the intensity of the zinc peak assigned to Crs5 but the intensities of LMM zinc complexes were unaffected. Phosphorus-detected chromatograms were dominated by peaks at apparent masses 400-800 Da, with minor peaks at 1000-1500 Da in some batches. Sulfur chromatograms contained a low-intensity peak that comigrated with a glutathione standard; quantification suggested a GSH concentration in the cytosol of ca. 13 mM. A second LMM sulfur peak that migrated at an apparent mass of 100 Da was also evident.

The cellular economy of the Saccharomyces cerevisiae zinc proteome

Zinc is an essential cofactor for many proteins. A key mechanism of zinc homeostasis during deficiency is "zinc sparing" in which specific zinc-binding proteins are repressed to reduce the cellular requirement. In this report, we evaluated zinc sparing across the zinc proteome of Saccharomyces cerevisiae. The yeast zinc proteome of 582 known or potential zinc-binding proteins was identified using a bioinformatics analysis that combined global domain searches with local motif searches. Protein abundance was determined by mass spectrometry. In zinc-replete cells, we detected over 2500 proteins among which 229 were zinc proteins. Based on copy number estimates and binding stoichiometries, a replete cell contains ∼9 million zinc-binding sites on proteins. During zinc deficiency, many zinc proteins decreased in abundance and the zinc-binding requirement decreased to ∼5 million zinc atoms per cell. Many of these effects were due at least in part to changes in mRNA levels rather than simply protein degradation. Measurements of cellular zinc content showed that the level of zinc atoms per cell dropped from over 20 million in replete cells to only 1.7 million in deficient cells. These results confirmed the ability of replete cells to store excess zinc and suggested that the majority of zinc-binding sites on proteins in deficient cells are either unmetalated or mismetalated. Our analysis of two abundant zinc proteins, Fba1 aldolase and Met6 methionine synthetase, supported that hypothesis. Thus, we have discovered widespread zinc sparing mechanisms and obtained evidence of a high accumulation of zinc proteins that lack their cofactor during deficiency.

Chromatographic separation of similar post-translationally modified metallothioneins reveals the changing conformations of apo-MT upon cysteine alkylation by high resolution LC-ESI-MS

Metallothioneins (MTs) are a class of small cysteine-rich proteins essential for Zn and Cu homeostasis, heavy metal detoxification, and cellular redox chemistry. Herein, we describe the separation and characterization of MTs differentially modified with N-ethylmaleimide (NEM) by liquid chromatography-mass spectrometry (LC-MS). The full-length recombinant MT isoform 1a as well as is isolated domain fragments were first alkylated, then separated on column with subsequent detection by ultra-high resolution ESI-MS. Different behavior was observed for the three peptides with the full-length protein and the isolated α-domain exhibiting similar separation characteristics. For the isolated β-domain, the smallest peptide with 9 cysteines in the sequence, each alkylated species was well separated, indicating large changes in protein conformation. For the full-length (20 cysteines in the sequence) and α-domain (11 cysteiens in the sequence) peptides, the apo- and lightly alkylated species co-eluted, indicating similar structural properties. However, the more extensively alkylated species were well separated from each other, indicating the sequential unfolding of the apo-MT peptides and providing evidence for the mechanistic explanation for the cooperative alkylation reaction observed for NEM and other bulky and hydrophobic alkylation reagents. We show for the first time clear separation of highly similar MTs, differing by only +125 Da, and can infer structural properties from the LC-MS data, analogous to more complicated and less ubiquitous ion-mobility experiments. The data suggest a compact globular structure for each of the apo-MTs, but where the β-domain is more easily unfolded. This differential folding stability may have biological implications in terms of domain-specific participation of MT in cellular redox chemistry and resulting metal release.

Acute toxic effects of zinc and mercury on survival, standard metabolism, and metal accumulation in juvenile ridgetail white prawn, Exopalaemon carinicauda

Ridgetail white prawn (Exopalaemon carinicauda) is widely distributed in Chinese coastal zones, especially in the Yellow Sea and Bohai Sea. It is not only considered as an important economic species in China, but also taken as a potential indicator species for the environmental pollution in the estuaries. At present, the responses of this species to environmental toxicants, including trace metal are not well understood. In this study, the acute toxic effects of zinc (Zn) and mercury (Hg) on the survival, oxygen consumption, ammonia-N excretion, and metal accumulation were investigated in the juveniles of E. carinicauda. The median lethal concentrations (LC₅₀) of Zn were 76.4, 44.0, 30.2, and 17.2mg/L, respectively, after the juveniles were exposed in for 24, 48, 72, and 96h, and the LC₅₀ of H_g was 0.212, 0.096, 0.084, and 0.065mg/L under the same exposure duration. The juveniles decreased the oxygen consumption by 51.4%, and increased ammonia-N excretion by 129% when they were exposed in Zn at the concentration of 76.4mg/L compared with their controls without Zn exposure, therefore the O:N ratio decreased by 82.9% compared with the control. Hg exposure with the concentration of 0.212mg/L caused the inhibition of oxygen consumption by 48.1% and increasement of ammonia-N excretion by 161%, and the atomic ratio of consumed oxygen to excreted ammonia-nitrogen (O:N ratio) decreased by 80.6% in the juveniles in comparison with the control. A concentration-dependent accumulation of heavy metals was observed in the gills, hepatopancreas and muscles of the experimental animals, with a maximum accumulation of 16.3 folds for Zn and 72.8 fold for Hg in the gills of the juveniles after 24h exposure. The data obtained from the present study would provide useful information for help further understanding on the toxicological responses of this species to trace metals.

Analysis of Metal-Binding Features of the Wild Type and Two Domain-Truncated Mutant Variants of Littorina littorea Metallothionein Reveals Its Cd-Specific Character

After the resolution of the 3D structure of the Cd₉-aggregate of the Littorina littorea metallothionein (MT), we report here a detailed analysis of the metal binding capabilities of the wild type MT, LlwtMT, and of two truncated mutants lacking either the N-terminal domain, Lltr2MT, or both the N-terminal domain, plus four extra flanking residues (SSVF), Lltr1MT. The recombinant synthesis and in vitro studies of these three proteins revealed that LlwtMT forms unique M₉-LlwtMT complexes with Zn(II) and Cd(II), while yielding a complex mixture of heteronuclear Zn,Cu-LlwtMT species with Cu(I). As expected, the truncated mutants gave rise to unique M₆-LltrMT complexes and Zn,Cu-LltrMT mixtures of lower stoichiometry with respect to LlwtMT, with the SSVF fragment having an influence on their metal binding performance. Our results also revealed a major specificity, and therefore a better metal-coordinating performance of the three proteins for Cd(II) than for Zn(II), although the analysis of the Zn(II)/Cd(II) displacement reaction clearly demonstrates a lack of any type of cooperativity in Cd(II) binding. Contrarily, the analysis of their Cu(I) binding abilities revealed that every LlMT domain is prone to build Cu₄-aggregates, the whole MT working by modules analogously to, as previously described, certain fungal MTs, like those of C. neoformans and T. mesenterica. It is concluded that the Littorina littorea MT is a Cd-specific protein that (beyond its extended binding capacity through an additional Cd-binding domain) confers to Littorina littorea a particular adaptive advantage in its changeable marine habitat.

Biomphalaria glabrata Metallothionein: Lacking Metal Specificity of the Protein and Missing Gene Upregulation Suggest Metal Sequestration by Exchange Instead of through Selective Binding

The wild-type metallothionein (MT) of the freshwater snail Biomphalaria glabrata and a natural allelic mutant of it in which a lysine residue was replaced by an asparagine residue, were recombinantly expressed and analyzed for their metal-binding features with respect to Cd²⁺, Zn²⁺ and Cu⁺, applying spectroscopic and mass-spectrometric methods. In addition, the upregulation of the Biomphalaria glabrataMT gene was assessed by quantitative real-time detection PCR. The two recombinant proteins revealed to be very similar in most of their metal binding features. They lacked a clear metal-binding preference for any of the three metal ions assayed-which, to this degree, is clearly unprecedented in the world of Gastropoda MTs. There were, however, slight differences in copper-binding abilities between the two allelic variants. Overall, the missing metal specificity of the two recombinant MTs goes hand in hand with lacking upregulation of the respective MT gene. This suggests that in vivo, the Biomphalaria glabrata MT may be more important for metal replacement reactions through a constitutively abundant form, rather than for metal sequestration by high binding specificity. There are indications that the MT of Biomphalaria glabrata may share its unspecific features with MTs from other freshwater snails of the Hygrophila family.

Functional analysis of two genes coding for distinct cation diffusion facilitators of the ectomycorrhizal Zn-accumulating fungus Russula atropurpurea

Russula atropurpurea can accumulate remarkably high concentrations of Zn in its sporocarps. We have previously demonstrated that 40 % of the intracellular Zn in this species is sequestered by MT-like RaZBP peptides. To see what other mechanisms for the handling of the accumulated Zn are available to R. atropurpurea, we searched its transcriptome for cDNAs coding for transporters of the cation diffusion facilitator (CDF) family. The transcriptome search enabled us to identify RaCDF1 and RaCDF2, which were further subjected to functional studies in metal sensitive Saccharomyces cerevisiae. The expression of RaCDF1 and its translational fusion with green fluorescent protein (GFP) protected the yeasts against Zn and Co, but not Cd or Mn, toxicity and led to increased Zn accumulation in the cells. The GFP fluorescence, observed in the RaCDF1::GFP-expressing yeasts on tonoplasts, indicated that the RaCDF1-mediated Zn and Co tolerance was a result of vacuolar sequestration of the metals. The expression of RaCDF2 supported Zn, but not Mn, tolerance in the yeasts and reduced the cellular uptake of Zn, which is congruent with the proposed idea of the Zn-efflux function of RaCDF2, supported by the localization of GFP-derived fluorescence on the plasma membrane of the yeasts expressing functional RaCDF2::GFP. Contrarily, RaCDF2 increased the sensitivity to Co and Cd in the yeasts and significantly promoted Cd uptake, which suggested that it can act as a bidirectional metal transporter. The notion that RaCDF1 and RaCDF2 are genuine CDF transporters in R. atropurputrea was further reinforced by the fact that the RaCDF-associated metal tolerance and uptake phenotypes were lost upon the replacement of histidyl (in RaCDF1) and aspartyl (in RaCDF2), which are highly conserved in the second transmembrane domain and known to be essential for the function of CDF proteins.

The Fungus Tremella mesenterica Encodes the Longest Metallothionein Currently Known: Gene, Protein and Metal Binding Characterization

Fungal Cu-thioneins, and among them, the paradigmatic Neurospora crassa metallothionein (MT) (26 residues), were once considered as the shortest MTs -the ubiquitous, versatile metal-binding proteins- among all organisms, and thus representatives of their primeval forms. Nowadays, fungal MTs of diverse lengths and sequence features are known, following the huge heterogeneity of the Kingdom of Fungi. At the opposite end of N. crassa MT, the recently reported Cryptococcus neoformans CnMT1 and CnMT2 (122 and 186 aa) constitute the longest reported fungal MTs, having been identified as virulence factors of this pathogen. CnMTs are high-capacity Cu-thioneins that appear to be built by tandem amplification of a basic unit, a 7-Cys segment homologous to N. crassa MT. Here, we report the in silico, in vivo and in vitro study of a still longer fungal MT, belonging to Tremella mesenterica (TmMT), a saprophytic ascomycete. The TmMT gene has 10 exons, and it yields a 779-bp mature transcript that encodes a 257 residue-long protein. This MT is also built by repeated fragments, but of variable number of Cys: six units of the 7-Cys building blocks-CXCX₃CSCPPGXCXCAXCP-, two fragments of six Cys, plus three Cys at the N-terminus. TmMT metal binding abilities have been analyzed through the spectrophotometric and spectrometric characterization of its recombinant Zn-, Cd- and Cu-complexes. Results allow it to be unambiguous classified as a Cu-thionein, also of extraordinary coordinating capacity. According to this feature, when the TmMT cDNA is expressed in MT-devoid yeast cells, it is capable of restoring a high Cu tolerance level. Since it is not obvious that T. mesenterica shares the same physiological needs for a high capacity Cu-binding protein with C. neoformans, the existence of this peculiar MT might be better explained on the basis of a possible role in Cu-handling for the Cu-enzymes responsible in lignin degradation pathways.

Does Variation of the Inter-Domain Linker Sequence Modulate the Metal Binding Behaviour of Helix pomatia Cd-Metallothionein?

Snail metallothioneins (MTs) constitute an ideal model to study structure/function relationships in these metal-binding polypeptides. Helix pomatia harbours three MT isoforms: the highly specific CdMT and CuMT, and an unspecific Cd/CuMT, which represent paralogous proteins with extremely different metal binding preferences while sharing high sequence similarity. Preceding work allowed assessing that, although, the Cys residues are responsible for metal ion coordination, metal specificity or preference is achieved by diversification of the amino acids interspersed between them. The metal-specific MT polypeptides fold into unique, energetically-optimized complexes of defined metal content, when binding their cognate metal ions, while they produce a mixture of complexes, none of them representing a clear energy minimum, with non-cognate metal ions. Another critical, and so far mostly unexplored, region is the stretch linking the individual MT domains, each of which represents an independent metal cluster. In this work, we have designed and analyzed two HpCdMT constructs with substituted linker segments, and determined their coordination behavior when exposed to both cognate and non-cognate metal ions. Results unequivocally show that neither length nor composition of the inter-domain linker alter the features of the Zn(II)- and Cd(II)-complexes, but surprisingly that they influence their ability to bind Cu(I), the non-cognate metal ion.

Understanding the 7-Cys module amplification of C. neoformans metallothioneins: how high capacity Cu-binding polypeptides are built to neutralize host nutritional immunity

Cryptococcus neoformans metallothioneins (MTs), CnMT1 and CnMT2, have been identified as essential infectivity and virulence factors of this pathogen. Both MTs are unusually long Cu-thioneins, exhibiting protein architecture and metal-binding abilities compatible with the hypothesis of resulting from three and five tandem repetitions of 7-Cys motives, respectively, each of them folding into Cu5-clusters. Through the study of the Zn(II)- and Cu(I)-binding capabilities of several CnMT1 truncated mutants, we show that a 7-Cys segment of CnMT1 folds into Cu5-species, of additive capacity when joined in tandem. All the obtained Cu-complexes share practically similar architectural features, if judging by their almost equivalent CD fingerprints, and they also share their capacity to restore copper tolerance in MT-devoid yeast cells. Besides the analysis of the modular composition of these long fungal MTs, we evaluate the features of the Cys-rich stretch spacer and flanking sequences that allow the construction of stable metal clusters by adjacent union of binding modules. Overall, our data support a mechanism by which some microbial MTs may have evolved to enlarge their original metal co-ordination capacity under the specific selective pressure of counteracting the Cu-based immunity mechanisms evolved by the infected hosts.

Hints for metal-preference protein sequence determinants: different metal binding features of the five tetrahymena thermophila metallothioneins

The metal binding preference of metallothioneins (MTs) groups them in two extreme subsets, the Zn/Cd- and the Cu-thioneins. Ciliates harbor the largest MT gene/protein family reported so far, including 5 paralogs that exhibit relatively low sequence similarity, excepting MTT2 and MTT4. In Tetrahymena thermophila, three MTs (MTT1, MTT3 and MTT5) were considered Cd-thioneins and two (MTT2 and MTT4) Cu-thioneins, according to gene expression inducibility and phylogenetic analysis. In this study, the metal-binding abilities of the five MTT proteins were characterized, to obtain information about the folding and stability of their cognate- and non-cognate metal complexes, and to characterize the T. thermophila MT system at protein level. Hence, the five MTTs were recombinantly synthesized as Zn²⁺-, Cd²⁺- or Cu⁺-complexes, which were analyzed by electrospray mass spectrometry (ESI-MS), circular dichroism (CD), and UV-vis spectrophotometry. Among the Cd-thioneins, MTT1 and MTT5 were optimal for Cd²⁺ coordination, yielding unique Cd₁₇- and Cd₈- complexes, respectively. When binding Zn²⁺, they rendered a mixture of Zn-species. Only MTT5 was capable to coordinate Cu⁺, although yielding heteronuclear Zn-, Cu-species or highly unstable Cu-homometallic species. MTT3 exhibited poor binding abilities both for Cd²⁺ and for Cu⁺, and although not optimally, it yielded the best result when coordinating Zn²⁺. The two Cu-thioneins, MTT2 and MTT4 isoforms formed homometallic Cu-complexes (major Cu₂₀-MTT) upon synthesis in Cu-supplemented hosts. Contrarily, they were unable to fold into stable Cd-complexes, while Zn-MTT species were only recovered for MTT4 (major Zn₁₀-MTT4). Thus, the metal binding preferences of the five T. thermophila MTs correlate well with their previous classification as Cd- and Cu-thioneins, and globally, they can be classified from Zn/Cd- to Cu-thioneins according to the gradation: MTT1>MTT5>MTT3>MTT4>MTT2. The main mechanisms underlying the evolution and specialization of the MTT metal binding preferences may have been internal tandem duplications, presence of doublet and triplet Cys patterns in Zn/Cd-thioneins, and optimization of site specific amino acid determinants (Lys for Zn/Cd- and Asn for Cu-coordination).

Full characterization of the Cu-, Zn-, and Cd-binding properties of CnMT1 and CnMT2, two metallothioneins of the pathogenic fungus Cryptococcus neoformans acting as virulence factors

We report here the full characterization of the metal binding abilities of CnMT1 and CnMT2, two Cryptococcus neoformans proteins recently identified as metallothioneins (MTs), which have been shown to play a crucial role in the virulence and pathogenicity of this human-infecting fungus. In this work, we first performed a thorough in silico study of the CnMT1 and CnMT2 genes, cDNAs and corresponding encoded products. Subsequently, the Zn(II)-, Cd(II)- and Cu(I) binding abilities of both proteins were fully determined through the analysis of the metal-to-protein stoichiometries and the structural features (determined by ESI-MS, CD, ICP-AES and UV-vis spectroscopies) of the corresponding recombinant Zn-, Cd- and Cu-MT preparations synthesized in metal-enriched media. Finally, the analysis of the Zn/Cd and Zn/Cu replacement processes of the respective Zn-MT complexes when allowed to react with Cd(II) or Cu(I) aqueous solutions was performed. Comprehensive consideration of all gathered results allows us to consider both isoforms as genuine copper-thioneins, and led to the identification of unprecedented Cu5-core clusters in MTs. CnMT1 and CnMT2 polypeptides appear to be evolutionarily related to the small fungal MTs, probably by ancient tandem-duplication events responding to a highly selective pressure to chelate copper, and far from the properties of Zn- and Cd-thioneins. Finally, we propose a modular structure of the Cu-CnMT1 and Cu-CnMT2 complexes on the basis of Cu5 clusters, concordantly with the modular structure of the sequence of CnMT1 and CnMT2, constituted by three and five Cys-rich units, respectively.

Cognate and noncognate metal ion coordination in metal-specific metallothioneins: the Helix pomatia system as a model

The Helix pomatia metallothionein (MT) system, namely, its two highly specific forms, HpCdMT and HpCuMT, has offered once again an optimum model to study metal-protein specificity. The present work investigates the most unexplored aspect of the coordination behavior of MT polypeptides with respect to either cognate or noncognate metal ions, as opposed to the standard studies of cognate metal ion coordination. To this end, we analyzed the in vivo synthesis of the corresponding complexes with their noncognate metals, and we performed a detailed spectroscopic and spectrometric study of the Zn(2+)/Cd(2+) and Zn(2+)/Cu(+) in vitro replacement reactions on the initial Zn-HpMT species. An HpCuMTAla site-directed mutant, exhibiting differential Cu(+)-binding abilities in vivo, was also included in this study. We demonstrate that when an MT binds its cognate metal, it yields well-folded complexes of limited stoichiometry, representative of minimal-energy conformations. In contrast, the incorporation of noncognate metal ions is better attributed to an unspecific reaction of cysteinic thiolate groups with metal ions, which is dependent on their concentration in the surrounding milieu, where no minimal-energy structure is reached, and otherwise, the MT peptide acts as a multidentate ligand that will bind metal ions until its capacity has been saturated. Additionally, we suggest that previous binding of an MT polypeptide with its noncognate metal ion (e.g., binding of Zn(2+) to the HpCuMT isoform) may preclude the correct folding of the complex with its cognate metal ion.

Cadmium

Cadmium is not an essential element for life. It is geologically marginal but anthropogenic activities have contributed significantly to its dispersion in the environment and to cadmium exposure of living species. The natural speciation of the divalent cation Cd2+ is dominated by its high propensity to bind to sulfur ligands, but Cd2+ may also occupy sites providing imidazole and carboxylate ligands. It binds to cell walls by passive adsorption (bio-sorption) and it may interact with surface receptors. Cellular uptake can occur by ion mimicry through a variety of transporters of essential divalent cations, but not always. Once inside cells, Cd2+ preferentially binds to thiol-rich molecules. It can accumulate in intracellular vesicles. It may also be transported over long distances within multicellular organisms and be trapped in locations devoid of efficient excretion systems. These locations include the renal cortex of animals and the leaves of hyper-accumulating plants. No specific regulatory mechanism monitors Cd2+ cellular concentrations. Thiol recruitment by cadmium is a major interference mechanism with many signalling pathways that rely on thiolate-disulfide equilibria and other redox-related processes. Cadmium thus compromises the antioxidant intracellular response that relies heavily on molecules with reactive thiolates. These biochemical features dominate cadmium toxicity, which is complex because of the diversity of the biological targets and the consequent pleiotropic effects. This chapter compares the cadmium-handling systems known throughout phylogeny and highlights the basic principles underlying the impact of cadmium in biology.

Differential expression of metallothioneins in response to heavy metals and their involvement in metal tolerance in the symbiotic basidiomycete Laccaria bicolor

Cysteine-rich peptides such as metallothioneins (MTs) are involved in metal homeostasis and detoxification in many eukaryotes. We report the characterization and expression of two MT genes, LbMT1 and LbMT2 from the ectomycorrhizal fungus Laccaria bicolor under metal stress conditions. LbMT1 and LbMT2 differ with respect to the length of the encoded peptides (58 versus 37 aa, respectively) and also by their expression patterns in response to metals. The expression levels of both LbMT1 and LbMT2 increased as a function of increased external Cu concentration, the expression levels for LbMT2 were always significantly higher compared with those of LbMT1. Only LbMT1, but not LbMT2, responded to Cd supply in the range of 25-100 µM while Zn did not affect the transcription of either LbMT1 or LbMT2. Both genes also responded to oxidative stress, but to a lesser extent compared to their responses to either Cu or Cd stress. Heterologous complementation assays in metal-sensitive yeast mutants indicated that both LbMT1 and LbMT2 encode peptides capable of conferring higher tolerance to both Cu and Cd. The present study identified LbMTs as potential determinants of the response of this mycorrhizal fungus to Cu and Cd stress.

Intracellular sequestration of zinc, cadmium and silver in Hebeloma mesophaeum and characterization of its metallothionein genes

Sequestration of intracellular heavy metals in eukaryotes involves compartmentalization and binding with cytosolic, cysteine-rich metallothionein (MT) peptides. We examined the roles of these processes in handling of zinc (Zn), cadmium (Cd) and silver (Ag) in sporocarps and a metal-exposed extraradical mycelium of Hebeloma mesophaeum, the Zn-accumulating ectomycorrhizal (EM) species frequently associated with metal disturbed sites. Size exclusion chromatography revealed that the majority of Zn and Cd in the sporocarps and mycelium was contained in a low molecular mass fraction attributable to compartmentalized metal. The staining of hyphal cells with the Zn-specific Zinquin and Cd-specific Leadmium fluorescent tracers labeled Zn and Cd in small, punctuated vesicles and vacuoles, respectively. By contrast, the sporocarp and mycelium Ag was associated with cysteine-rich, 5-kDa peptides. The peptides of the same size were also identified in minor Zn and Cd complexes from the metal-exposed mycelium. We have further isolated and characterized HmMT1, HmMT2 and HmMT3 genes coding for different 5-kDa MTs of H. mesophaeum collected at a lead smelter site. Heterologous complementation assays in metal-sensitive yeast mutants indicated that HmMTs encode functional, metal-specific peptides: only HmMT1 was able to complement sensitivity to Zn; HmMT1 conferred higher tolerance to Cd and Cu than HmMT2 or HmMT3; and both HmMT2 and HmMT3, but not HmMT1, conferred increased tolerance to Ag. The presence of HmMT1 and HmMT3, but not HmMT2, was also confirmed in a H. mesophaeum isolate from an unpolluted site. Gene expression analysis in the extraradical mycelium of this isolate revealed that the transcription of HmMT1 was preferentially induced in the presence of Zn and Cd, while Ag was a stronger inducer of HmMT3. Altogether, these results improve our understanding of the handling of intracellular Zn, Cd and Ag in Hebeloma and represent the first evidence suggesting involvement of MTs in sequestration of Zn in EM fungi.

In vivo-folded metal-metallothionein 3 complexes reveal the Cu-thionein rather than Zn-thionein character of this brain-specific mammalian metallothionein

Metallothionein-3 (MT3) is one of the four mammalian metallothioneins (MT), and is constitutively synthesized in the brain. MT3 acts both intracellularly and extracellularly in this organ, performing functions related to neuronal growth and physiological metal (Zn and Cu) handling. It appears to be involved in the prevention of neurodegenerative disorders caused by insoluble Cu-peptide aggregates, as it triggers a Zn-Cu swap that may counteract the deleterious presence of copper in neural tissues. The literature data on MT3 coordination come from studies either on apo-MT3 reconstitution or the reaction of Zn-MT3 with Cu(2+) , an ion that is hardly present inside cells. To ascertain the MT3 metal-binding features in a scenario closer to the reductive cell cytoplasm, a study of the recombinant Zn(2+) , Cd(2+) and Cu(+) complexes of MT3, βMT3, and αMT3, as well as the in vitro Zn(2+) -Cd(2+) and Zn(2+) -Cu(+) replacement processes, is presented here. We conclude that MT3 has a Cu-thionein character that is stronger than that of the MT1 and MT2 isoforms - also present in the mammalian brain - which is mainly contributed by its β domain. In contrast, the α domain retains a high capacity to bind Zn(2+) ions, and, consequently, the entire MT3 peptide shows a peculiar dual ability to handle both metal ions. The nature of the formed Cu(+) -MT3 complexes oscillates from heterometallic Cu6 Zn4 -MT3 to homometallic Cu10 -MT3 major species, in a narrow Cu concentration range. Therefore, the entire MT3 peptide shows a high capacity to bind Cu(+) , provided that this occurs in a nonoxidative milieux. This reflects a peculiar property of this MT isoform, which accurately senses different Cu contents in the environment in which it is synthesized.

Metallothionein-like peptides involved in sequestration of Zn in the Zn-accumulating ectomycorrhizal fungus Russula atropurpurea

The first evidence of the existence of gene-encoded Zn-binding peptides that sequester a substantial portion of intracellular Zn in ectomycorrhizal fungi under natural conditions.

Overexpression of ctr1Δ300, a high-affinity copper transporter with deletion of the cytosolic C-terminus in Saccharomyces cerevisiae under excess copper, leads to disruption of transition metal homeostasis and transcriptional remodelling of cellular processes

In an approach to generating Saccharomyces cerevisiae strains with increased intracellular copper amounts for technical applications, we overexpressed the copper transporter CTR1 and a variant of CTR1 with a truncation in the C-terminus after the 300th amino acid (ctr1Δ300). We determined the copper sensitivity of the generated strains and used inductively coupled plasma spectrometry analysis (ICP-OES and ICP-MS) to investigate the effects of overexpression of both constructs under excess copper on the cellular content of different elements in S. cerevisiae. In addition, we performed DNA microarray analysis to obtain the gene expression profile under the changed element contents. Overexpression of CTR1 increased the copper content in the cells to 160% and 78 genes were differentially regulated. Overexpression of the truncated ctr1Δ300 resulted in an increased copper, iron and zinc content of > 200% and 980 genes showed differential expression. We found that transition metal ion homeostasis was disrupted in ctr1Δ300-overexpressing strains under excess copper and that this was combined with a transcriptional remodelling of cellular processes.

The sea urchin metallothionein system: Comparative evaluation of the SpMTA and SpMTB metal-binding preferences

Metallothioneins (MTs) constitute a superfamily of ubiquitous metal-binding proteins of low molecular weight and high Cys content. They are involved in metal homeostasis and detoxification, amongst other proposed biological functions. Two MT isoforms (SpMTA and SpMTB) have been reported in the echinoderm Strongylocentrotus purpuratus (sea urchin), both containing 20 Cys residues and presenting extremely similar sequences, although showing distinct tissular and ontogenic expression patterns. Although exhaustive information is available for the Cd(II)-SpMTA complex, this including the full resolution of its 3D structure, no data has been reported concerning either SpMTA Zn(II) and Cu(I) binding properties, or the characterization of SpMTB at protein level. In this work, both the SpMTA and SpMTB isoforms, as well as their separate α and β domains, have been recombinantly synthesized in the presence of Zn(II), Cd(II) or Cu(II), and the corresponding metal complexes have been analyzed using electrospray mass spectrometry, and CD, ICP-AES and UV-vis spectroscopies. The results clearly show a better performance of isoform A when binding Zn(II) and Cd(II), and of isoform B when coordinating Cu(I). Thus, our results confirm the differential metal binding preference of SpMTA and SpMTB, which, together with the reported induction pattern of the respective genes, highlights how also in Echinodermata the MT polymorphism may be linked to the evolution of different physiological roles.

Metal dealing at the origin of the Chordata phylum: the metallothionein system and metal overload response in amphioxus

Non-vertebrate chordates, specifically amphioxus, are considered of the utmost interest for gaining insight into the evolutionary trends, i.e. differentiation and specialization, of gene/protein systems. In this work, MTs (metallothioneins), the most important metal binding proteins, are characterized for the first time in the cephalochordate subphylum at both gene and protein level, together with the main features defining the amphioxus response to cadmium and copper overload. Two MT genes (BfMT1 and BfMT2) have been identified in a contiguous region of the genome, as well as several ARE (antioxidant response element) and MRE (metal response element) located upstream the transcribed region. Their corresponding cDNAs exhibit identical sequence in the two lancelet species (B. floridae and B. lanceolatum), BfMT2 cDNA resulting from an alternative splicing event. BfMT1 is a polyvalent metal binding peptide that coordinates any of the studied metal ions (Zn, Cd or Cu) rendering complexes stable enough to last in physiological environments, which is fully concordant with the constitutive expression of its gene, and therefore, with a metal homeostasis housekeeping role. On the contrary, BfMT2 exhibits a clear ability to coordinate Cd(II) ions, while it is absolutely unable to fold into stable Cu (I) complexes, even as mixed species. This identifies it as an essential detoxification agent, which is consequently only induced in emergency situations. The cephalochordate MTs are not directly related to vertebrate MTs, neither by gene structure, protein similarity nor metal-binding behavior of the encoded peptides. The closest relative is the echinoderm MT, which confirm proposed phylogenetic relationships between these two groups. The current findings support the existence in most organisms of two types of MTs as for their metal binding preferences, devoted to different biological functions: multivalent MTs for housekeeping roles, and specialized MTs that evolve either as Cd-thioneins or Cu-thioneins, according to the ecophysiological needs of each kind of organisms.

Amplification of the CUP1 gene is associated with evolution of copper tolerance in Saccharomyces cerevisiae

In living organisms, copper (Cu) contributes to essential functions but at high concentrations it may elicit toxic effects. Cu-tolerant yeast strains are of relevance for both biotechnological applications and studying physiological and molecular mechanisms involved in stress resistance. One way to obtain tolerant strains is to exploit experimental methods that rely on the principles of natural evolution (evolutionary engineering) and allow for the development of complex phenotypic traits. However, in most cases, the molecular and physiological basis of the phenotypic changes produced have not yet been unravelled. We investigated the determinants of Cu resistance in a Saccharomyces cerevisiae strain that was evolved to tolerate up to 2.5 g CuSO(4) l(-1) in the culture medium. We found that the content of intracellular Cu and the expression levels of several genes encoding proteins involved in Cu metabolism and oxidative stress response were similar in the Cu-tolerant (evolved) and the Cu-sensitive (non-evolved) strain. The major difference detected in the two strains was the copy number of the gene CUP1, which encodes a metallothionein. In evolved cells, a sevenfold amplification of CUP1 was observed, accounting for its strongly and steadily increased expression. Our results implicate CUP1 in protection of the evolved S. cerevisiae cells against Cu toxicity. In these cells, robustness towards Cu is stably inheritable and can be reproducibly selected by controlling environmental conditions. This finding corroborates the effectiveness of laboratory evolution of whole cells as a tool to develop microbial strains for biotechnological applications.

Enhanced resistance of yeast mutants deficient in low-affinity iron and zinc transporters to stannous-induced toxicity

Tin or stannous (Sn(2+)) compounds are used as catalysts, stabilizers in plastic industries, wood preservatives, agricultural biocides and nuclear medicine. In order to verify the Sn(2+) up-take and toxicity in yeast cells we utilized a multi-elemental analysis known as particle-induced X-ray emission (PIXE) along with cell survival assays and quantitative real-time PCR. The detection of Sn(2+) by PIXE was possible only in yeast cells in stationary phase of growth (STAT cells) that survive at 25mM Sn(2+) concentration. Yeast cells in exponential phase of growth (LOG cells) tolerate only micro-molar Sn(2+) concentrations that result in intracellular concentration below of the method detection limit. Our PIXE analysis showed that STAT XV185-14c yeast cells demonstrate a significant loss of intracellular elements such as Mg, Zn, S, Fe and an increase in P levels after 1h exposure to SnCl(2). The survival assay showed enhanced tolerance of LOG yeast cells lacking the low-affinity iron and zinc transporters to stannous treatment, suggesting the possible involvement in Sn(2+) uptake. Moreover, our qRT-PCR data showed that Sn(2+) treatment could generate reactive oxygen species as it induces activation of many stress-response genes, including SOD1, YAP1, and APN1.

The metal binding abilities of Megathura crenulata metallothionein (McMT) in the frame of gastropoda MTs

Metallothioneins (MTs) are proteins that play a major role in metal homeostasis and/or detoxification in all kind of organisms. The MT gene/protein system of gastropod molluscs provides an invaluable model to study the diversification mechanisms that have enabled MTs to achieve metal-binding specificity through evolution. Most pulmonate gastropods, particularly terrestrial snails, harbor three paralogous isogenes encoding three MT isoforms with different metal binding preferences: the highly specific CdMT and CuMT isoforms, for cadmium and copper respectively, and the unspecific Cd/CuMT isoform. Megathura crenulata is a non-pulmonate gastropod in which only one MT isogene has so far been reported. In order to elucidate the metal binding character of the corresponding peptide (McMT), it has been recombinantly synthesized in the presence of Cd(2+), Zn(2+) or Cu(2+), and the corresponding metal complexes have been analyzed using electrospray mass spectrometry, and CD and UV-visible spectroscopy. The metal-binding traits exhibited by McMT revealed that it is an unspecific MT, similarly to the pulmonate Cd/CuMT isoforms. This is in full concordance with the protein sequence distance analysis in relation to other gastropod MTs.

Zinc and human health: an update

The importance of micronutrients in health and nutrition is undisputable, and among them, zinc is an essential element whose significance to health is increasingly appreciated and whose deficiency may play an important role in the appearance of diseases. Zinc is one of the most important trace elements in the organism, with three major biological roles, as catalyst, structural, and regulatory ion. Zinc-binding motifs are found in many proteins encoded by the human genome physiologically, and free zinc is mainly regulated at the single-cell level. Zinc has critical effect in homeostasis, in immune function, in oxidative stress, in apoptosis, and in aging, and significant disorders of great public health interest are associated with zinc deficiency. In many chronic diseases, including atherosclerosis, several malignancies, neurological disorders, autoimmune diseases, aging, age-related degenerative diseases, and Wilson's disease, the concurrent zinc deficiency may complicate the clinical features, affect adversely immunological status, increase oxidative stress, and lead to the generation of inflammatory cytokines. In these diseases, oxidative stress and chronic inflammation may play important causative roles. It is therefore important that status of zinc is assessed in any case and zinc deficiency is corrected, since the unique properties of zinc may have significant therapeutic benefits in these diseases. In the present paper, we review the zinc as a multipurpose trace element, its biological role in homeostasis, proliferation and apoptosis and its role in immunity and in chronic diseases, such as cancer, diabetes, depression, Wilson's disease, Alzheimer's disease, and other age-related diseases.

Metallothionein protein evolution: a miniassay

Metallothionein (MT) evolution is one of the most obscure yet fascinating aspects of the study of these atypical metal-binding peptides. The different members of the extremely heterogeneous MT protein superfamily probably evolved through a web of duplication, functional differentiation, and/or convergence events leading to the current scenario, which is particularly hard to interpret in terms of molecular evolution. Difficulties in drawing straight evolutionary relationships are reflected in the lack of definite MT classification criteria. Presently, MTs are categorized either according to a pure taxonomic clustering or depending on their metal binding preferences and specificities. Extremely well documented MT revisions were recently published. But beyond classic approaches, this review of MT protein evolution will bring together new aspects that have seldom been discussed before. Hence, the emergence of life on our planet, since metal ion utilization is accepted to be at the root of the emergence of living organisms, and global trends that underlie structural and functional MT diversification, will be presented. Major efforts are currently being devoted to identifying rules for function-constrained MT evolution that may be applied to different groups of organisms.

Three metallothionein isoforms and sequestration of intracellular silver in the hyperaccumulator Amanita strobiliformis

Metallothioneins (MTs) are cysteine-rich peptides involved in heavy metal tolerance of many eukaryotes. Here, we examined their involvement in intracellular binding of silver (Ag) in the ectomycorrhizal fungus Amanita strobiliformis. The Ag complexes and their peptide ligands were characterized using chromatography and mass spectrometry. The full-length coding sequences obtained from a cDNA library were used for complementation assays in yeast mutant strains. Abundance of respective transcripts in A. strobiliformis was measured by quantitative real-time reverse-transcribed polymerase chain reaction (qRT-PCR). Ag-speciation analyses showed that intracellular Ag was in wild-grown fruit bodies and cultured extraradical mycelia of A. strobiliformis sequestered by metallothioneins. The determined sequence of the peptide facilitated isolation of three cDNA clones, AsMT1a, AsMT1b and AsMT1c. These encode isomorphic MTs consisting of 34 amino acid residues and sharing 82% identity. In mycelia the expression of AsMT1s is induced by Ag. All AsMT1s expressed in yeasts complemented hypersensitivity of mutants to cadmium (Cd) and copper (Cu) and formed Ag complexes. Only the Ag-AsMT1a complex was detected in the A. strobiliformis fruit body in which AsMT1a was the prevailing transcript. The present study identified the existence of metallothionein isoforms in ectomycorrhizal fungi. We demonstrated that intracellular sequestration of Ag in fruit bodies and mycelia of hyperaccumulating A. strobiliformis is dominated by metallothioneins.

How Saccharomyces cerevisiae copes with toxic metals and metalloids

Toxic metals and metalloids are widespread in nature and can locally reach fairly high concentrations. To ensure cellular protection and survival in such environments, all organisms possess systems to evade toxicity and acquire tolerance. This review provides an overview of the molecular mechanisms that contribute to metal toxicity, detoxification and tolerance acquisition in budding yeast Saccharomyces cerevisiae. We mainly focus on the metals/metalloids arsenic, cadmium, antimony, mercury, chromium and selenium, and emphasize recent findings on sensing and signalling mechanisms and on the regulation of tolerance and detoxification systems that safeguard cellular and genetic integrity.

Shaping mechanisms of metal specificity in a family of metazoan metallothioneins: evolutionary differentiation of mollusc metallothioneins

Background

The degree of metal binding specificity in metalloproteins such as metallothioneins (MTs) can be crucial for their functional accuracy. Unlike most other animal species, pulmonate molluscs possess homometallic MT isoforms loaded with Cu⁺ or Cd²⁺. They have, so far, been obtained as native metal-MT complexes from snail tissues, where they are involved in the metabolism of the metal ion species bound to the respective isoform. However, it has not as yet been discerned if their specific metal occupation is the result of a rigid control of metal availability, or isoform expression programming in the hosting tissues or of structural differences of the respective peptides determining the coordinative options for the different metal ions. In this study, the Roman snail (Helix pomatia) Cu-loaded and Cd-loaded isoforms (HpCuMT and HpCdMT) were used as model molecules in order to elucidate the biochemical and evolutionary mechanisms permitting pulmonate MTs to achieve specificity for their cognate metal ion.

Results

HpCuMT and HpCdMT were recombinantly synthesized in the presence of Cd²⁺, Zn²⁺ or Cu²⁺ and corresponding metal complexes analysed by electrospray mass spectrometry and circular dichroism (CD) and ultra violet-visible (UV-Vis) spectrophotometry. Both MT isoforms were only able to form unique, homometallic and stable complexes (Cd₆-HpCdMT and Cu₁₂-HpCuMT) with their cognate metal ions. Yeast complementation assays demonstrated that the two isoforms assumed metal-specific functions, in agreement with their binding preferences, in heterologous eukaryotic environments. In the snail organism, the functional metal specificity of HpCdMT and HpCuMT was contributed by metal-specific transcription programming and cell-specific expression. Sequence elucidation and phylogenetic analysis of MT isoforms from a number of snail species revealed that they possess an unspecific and two metal-specific MT isoforms, whose metal specificity was achieved exclusively by evolutionary modulation of non-cysteine amino acid positions.

Conclusion

The Roman snail HpCdMT and HpCuMT isoforms can thus be regarded as prototypes of isoform families that evolved genuine metal-specificity within pulmonate molluscs. Diversification into these isoforms may have been initiated by gene duplication, followed by speciation and selection towards opposite needs for protecting copper-dominated metabolic pathways from nonessential cadmium. The mechanisms enabling these proteins to be metal-specific could also be relevant for other metalloproteins.