Metallothioneins with unusual residues: histidines as modulators of zinc affinity and reactivity

Homo- and Heteronuclear Group 12 Metallothionein Type B Cluster Analogs: Synthesis, Structure, 1H NMR and ESI-MS

Metallothioneins (MTs) are a ubiquitous class of small cysteine-rich metal-binding proteins involved in metal homeostasis and detoxification with highly versatile metal binding properties. Despite the long-standing association of MT with M₃S₃ and M₄S₅ metal clusters, synthetic complexes with these core architectures are exceptionally rare. Here, we demonstrate an approach to synthesizing and characterizing aggregates of group 12 metal ions with monocyclic M₃S₃ cores in acetonitrile solution without the protection of a protein. Multidentate monothiol ligand N,N-bis(2-pyridylmethyl)-2-aminoethanethiol (L1H) provided [Cd₃(L1)₃](ClO₄)₃ (1), the first structurally characterized nonproteinaceous aggregate with a metallothionein-like monocyclic Cd₃S₃ core. In addition, [Zn₃(L1)₃](ClO₄)₃·4CH₃CN (2·4CH₃CN) was characterized by X-ray crystallography. The complex cations of 1 and 2 had comparable structures despite being nonisomorphic. Variable temperature and concentration ¹H NMR were used to investigate aggregation equilibria of 1, 2, and a precipitate with composition "Hg(L1)(ClO₄)" (3). Cryogenic ¹H NMR studies of 3 revealed a J(¹⁹⁹Hg¹H) coupling constant pattern consistent with an aggregate possessing a cyclic core. ESI-MS was used for gas-phase characterization of 1-3, as well as mixed-metal [M₂M'(L1)₃(ClO₄)₂]⁺ ions prepared in situ by pairwise acetonitrile solution combinations of the group 12 complexes of L1. Access to synthetic variants of metallothionein-like group 12 aggregates provides an additional approach to understanding their behavior.

PmtA Regulates Pyocyanin Expression and Biofilm Formation in Pseudomonas aeruginosa

The opportunistic pathogen Pseudomonas aeruginosa expresses a small molecular weight, cysteine-rich protein (PmtA), identified as a metallothionein (MT) protein family member. The MT family proteins have been well-characterized in eukaryotes as essential for zinc and copper homeostasis, protection against oxidative stress, and the ability to modify a variety of immune activities. Bacterial MTs share sequence homology, antioxidant chemistry, and heavy metal-binding capacity with eukaryotic MTs, however, the impact of bacterial MTs on virulence and infection have not been well-studied. In the present study, we investigated the role of PmtA in P. aeruginosa PAO1 using a PmtA-deficient strain (ΔpmtA). Here we demonstrated the virulence factor, pyocyanin, relies on the expression of PmtA. We showed that PmtA may be protective against oxidative stress, as an alternative antioxidant, glutathione, can rescue pyocyanin expression. Furthermore, the expression of phzM, which encodes a pyocyanin precursor enzyme, was decreased in the ΔpmtA mutant during early stationary phase. Upregulated pmtA expression was previously detected in confluent biofilms, which are essential for chronic infection, and we observed that the ΔpmtA mutant was disrupted for biofilm formation. As biofilms also modulate antibiotic susceptibility, we examined the ΔpmtA mutant susceptibility to antibiotics and found that the ΔpmtA mutant is more susceptible to cefepime and ciprofloxacin than the wild-type strain. Finally, we observed that the deletion of pmtA results in decreased virulence in a waxworm model. Taken together, our results support the conclusion that PmtA is necessary for the full virulence of P. aeruginosa and may represent a potential target for therapeutic intervention.

The Response of the Soil Microbiome to Contamination with Cadmium, Cobalt and Nickel in Soil Sown with Brassica napus

Soil fertility is determined by biological diversity at all levels of life, from genes to entire biocenoses. The aim of this study was to evaluate bacterial diversity in soil contaminated with Cd2+, Co2+ and Ni2+ and sown with Brassica napus. This is an important consideration because soil-dwelling microorganisms support phytoremediation and minimize the adverse effects of heavy metals on the environment. Microbial counts, the influence (IFHM) of Cd2+, Co2+ and Ni2+ on microorganisms, the colony development (CD) index, the ecophysiological diversity (EP) index and genetic diversity of bacteria were determined under controlled conditions. Soil contamination with Cd2+, Co2+ and Ni2+ significantly influenced microbial diversity and increased the values of CD and EP indices. The tested heavy metals decreased the genetic diversity of bacteria, in particular in the phyla Actinobacteria and Proteobacteria. Bacteria of the genera Arthrobacter, Devosia, Kaistobacter, Paenibacillus, Phycicoccus, Rhodoplanes and Thermomonas were identified in both contaminated and non-contaminated soil. These bacteria are highly resistant to soil contamination with Cd2+, Co2+ and Ni2+.

Acute Toxicity of Divalent Mercury to Bacteria Explained by the Formation of Dicysteinate and Tetracysteinate Complexes Bound to Proteins in Escherichia coli and Bacillus subtilis

Bacteria are the most abundant organisms on Earth and also the major life form affected by mercury (Hg) poisoning in aquatic and terrestrial food webs. In this study, we applied high energy-resolution X-ray absorption near edge structure (HR-XANES) spectroscopy to bacteria with intracellular concentrations of Hg as low as 0.7 ng/mg (ppm) for identifying the intracellular molecular forms and trafficking pathways of Hg in bacteria at environmentally relevant concentrations. Gram-positive Bacillus subtilis and Gram-negative Escherichia coli were exposed to three Hg species: HgCl₂, Hg-dicysteinate (Hg(Cys)₂), and Hg-dithioglycolate (Hg(TGA)₂). In all cases, Hg was transformed into new two- and four-coordinate cysteinate complexes, interpreted to be bound, respectively, to the consensus metal-binding CXXC motif and zinc finger domains of proteins, with glutathione acting as a transfer ligand. Replacement of zinc cofactors essential to gene regulatory proteins with Hg would inhibit vital functions such as DNA transcription and repair and is suggested to be a main cause of Hg genotoxicity.

Two Unconventional Metallothioneins in the Apple Snail Pomacea bridgesii Have Lost Their Metal Specificity during Adaptation to Freshwater Habitats

Metallothioneins (MTs) are a diverse group of proteins responsible for the control of metal homeostasis and detoxification. To investigate the impact that environmental conditions might have had on the metal-binding abilities of these proteins, we have characterized the MTs from the apple snail Pomacea bridgesii, a gastropod species belonging to the class of Caenogastropoda with an amphibious lifestyle facing diverse situations of metal bioavailability. P. bridgesii has two structurally divergent MTs, named PbrMT1 and PbrMT2, that are longer than other gastropod MTs due to the presence of extra sequence motifs and metal-binding domains. We have characterized the Zn(II), Cd(II), and Cu(I) binding abilities of these two MTs after their heterologous expression in E. coli. Our results have revealed that despite their structural differences, both MTs share an unspecific metal-binding character, and a great ability to cope with elevated amounts of different metal ions. Our analyses have also revealed slight divergences in their metal-binding features: PbrMT1 shows a more pronounced Zn(II)-thionein character than PbrMT2, while the latter has a stronger Cu(I)-thionein character. The characterization of these two unconventional PbrMTs supports the loss of the metal-binding specificity during the evolution of the MTs of the Ampullariid family, and further suggests an evolutionary link of this loss with the adaptation of these gastropod lineages to metal-poor freshwater habitats.

Revealing differences in the chemical form of zinc in brain tissue using K-edge X-ray absorption near-edge structure spectroscopy

Zinc is a prominent trace metal required for normal memory function. Memory loss and cognitive decline during natural ageing and neurodegenerative disease have been associated with altered brain-Zn homeostasis. Yet, the exact chemical pathways through which Zn influences memory function during health, natural ageing, or neurodegenerative disease remain unknown. The gap in the literature may in part be due to the difficulty to simultaneously image, and therefore, study the different chemical forms of Zn within the brain (or biological samples in general). To this extent, we have begun developing and optimising protocols that incorporate X-ray absorption near-edge structure (XANES) spectroscopic analysis of tissue at the Zn K-edge as an analytical tool to study Zn speciation in the brain. XANES is ideally suited for this task as all chemical forms of Zn are detected, the technique requires minimal sample preparation that may otherwise redistribute or alter the chemical form of Zn, and the Zn K-edge has known sensitivity to coordination geometry and ligand type. Herein, we report our initial results where we fit K-edge spectra collected from micro-dissected flash-frozen brain tissue, to a spectral library prepared from standard solutions, to demonstrate differences in the chemical form of Zn that exist between two brain regions, the hippocampus and cerebellum. Lastly, we have used an X-ray microprobe to demonstrate differences in Zn speciation within sub-regions of thin air-dried sections of the murine hippocampus; but, the corresponding results highlight that the chemical form of Zn is easily perturbed by sample preparation such as tissue sectioning or air-drying, which must be a critical consideration for future work.

Diversity, structure and regulation of microbial metallothionein: metal resistance and possible applications in sequestration of toxic metals

Metallothioneins (MTs) are a group of cysteine-rich, universal, low molecular weight proteins distributed widely in almost all major taxonomic groups ranging from tiny microbes to highly organized vertebrates. The primary function of this protein is storage, transportation and binding of metals, which enable microorganisms to detoxify heavy metals. In the microbial world, these peptides were first identified in a cyanobacterium Synechococcus as the SmtA protein which exhibits high affinity towards rising level of zinc and cadmium to preserve metal homeostasis in a cell. In yeast, MTs aid in reserving copper and confer protection against copper toxicity by chelating excess copper ions in a cell. Two MTs, CUP1 and Crs5, originating from Saccharomyces cerevisiae predominantly bind to copper though are capable of binding with zinc and cadmium ions. MT superfamily 7 is found in ciliated protozoa which show high affinity towards copper and cadmium. Several tools and techniques, such as western blot, capillary electrophoresis, inductively coupled plasma, atomic emission spectroscopy and high performance liquid chromatography, have been extensively utilized for the detection and quantification of microbial MTs which are utilized for the efficient remediation and sequestration of heavy metals from a contaminated environment.

Selenium cytotoxicity in Tetrahymena thermophila: New clues about its biological effects and cellular resistance mechanisms

Selenium is an essential micronutrient but at high concentrations can produce severe cytotoxicity and genomic damage. We have evaluated the cytotoxicity, ultrastructural and mitochondrial alterations of the two main selenium inorganic species; selenite and selenate, in the eukaryotic microorganism Tetrahymena thermophila. In this ciliate, selenite is more toxic than selenate. Their LC₅₀ values were calculated as 27.65 μM for Se(IV) and 56.88 mM for Se(VI). Significant levels of peroxides/hydroperoxides are induced under low-moderate selenite or selenate concentrations. Se(VI) exposures induce an immediate mitochondrial membrane depolarization. Selenium treated cells show an intense vacuolization and some of them present numerous discrete and small electrondense particles, probably selenium deposits. Mitochondrial fusion, an intense swelling in peripheral mitochondria and mitophagy are detected in selenium treated cells, especially in those exposed to Se (IV). qRT-PCR analysis of diverse genes, encoding relevant antioxidant enzymes or other proteins, like metallothioneins, involved in an environmental general stress response, have shown that they may be crucial against Se(IV) and/or Se (VI) cytotoxicity.

Analysis of the soybean metallothionein system under free radical stress: protein modification connected to lipid membrane damage

Metallothioneins are small Cys-rich peptides capable of coordinating metal ions, and proposed to be involved in radical stress. The four Zn(ii)-GmMT complexes of soybean (Glycine max) were recombinantly synthesised and exposed to oxidative (HO˙) and reductive (H˙ atoms and eaq-) stress conditions. Gamma-irradiation was used to simulate the endogenous formation of the reactive species in both aqueous solutions and unsaturated lipid vesicle suspensions, a biomimetic model that showed that tandem protein/lipid damage occurs, in particular under reductive radical stress. This is due to the formation of diffusible sulphur-centred radicals, which migrate from the aqueous phase to the lipid bilayer and are thus able to transform the cis double bond of the oleate moiety into the trans isomer. Among the amino acid residues present in GmMTs, Cys is one of the most sensitive residues towards the attack of free radicals, thus suggesting metal-clusters to be good interceptors of free radicals. Also Met, Tyr and Phe residues are sensitive amino acid sites of attack under both oxidative and reductive conditions. The modification of the Zn(ii)-GmMT complexes, in particular isoform 2, was monitored by Raman spectroscopy and mass spectrometry. Free radical stress on the Zn(ii)-GmMT complexes is able to induce significant structural changes such as partial deconstruction and/or rearrangement of the metal clusters, but not the complete demetallation of the proteins nor breaking of the backbone, thus confirming their capability to act as protectors under free radical stress conditions.

Metallothionein diversity and distribution in the tree of life: a multifunctional protein

Metallothioneins are diverse, but not represented yet in all phyla. Moreover, they play a central role as a [MT:T:TO] protein system.

A histidine-rich Pseudomonas metallothionein with a disordered tail displays higher binding capacity for cadmium than zinc

The NMR solution structure of a Pseudomonas metallothionein reveals a different binding capacity for Zn^II and Cd^II ions that results in two novel metal-cluster topologies. Replacement of a non-coordinating residue by histidine decreases the kinetic lability of the cluster. All three structures reported show an identical protein fold.

Extreme metal adapted, knockout and knockdown strains reveal a coordinated gene expression among different Tetrahymena thermophila metallothionein isoforms

Metallothioneins (MT) constitute a superfamily of small cytosolic proteins that are able to bind metal cations through numerous cysteine (Cys) residues. Like other organisms the ciliate Tetrahymena thermophila presents several MT isoforms, which have been classified into two subfamilies (Cd- and Cu-metallothioneins). The main aim of this study was to examine the specific functions and transcriptional regulation of the five MT isoforms present in T. thermophila, by using several strains of this ciliate. After a laboratory evolution experiment over more than two years, three different T. thermophila strains adapted to extreme metal stress (Cd²⁺, Cu²⁺ or Pb²⁺) were obtained. In addition, three knockout and/or knockdown strains for different metallothionein (MT) genes were generated. These strains were then analyzed for expression of the individual MT isoforms. Our results provide a strong basis for assigning differential roles to the set of MT isoforms. MTT1 appears to have a key role in adaptation to Cd. In contrast, MTT2/4 are crucial for Cu-adaptation and MTT5 appears to be important for Pb-adaptation and might be considered as an “alarm” MT gene for responding to metal stress. Moreover, results indicate that likely a coordinated transcriptional regulation exists between the MT genes, particularly among MTT1, MTT5 and MTT2/4. MTT5 appears to be an essential gene, a first such report in any organism of an essential MT gene.

Metallothionein from Wild Populations of the African Catfish Clarias gariepinus: From Sequence, Protein Expression and Metal Binding Properties to Transcriptional Biomarker of Metal Pollution

Anthropogenic pollution with heavy metals is an on-going concern throughout the world, and methods to monitor release and impact of heavy metals are of high importance. With a view to probe its suitability as molecular biomarker of metal pollution, this study has determined a coding sequence for metallothionein of the African sharptooth catfish Clarias gariepinus. The gene product was recombinantly expressed in Escherichia coli in presence of Zn(II), Cd(II), or Cu, and characterised by Electrospray Ionisation Mass Spectrometry and elemental analysis. C. gariepinus MT displays typical features of fish MTs, including 20 conserved cysteines, and seven bound divalent cations (Zn(II) or Cd(II)) when saturated. Livers from wild C. gariepinus fish collected in all three seasons from four different sites on the Kafue River of Zambia were analysed for their metal contents and for MT expression levels by quantitative PCR. Significant correlations were found between Zn and Cu levels and MT expression in livers, with MT expression clearly highest at the most polluted site, Chililabombwe, which is situated in the Copperbelt region. Based on our findings, hepatic expression of MT from C. gariepinus may be further developed as a major molecular biomarker of heavy metal pollution resulting from mining activities in this region.

Metallothionein assisted periplasmic lead sequestration as lead sulfite by Providencia vermicola strain SJ2A

Lead resistant Providencia vermicola strain SJ2A was isolated from the waste of a battery manufacturing industry which could tolerate upto 3.0mM lead nitrate in the minimal medium. Interestingly, this isolate showed presence of a plasmid borne metallothionein gene, bmtA that matched significantly (96%) with that of Pseudomonas aeruginosa. Scanning electron micrographs of bacterial cells exposed to lead revealed a unique alteration in the cell morphology from rods to long inter-connected filaments. On the other hand, electron dispersive X-ray spectroscopy (EDX) clearly indicated no significant lead adsorption therefore, we speculated intracellular sequestration in this bacterial strain. Transmission electron micrographs of the bacterial cells exposed to lead evidently demonstrated periplasmic sequestration of lead which was also supported by Fourier transformed infrared spectroscopic (FTIR) analysis. The bacterium internalised 155.12mg Pb²⁺/g biomass as determined by atomic absorption spectroscopy. Subsequently, the accumulated lead was identified as lead sulfite by X-ray diffraction studies. Therefore P. vermicola strain SJ2A has potential to bioremediate lead contaminated environmental sites.

Reduction of (chloro)-μ-nitrido-bis[(tetra-tert-butyl-phthalocyaninato)iron(IV)] with organic N-bases

The interaction of (chloro)-[Formula: see text]-nitrido-bis[(tetra-tert-butyl-phthalocyaninato)iron(IV)] Cl(FePc)2N with organic [Formula: see text]-bases L as electron–donors (L [Formula: see text] diethylamine, imidazole, 1-methylimidazole, 2-methylimidazole) with the formation of one-electron reduced species (L)PcFe[Formula: see text]–N[Formula: see text]Fe[Formula: see text]Pc(L) was investigated in benzene at 298 K by UV-visible spectroscopy. The reaction was established to be stepwise process including fast reversible axial binding of two substrate molecules onto iron cations followed by slow one-electron metal-centered reduction. The results of IR, ESI-MS and EPR study support the formation of final product with Fe[Formula: see text]–N[Formula: see text]Fe[Formula: see text] unit and two substrate molecules in the first coordination sphere. The reaction kinetics was studied, the pre-equilibrium constants [Formula: see text] and rate constants [Formula: see text] were obtained. The [Formula: see text] and [Formula: see text] values were found to be linearly correlated with basicity of substrates p[Formula: see text]. The possibility of the transition Fe[Formula: see text] Fe[Formula: see text] is promoted by electron–donor properties of substrate combined with the presence of both electron-rich macrocycle and [Formula: see text]-backdonation Fe [Formula: see text] N[Formula: see text].

The Tetrahymena metallothionein gene family: twenty-one new cDNAs, molecular characterization, phylogenetic study and comparative analysis of the gene expression under different abiotic stressors

Background

Ciliate metallothioneins (MTs) are included in family 7 of the MT superfamily. This family has been divided into two main subfamilies: 7a or CdMTs and 7b or CuMTs. All ciliate MTs reported have been isolated from different Tetrahymena species and present unique features with regard to standard MTs. Likewise, an expression analysis has been carried out on some of MT genes under metal stress, corroborating their classification into two subfamilies.

Results

We isolated 21 new cDNAs from different Tetrahymena species to obtain a wider view of the biodiversity of these conserved genes. Structural analysis (cysteine patterns) and an updated phylogenetic study both corroborated the previous classification into two subfamilies. A new CuMT from a Tetrahymena-related species Ichthyophthirius multifiliis was also included in this general analysis. We detected a certain tendency towards the presentation of a CdMT tri-modular structure in Borealis group species with respect to Australis group. We report for the first time a semi-complete paralog duplication of a CdMT gene originating a new CdMT gene isoform in T. malaccensis. An asymmetry of the codon usage for glutamine residues was detected between Cd- and CuMTs, and the phylogenetic implications are discussed. A comparative gene expression analysis of several MT genes by qRT-PCR revealed differential behavior among them under different abiotic stressors in the same Tetrahymena species.

Conclusions

The Tetrahymena metallothionein family represents a quite conserved protein structure group with unique features with respect to standard MTs. Both Cd- and CuMT subfamilies present very defined and differentiated characteristics at several levels: cysteine patterns, modular structure, glutamine codon usage and gene expression under metal stress, among others. Gene duplication through evolution seems to be the major genetic mechanism for creating new MT gene isoforms and increasing their functional diversity.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2658-6) contains supplementary material, which is available to authorized users.

Molecular Characterization of a Copper Metallothionein Gene From a Ciliate Tetrahymena farahensis

A new copper metallothionein (TfCuMT) gene has been identified from a locally isolated ciliate Tetrahymena farahensis. It contains 327 nucleotides encoding a peptide chain of 108 amino acids and belongs to class MTT2 and subfamily 7b. Amplification from both gDNA and mRNA confirmed the intronless nature of this gene. Like most of the metallohtioneins, cysteine residues contribute nearly 30% content with the specific CKC motifs. Structural repeats present in peptide sequence of TfCuMT indicate internal duplication of gene at some stage of gene evolution. The predicted model of copper metallothionein protein showed that copper ions are mainly chelated by thiol sulfur of cysteine residues and are embedded in the folds of polypeptide chain. For in vivo expression of TfCuMT in Escherichia coli host cells the classical stop codons, which coded for glutamine in the ciliate were mutated to CAA and CAG through site directed mutagenesis. The mutated gene showed higher expression in pET28a expression vector compared with pET21a. Optimum expression was obtained after 6-8 h of 0.1 mM IPTG induction. Stability of His tagged TfCuMT in 5% SDS was low, with half-life of about 104 min. Presence of 1.0 μM copper increased the expression level by 1.65-fold. Presence of 100 μM Cysteine in culture medium caused 2.4-fold increase in expression level. His tagged TfCuMT was purified through affinity chromatography using NTN-His binding resin in the presence of 0.1 M imidazole and NaCl. The modeled structure of the TfCuMT showed a cleft for Cu binding with correct orientation of Cys residues in the motif CKC. J. Cell. Biochem. 117: 1843-1854, 2016. © 2016 Wiley Periodicals, Inc.

Mixed planting with a leguminous plant outperforms bacteria in promoting growth of a metal remediating plant through histidine synthesis

The effectiveness of plant growth promoting bacteria (PGPB) in improving metal phytoremediation is still limited by stunted plant growth under high soil metal concentrations. Meanwhile, mixed planting with leguminous plants is known to improve yield in nutrient deficient soils but the use of a metal tolerant legume to enhance metal tolerance of a phytoremediator has not been explored. We compared the use of Pseudomonas brassicacearum, Rhizobium leguminosarum, and the metal tolerant leguminous plant Vicia sativa to promote the growth of Brassica juncea in soil contaminated with 400 mg Zn kg(-1), and used synchrotron based microfocus X-ray absorption spectroscopy to probe Zn speciation in plant roots. B. juncea grew better when planted with V. sativa than when inoculated with PGPB. By combining PGPB with mixed planting, B. juncea recovered full growth while also achieving soil remediation efficiency of >75%, the maximum ever demonstrated for B. juncea. μXANES analysis of V. sativa suggested possible root exudation of the Zn chelates histidine and cysteine were responsible for reducing Zn toxicity. We propose the exploration of a legume-assisted-phytoremediation system as a more effective alternative to PGPB for Zn bioremediation.

Jacks of metal/metalloid chelation trade in plants-an overview

Varied environmental compartments including soils are being contaminated by a myriad toxic metal(loid)s (hereafter termed as "metal/s") mainly through anthropogenic activities. These metals may contaminate food chain and bring irreparable consequences in human. Plant-based approach (phytoremediation) stands second to none among bioremediation technologies meant for sustainable cleanup of soils/sites with metal-contamination. In turn, the capacity of plants to tolerate potential consequences caused by the extracted/accumulated metals decides the effectiveness and success of phytoremediation system. Chelation is among the potential mechanisms that largely govern metal-tolerance in plant cells by maintaining low concentrations of free metals in cytoplasm. Metal-chelation can be performed by compounds of both thiol origin (such as GSH, glutathione; PCs, phytochelatins; MTs, metallothioneins) and non-thiol origin (such as histidine, nicotianamine, organic acids). This paper presents an appraisal of recent reports on both thiol and non-thiol compounds in an effort to shed light on the significance of these compounds in plant-metal tolerance, as well as to provide scientific clues for the advancement of metal-phytoextraction strategies.

Advances in the molecular understanding of biological zinc transport

Recognition of the importance of zinc homeostasis for health has driven a surge in structural data on major zinc-transporting proteins.

His-containing plant metallothioneins: comparative study of divalent metal-ion binding by plant MT3 and MT4 isoforms

Metallothioneins (MTs) are a superfamily of Cys-rich, low-molecular weight metalloproteins that bind heavy metal ions. These cytosolic metallopeptides, which exist in most living organisms, are thought to be involved in metal homeostasis, metal detoxification, and oxidative stress protection. In this work, we characterise the Zn(II)- and Cd(II)-binding abilities of plant type 3 and type 4 MTs identified in soybean and sunflower, both of them being His-containing peptides. The recombinant metal-MT complexes synthesised in Zn(II) or Cd(II)-enriched Escherichia coli cultures have been analysed by ESI-MS, and CD, ICP-AES, and UV spectroscopies. His-to-Ala type 3 MT mutants have also been constructed and synthesised for the study of the role of His in divalent metal ion coordination. The results show comparable divalent metal-binding capacities for the MTs of type 3, and suggest, for the first time, the participation of their conserved C-term His residues in metal binding. Interesting features for the Zn(II)-binding abilities of type 4 MTs are also reported, as their variable His content may be considered crucial for their biological performance.

Heterologous expression and metal-binding characterization of a type 1 metallothionein isoform (OsMTI-1b) from rice (Oryza sativa)

Metallothioneins (MTs) are ubiquitous, low molecular mass and cysteine-rich proteins that play important roles in maintaining intracellular metal homeostasis, eliminating metal toxification and protecting the cells against oxidative damages. MTs are able to bind metal ions through the thiol groups of their cysteine residues. Plants have several MT isoforms which are classified into four types based on the arrangement of cysteine residues. In the present study, a rice (Oryza sativa) gene encoding type 1 MT isoform, OsMTI-1b, was inserted in vector pET41a and overexpressed in Escherichia coli as carboxy-terminal extensions of glutathione-S-transferase (GST). The recombinant protein GST-OsMTI-1b was purified using affinity chromatography and its ability to bind with Ni(2+), Cd(2+), Zn(2+) and Cu(2+) ions was analyzed. The results demonstrated that this isoform has ability to bind Ni(2+), Cd(2+) and Zn(2+) ions in vitro, whereas it has no substantial ability to bind Cu(2+) ions. From competitive reaction with 5,5'-dithiobis(2-nitrobenzoic acid), DTNB, the affinity of metal ions for recombinant form of GST-OsMTI-1b was as follows: Ni(2+)/Cd(2+) > Zn(2+) > Cu(2+).

Non-enzymatic modifications in metallothioneins connected to lipid membrane damages: structural and biomimetic studies under reductive radical stress

Metallothioneins (MTs) are small cysteine-rich proteins with the ability to coordinate heavy metal atoms through metal-thiolate bonds, which are widely distributed among the animal and plant kingdoms. Multifunctional roles for MTs have been proposed, including their ability to scavenger various radicals and reactive oxygen species. In the present article we summarize available information of four MT polypeptides from different organisms, forming metal complexes with Zn(II), Cd(II) or Cu (I) ions. Non-enzymatic modifications of MTs under ionizing radiations and their consequences on the lipidic membrane compartment were studied by Raman spectroscopy and a biomimetic model, respectively. The latter is based on liposome technology and allows to measure the trans unsaturated fatty acid content as a result of reductive radical stress on MTs.

BIOLOGICAL SIGNIFICANCE

The effect of radical stress on the cell metabolism and functions is a very active field of research connecting various disciplines in life sciences. In this contest tandem radical damage has been the subject of recent investigations that pointed out its harmfulness in the general scenario of establishing the consequences of radical stress. By using biomimetic models of tandem damage we have for the first time tested the capability of metallothioneins (MTs), small metalloproteins rich of Cys residues, to damage another cell compartment like lipid membranes when they are undergone to reductive radical stress. The connection of MT reactivity with membrane lipid transformation can give a contribution to the puzzling context of radical stress occurring to biomolecules and the role as biological signaling. To this purpose, MT polypeptides from different organisms, exhibiting different sequence peculiarities, have been analyzed here. The spectroscopic analysis of these systems has allowed to identify modifications affecting metal-thiolate clusters, cystines, and Met residues, acting as efficient interceptors of reducing radical species. The chemical mechanism involving sulfur-containing moieties under reductive conditions discloses new scenarios that bring to the loss of sulfur-centered radicals by desulfurization reactions that change the natural sequences of MTs. Ala is a genetically coded amino acid, therefore the mutation of Cys to Ala occurring to a sequence by the radical process so far discussed, corresponds to a post-translational modification. Research on such mutation connected also to a free radical stress will be important to contribute for a complete picture of the degeneration associated to diseases and aging. Analogously, the Met to Aba mutation occurring after reductive stress transforms a natural amino acid into a natural, non-genetically-coded congener. Aba corresponds to a homologation of the alkyl chains normally present in genetically codified amino acids, such as methyl (in Ala) and isopropyl (in Leu), with an ethyl unit. Based on alkyl substitution, this modification can therefore be studied in order to understand its general consequences on the structure-activity relationships in proteins and, in particular, on molecular interactions. This article is part of a Special issue entitled: Posttranslational Protein modifications in biology and Medicine.

Lessons on the critical interplay between zinc binding and protein structure and dynamics

Zinc is one of the most important micronutrients for virtually all living organisms, and hence, it is important to understand the molecular mechanisms for its homeostasis. Besides proteins involved in transmembrane transport, both extra- and intracellular zinc-binding proteins play important roles in the respective metabolic networks. Important examples for extracellular zinc transporters are mammalian serum albumins, and for intracellular zinc handling, certain metallothioneins are of relevance. The availability of protein structures including relevant metal binding sites is a fundamental prerequisite to decipher the mechanisms that govern zinc binding dynamics in these proteins, but their determination can prove to be surprisingly challenging. Due to the spectroscopic silence of Zn(2+), combinations of biophysical techniques including electrospray ionisation mass spectrometry (ESI-MS) and multinuclear NMR, isothermal titration calorimetry (ITC) and extended X-ray absorption fine structure (EXAFS) spectroscopy, coupled with site-directed mutagenesis and molecular modelling have proven to be valuable approaches to understand not only the zinc-binding properties of metallothioneins and albumins, but also the influence of other physiologically relevant competing agents. These studies have demonstrated why the bacterial metallothionein SmtA contains a site inert towards exchange with Cd(2+), why the plant metallothionein EC from wheat is partially unfolded in the presence of Cd(2+), and how fatty acids impact on the zinc-binding ability of mammalian serum albumins.

Heavy metal accumulation by recombinant mammalian metallothionein within Escherichia coli protects against elevated metal exposure

Metallothioneins (MTs) are ubiquitous metal-binding, cysteine-rich, small proteins known to provide protection against toxic heavy metals such as cadmium. In an attempt to increase the ability of bacterial cells to accumulate heavy metals, sheep MTII was produced in fusion with the maltose binding protein (MBP) and localized to the cytoplasmic or periplasmic compartments of Escherichia coli. For all metals tested, higher levels of bioaccumulation were measured with strains over-expressing MBP-MT in comparison with control strains. A marked bioaccumulation of Cd, As, Hg and Zn was observed in the strain over-expressing MBP-MT in the cytoplasm, whereas Cu was accumulated to higher levels when MBP-MT was over-expressed in the periplasm. Metal export systems may also play a role in this bioaccumulation. To illustrate this, we over-expressed MBP-MT in the cytoplasm of two mutant strains of E. coli affected in metal export. The first, deficient in the transporter ZntA described to export numerous divalent metal ions, showed increasing quantities of Zn, Cd, Hg and Pb being bioaccumulated. The second, strain LF20012, deficient in As export, showed that As was bioaccumulated in the form of arsenite. Furthermore, high quantities of accumulated metals, chelated by MBP-MT in the cytoplasm, conferred greater metal resistance levels to the cells in the presence of added toxic metals, such as Cd or Hg, while other metals showed toxic effects when the export systems were deficient. The strain over-expressing MBP-MT in the cytoplasm, in combination, with disruption of metal export systems, could be used to develop strategies for bioremediation.

Mining genomes of marine cyanobacteria for elements of zinc homeostasis

Zinc is a recognized essential element for the majority of organisms, and is indispensable for the correct function of hundreds of enzymes and thousands of regulatory proteins. In aquatic photoautotrophs including cyanobacteria, zinc is thought to be required for carbonic anhydrase and alkaline phosphatase, although there is evidence that at least some carbonic anhydrases can be cambialistic, i.e., are able to acquire in vivo and function with different metal cofactors such as Co²⁺ and Cd²⁺. Given the global importance of marine phytoplankton, zinc availability in the oceans is likely to have an impact on both carbon and phosphorus cycles. Zinc concentrations in seawater vary over several orders of magnitude, and in the open oceans adopt a nutrient-like profile. Most studies on zinc handling by cyanobacteria have focused on freshwater strains and zinc toxicity; much less information is available on marine strains and zinc limitation. Several systems for zinc homeostasis have been characterized in the freshwater species Synechococcus sp. PCC 7942 and Synechocystis sp. PCC 6803, but little is known about zinc requirements or zinc handling by marine species. Comparative metallo-genomics has begun to explore not only the putative zinc proteome, but also specific protein families predicted to have an involvement in zinc homeostasis, including sensors for excess and limitation (SmtB and its homologs as well as Zur), uptake systems (ZnuABC), putative intracellular zinc chaperones (COG0523) and metallothioneins (BmtA), and efflux pumps (ZiaA and its homologs).

Nematode and snail metallothioneins

Metallobiologists have, at large, neglected soil dwelling invertebrates; exceptions are the nematode (Caenorhabditis elegans) and snails (Helix pomatia and Cantareus aspersus). This review aims to compare and contrast the molecular, protein and cellular mechanisms of the multifunctional nematode and snail metallothioneins (MTs). The C. elegans genome contains two MT genes, mtl-1, which is constitutively expressed in the pharynx and likely to act as an essential and/or toxic metal sensor, and mtl-2, which plays a negligible role under normal physiological conditions but is strongly induced (as mtl-1) in intestinal cells upon metal exposure. It has been possible to follow the intricate phenotypic responses upon the knockdown/knockout of single and multiple MT isoforms and we have started to decipher the multifunctional role of C. elegans MTs. The snails have contributed to our understanding regarding MT evolution and diversity, structure and metal-specific functionality. The H. pomatia and C. aspersus genomes contain at least three MT isoform genes. CdMT is responsible for cadmium detoxification, CuMT is involved in copper homeostasis and Cd/CuMT is a putative ancestral MT possibly only of minor importance in metal metabolism. Further investigations of nematode, snail and other invertebrate MTs will allow the development of alternative biomarker approaches and lead to an improved understanding of metallobiology, protein evolution and toxicogenomics.

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.

Plant metallothioneins--metal chelators with ROS scavenging activity?

Metallothioneins (MTs) are ubiquitous cysteine-rich proteins present in plants, animals, fungi and cyanobacteria. In plants, MTs are suggested to be involved in metal tolerance or homeostasis, as they are able to bind metal ions through the thiol groups of their cysteine residues. Recent reports show that MTs are also involved in the scavenging of reactive oxygen species (ROS). The interplay between these roles is not entirely clear. Plants have many MT isoforms with overlapping expression patterns, and no specific role for any of them has been assigned. This review is focused on recent findings on plant MTs.

The isolated Cys2His2 site in EC metallothionein mediates metal-specific protein folding

The selectivity of proteins involved in metal ion homeostasis is an important part of the puzzle to understand how cells allocate the correct metal ions to the correct proteins. Due to their similar ligand-binding properties, and their frequent co-existence in soils, essential zinc and toxic cadmium are a particularly challenging couple. Thus, minimisation of competition of Cd(2+) for Zn(2+) sites is of crucial importance for organisms that are in direct contact with soil. Amongst these, plants have an especially critical role, due to their importance for nutrition and energy. We have studied an embryo-specific, zinc-binding metallothionein (E(C)) from wheat by nuclear magnetic resonance, electrospray mass spectrometry, site-directed mutagenesis, and molecular modelling. Wheat E(C) exploits differences in affinities of Cys(4) and Cys(2)His(2) sites for Cd(2+) and Zn(2+) to achieve metal-selective protein folding. We propose that this may constitute a novel mechanism to discriminate between essential Zn(2+) and toxic Cd(2+).