Spectroscopic characterization of a fruit-specific metallothionein: M. acuminata MT3

Differentiated Zn(II) binding affinities in animal, plant, and bacterial metallothioneins define their zinc buffering capacity at physiological pZn

Metallothioneins (MTs) are small, Cys-rich proteins present in various but not all organisms, from bacteria to humans. They participate in zinc and copper metabolism, toxic metals detoxification, and protection against reactive species. Structurally, they contain one or multiple domains, capable of binding a variable number of metal ions. For experimental convenience, biochemical characterization of MTs is mainly performed on Cd(II)-loaded proteins, frequently omitting or limiting Zn(II) binding features and related functions. Here, by choosing 10 MTs with relatively well-characterized structures from animals, plants, and bacteria, we focused on poorly investigated Zn(II)-to-protein affinities, stability-structure relations, and the speciation of individual complexes. For that purpose, MTs were characterized in terms of stoichiometry, pH-dependent Zn(II) binding, and competition with chromogenic and fluorescent probes. To shed more light on protein folding and its relation with Zn(II) affinity, reactivity of variously Zn(II)-loaded MTs was studied by (5,5'-dithiobis(2-nitrobenzoic acid) oxidation in the presence of mild chelators. The results show that animal and plant MTs, despite their architectural differences, demonstrate the same affinities to Zn(II), varying from nano- to low picomolar range. Bacterial MTs bind Zn(II) more tightly but, importantly, with different affinities from low picomolar to low femtomolar range. The presence of weak, moderate, and tight zinc sites is related to the folding mechanisms and internal electrostatic interactions. Differentiated affinities of all MTs define their zinc buffering capacity required for Zn(II) donation and acceptance at various free Zn(II) concentrations (pZn levels). The data demonstrate critical roles of individual Zn(II)-depleted MT species in zinc buffering processes.

A metallothionein type 2 from Avicennia marina binds to iron and mediates hydrogen peroxide balance by activation of enzyme catalase

Metallothioneins (MTs) are low molecular weight, cysteine-rich, metal-binding proteins that are important for essential metal homeostasis, protection against oxidative stress, and buffering against toxic heavy metals. In this work the gene encoding an MT type 2 from Avicennia marina (Forssk.) Vierh. (AmMT2) was cloned into pET41a and transformed into the Escherichia coli strain Rosetta (DE3). Following the induction with isopropyl β-_D-1-thiogalactopyranoside, AmMT2 was expressed as glutathione-S-transferase (GST)-tagged fusion protein. The accumulation of Zn²⁺, Cu²⁺, Fe²⁺, Ni²⁺ and Cd²⁺ for strain R-AmMT2 was 4, 8, 5.4, 2 and 1.6 fold of control strain suggesting the role of AmMT2 in accumulation of metals. Particularly the strain R-AmMT2 was able to accumulate 30.7 mg per g dry weight. The cells expressing AmMT2 was more tolerant to hydrogen peroxide and had higher catalase (CAT) activity. To understand the mechanistic action of AmMT2 hydrogen peroxide tolerance, the activity of CAT in the E. coli protein extract was assayed after addition of pure Fe²⁺/GST-AmMT complex and Apo/GST-AmMT2 in vitro. Whereas, the activity of CAT did not change by the addition of Apo/GST-AmMT2, the activity of CAT significantly increased after addition of Fe²⁺/GST-AmMT2. These results show that AmMT2 activates CAT through Fe²⁺ transfer which subsequently causes the oxidative stress tolerance.

C-terminal Cysteines of CueR Act as Auxiliary Metal Site Ligands upon HgII Binding-A Mechanism To Prevent Transcriptional Activation by Divalent Metal Ions?

Intracellular CuI is controlled by the transcriptional regulator CueR, which effectively discriminates between monovalent and divalent metal ions. It is intriguing that HgII does not activate transcription, as bis-thiolate metal sites exhibit high affinity for HgII . Here the binding of HgII to CueR and a truncated variant, ΔC7-CueR, without the last 7 amino acids at the C-terminus including a conserved CCHH motif is explored. ESI-MS demonstrates that up to two HgII bind to CueR, while ΔC7-CueR accommodates only one HgII . 199m Hg PAC and UV absorption spectroscopy indicate HgS2 structure at both the functional and the CCHH metal site. However, at sub-equimolar concentrations of HgII at pH 8.0, the metal binding site displays an equilibrium between HgS2 and HgS3 , involving cysteines from both sites. We hypothesize that the C-terminal CCHH motif provides auxiliary ligands that coordinate to HgII and thereby prevents activation of transcription.

Unravelling the mechanistic details of metal binding to mammalian metallothioneins from stoichiometric, kinetic, and binding affinity data

Metallothioneins (MTs) are small, cysteine-rich proteins, found throughout Nature. Their ability to bind a number of different metals with a range of stoichiometric ratios means that this protein family is critically important for essential metal (Zn²⁺ and Cu⁺) homeostasis, metal storage, metal donation to nascent metalloenzymes as well as heavy metal detoxification. With its 20 cysteines, metallothionein is also considered to protect cells against oxidative stress. MT has been studied by a large number of researchers over the last 6 decades using a variety of spectroscopic techniques. The lack of distinguishing chromophores for the multitude of binding sites has made the evaluation of stoichiometric properties for different metals challenging. Initially, only ¹¹³Cd-NMR spectroscopy could provide strong evidence for the proposed cluster formation of Cd-MT. The extraordinary development of electrospray ionization mass spectrometry (ESI-MS), where all coexisting species in solution are observed, revolutionized MT research. Prior to the use of ESI-MS data, a range of "magic numbers" representing metal-to-MT molar ratios were reported from optical spectroscopic studies. The availability of ESI mass spectral data led to (i) the confirmation of cluster formation, (ii) a conceptual understanding of the cooperativity involved in multiple metal binding events, (iii) the presence of domain specificity between regions of the protein and (iv) mechanistic details involving both binding affinities and rate constants. The kinetic experiments identified the presence of multiple individual binding sites, each with a unique rate constant and an analogous binding affinity. The almost linear trend in rate constants as a function of bound As³⁺ provided a unique insight that became a critical step in the complete understanding of the mechanistic details of the metalation of MT. To fully define the biological function of this sulfur-rich protein it is necessary to determine kinetic rate constants and binding affinities for the essential metals. Recently, Zn²⁺ competition experiments between both of the isolated fragments (α and β) and the full-length protein (βα-MT 1a) as well as Zn²⁺ competition between βα-MT 1a and carbonic anhydrase were reported. From these data, the trend in binding affinities and the values of the K_f of the 7 bimolecular reactions involved in metalation were determined. From the analysis of ESI-MS data for Cu⁺ binding to βα-MT 1a at different pH-values, a trend in the 20 binding affinities for the complete metalation mechanism was reported. This review details a personal view of the historical development of the determination of stoichiometry for metal binding, the structure of the binding sites, the rates of the metalation reactions and the underlying binding affinities for each metalation step. We have attempted to summarize the experimental developments that led to the publication in May 2017 of the experimental determination of the 20 binding constants for the 20 sequential bimolecular reactions for Cu⁺ binding to the 20 Cys of apoMT as a function of pH that show the appearance and disappearance of clusters. We report both published data and in a series of tables an assembly of stoichiometries, and equilibrium constants for Zn²⁺ and Cu⁺ for many different metallothioneins.

Cyamopsis tetragonoloba type 1 metallothionein (CtMT1) gene is upregulated under drought stress and its protein product has an additional C-X-C motif and unique metal binding pattern

Metallothioneins (MTs) are involved in cellular homeostasis of essential metal ions and detoxification of nonessential metal ions. We report here the identification of four MT genes, CtMT1, CtMT2, CtMT3 and CtMT4, encoding CtMT1, CtMT2, CtMT3 and CtMT4 proteins, respectively, from the industrial guar crop. The primary structures of last three proteins were similar to those of respective MT proteins of other plants but the CtMT1 protein primary structure was different from the other plant MT1 proteins in having an additional C-X-C motif. The four MT genes showed tissue specific expression patterns suggesting their specific roles in different tissues. High expression of CtMT1 gene was observed in roots and nodules whereas CtMT2 and CtMT3 genes showed high expression in leaves. The expression of CtMT4 gene was high in seeds. The qRT-PCR studies revealed upregulation in expression of CtMT1 gene under drought stress. Recombinant CtMT1 protein was produced in E. coli Rosetta cells and purified by metal affinity chromatography. The purified protein showed antioxidant property and the order of its metal ion binding affinities was Cu²⁺ > Zn²⁺ > Fe²⁺ > Cd²⁺. This information about CtMT1 protein is expected to be useful in understanding its role in drought tolerance and other physiological processes of guar.

Electrochemical amphotericity and NIR absorption induced via the step-wise protonation of fused quinoxaline-tetrathiafulvalene-pyrroles

We describe an effective approach to producing electrochemical amphoteric character and tuning optical properties.

Further insights into the metal ion binding abilities and the metalation pathway of a plant metallothionein from Musa acuminata

The superfamily of metallothioneins (MTs) combines a diverse group of metalloproteins, sharing the characteristics of rather low molecular weight and high cysteine content. The latter provides MTs with the capability to coordinate thiophilic metal ions, in particular those with a d 10 electron configuration. The sub-family of plant MT3 proteins is only poorly characterized and there is a complete lack of three-dimensional structure information. Building upon our previous results on the Musa acuminata MT3 (musMT3) protein, the focus of the present work is to understand the metal cluster formation process, the role of the single histidine residue present in musMT3, and the metal ion binding affinity. We concentrate our efforts on the coordination of ZnII and CdII ions, using CoII as a spectroscopic probe for ZnII binding. The overall protein-fold is analysed with a combination of limited proteolytic digestion, mass spectrometry, and dynamic light scattering. Histidine coordination of metal ions is probed with extended X-ray absorption fine structure spectroscopy and CoII titration experiments. Initial experiments with isothermal titration calorimetry provide insights into the thermodynamics of metal ion binding.

Independent metal-thiolate cluster formation in C-terminal Cys-rich region of a rice type 1 metallothionein isoform

In this study we examined the independent self assembly of metal-binding in C-terminal Cys- rich region of a type 1 metallothionein (MT) isoform from rice (OsMTI-1b). To this end the N-terminal of OsMTI-1b (C-OsMTI-1b) was heterologously expressed in Escherichia coli as fusion protein with glutathione-S-transferase (GST). As compared with control (The E. coli cells containing pET41a without gene), transgenic E. coli cells expressing GST-C-OsMTI-1b accumulated more Ni²⁺, Cd²⁺, and Zn²⁺ from culture medium and showed increased tolerance against these metals. The recombinant GST-C-OsMTI-1b was purified using affinity chromatography. According to in vitro assays the protein GST-C-OsMTI-1b was able to form complexes with Ni²⁺, Cd²⁺ and Zn²⁺. These results demonstrate the formation of independent metal-thiolate cluster at C-terminal Cys-rich region of OsMTI-1b without participation of N-terminal Cys-rich region.

Oxidation of the N-terminal domain of the wheat metallothionein Ec -1 leads to the formation of three distinct disulfide bridges

Metallothioneins (MTs) are low molecular weight proteins, characterized by a high cysteine content and the ability to coordinate large amounts of d(10) metal ions, for example, Zn(II), Cd(II), and Cu(I), in form of metal-thiolate clusters. Depending on intracellular conditions such as redox potential or metal ion concentrations, MTs can occur in various states ranging from the fully metal-loaded holo- to the metal-free apo-form. The Cys thiolate groups in the apo-form can be either reduced or be involved in disulfide bridges. Although oxidation-mediated Zn(II) release might be a possible mechanism for the regulation of Zn(II) availability by MTs, no concise information regarding the associated pathways and the structure of oxidized apo-MT forms is available. Using the well-studied Zn2 γ-Ec -1 domain of the wheat Zn6 Ec -1 MT we attempt here to answer several question regarding the structure and biophysical properties of oxidized MT forms, such as: (1) does disulfide bond formation increase the stability against proteolysis, (2) is the overall peptide backbone fold similar for the holo- and the oxidized apo-MT form, and (3) are disulfide bridges specifically or randomly formed? Our investigations show that oxidation leads to three distinct disulfide bridges independently of the applied oxidation conditions and of the initial species used for oxidation, that is, the apo- or the holo-form. In addition, the oxidized apo-form is as stable against proteolysis as Zn2 γ-Ec -1, rendering the currently assumed degradation of oxidized MTs unlikely and suggesting a role of the oxidation process for the extension of protein lifetime in absence of sufficient amounts of metal ions. © 2016 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 106: 295-308, 2016.

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.

Isolation, molecular characterization and functional analysis of OeMT2, an olive metallothionein with a bioremediation potential

Metallothioneins are essential in plants for metal detoxification in addition to their other roles in plant life cycle. This study reports the characterization of an olive (Olea europaea L. cv. Ayvalik) metallothionein with respect to molecular and functional properties. A cDNA encoding a type 2 metallothionein from olive was isolated from a leaf cDNA library, characterized and named OeMT2 after its molecular and functional properties. OeMT2 was expressed in Escherichia coli, and a single protein band was confirmed by protein gel blot analysis. Metal tolerance ability of bacterial cells expressing OeMT2 was determined against 0.2 mM CdCl2, 0.4 mM CdCl2 and 1 mM CuSO4 in the growth medium. Metal ion contents of bacterial cells expressing OeMT2 were measured by ICP. Metal tolerance assays and ICP measurements suggested that OeMT2 effectively binds Cu and Cd. Molecular analysis of OeMT2 revealed two introns, three exons, a short 3' UTR and a long 5' UTR. Comparing the genomic sequences from 14 olive cultivars revealed OeMT2 had both intron and exon polymorphisms dividing the cultivars into three groups. Real-time PCR analysis demonstrated that OeMT2 expresses more or less the same amounts in all tissues of the olive tree examined. The genomic copy number of OeMT2 was also determined employing real-time PCR which suggested a single copy gene in the olive genome while three other MT2 members were determined from the draft olive genome sequences of Ayvalik cultivar and that of wild olive. This is the first report on molecular and functional characterization of an olive metallothionein and shows that OeMT2 expressed in E. coli has the capability of effectively binding toxic heavy metals. This may suggest that OeMT2 plays an important role in metal homeostasis in addition to a good potential for environmental and industrial usage.

Discrimination between two rice metallothionein isoforms belonging to type 1 and type 4 in metal-binding ability

Metallothioneins (MTs) are a superfamily of low-molecular-weight, cysteine (Cys)-rich proteins that are believed to play important roles in protection against metal toxicity and oxidative stress. Plants have several MT isoforms, which are classified into four types based on the arrangement of Cys residues. In this study, two rice (Oryza sativa) MT isoforms, OsMTI-1b and OsMTII-1a from type 1 and type 4, respectively, were heterologously expressed in Escherichia coli as carboxy-terminal extensions of glutathione-S-transferase (GST). Transformed cells expressing GST-OsMTI-1b showed increased tolerance to Ni(2+) , Cd(2+) , and Zn(2+) and accumulated more metal ions compared with cells expressing GST alone. However, heterologous expression of GST-OsMTII-1a had no significant effects on metal tolerance or ion accumulation. The UV absorption spectra and competitive reactions of in vitro Cd-incubated proteins with 5-5'-dithiobis(2-nitrobenzoic) acid revealed that GST-OsMTI-1b, but not GST-OsMTII-1a, is able to form Cd-thiolate clusters. Furthermore, heterologous expression of both GST-OsMTI-1b and GST-OsMTII-1a conferred H2 O2 tolerance to E. coli cells. Taken together, the results presented here show that two different rice MT isoforms belonging to type 1 and type 4 differ in Ni(2+) , Cd(2+) , and Zn(2+) binding abilities, but they may have overlapping function in protection of cells against oxidative stress.

Solution structure of the circular γ-domain analog from the wheat metallothionein E(c)-1

The first cyclic analog of a metallothionein (MT) was prepared and analyzed by UV and (magnetic) circular dichroism spectroscopy, ESI-MS as well as NMR spectroscopy. Results reveal that the evaluated cyclic γ-E_c-1 domain of the wheat MT E_c-1 retains its ability to coordinate two Zn(II) or Cd(II) ions and adopts a three-dimensional structure that is highly similar to the one of the linear wild-type form. However, the reduced flexibility of the protein backbone facilitates structure solution significantly and results in a certain stabilization of metal binding to the protein.

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+).

Role of pCeMT, a putative metallothionein from Colocasia esculenta, in response to metal stress

Metallothioneins (MTs) play a major role in metal homeostasis and/or detoxification in plants. In this study, a novel gene, pCeMT, was isolated from Colocasia esculenta and characterized. Our results indicate that Escherichia coli cells expressing pCeMT exhibited enhanced Cd, Cu, and Zn tolerance and accumulation compared with control cells. Furthermore, pCeMT-overexpressing tobacco seedlings displayed better growth under Cd, Cu, and Zn stresses and accumulated more Cd and Zn compared with the wild type. Interestingly, transgenic tobacco displayed markedly decreased hydrogen peroxide (H(2)O(2)) and lipid peroxidation levels under Cd, Cu, and Zn treatments. These results suggest that pCeMT could play an important role in the protection of plant cells from oxidative stress by reactive oxygen species (ROS) scavenging and in the detoxification of free metals by metal binding, leading to improved plant metal tolerance.

The response of the different soybean metallothionein isoforms to cadmium intoxication

Cadmium is a highly toxic heavy metal for both plants and animals. The presence of Cd in agricultural soils is of major concern regarding its entry into the food chain, since Cd compounds are readily taken up by plants, and accumulated in edible parts due to their high solubility. In this study, we first demonstrate the high capacity for Cd concentration of soybean grains. Consequently, we considered the study and characterization of the molecular determinants of Cd accumulation -such as metallothioneins (MT)- to be of major practical importance. We report here the first characterization of the soybean MT system, with the identification of nine genes (one of which is a truncated pseudogene), belonging to the four plant MT types. The most highly expressed of each type was chosen for further function analysis. All of them are expressed at high levels in soybean tissues: GmMT1, GmMT2 and GmMT3 in roots, shoots and seeds, and GmMT4 only in seeds. The corresponding recombinant soybean MTs, synthesized in Escherichia coli cells cultured in metal supplemented media, exhibit greater cadmium than zinc binding capacity. These results suggest a definite role of GmMTs in Cd(II) accumulation as one of the main responses of soybean to an overload of this metal.

Barley metallothioneins: MT3 and MT4 are localized in the grain aleurone layer and show differential zinc binding

Metallothioneins (MTs) are low-molecular-weight, cysteine-rich proteins believed to play a role in cytosolic zinc (Zn) and copper (Cu) homeostasis. However, evidence for the functional properties of MTs has been hampered by methodological problems in the isolation and characterization of the proteins. Here, we document that barley (Hordeum vulgare) MT3 and MT4 proteins exist in planta and that they differ in tissue localization as well as in metal coordination chemistry. Combined transcriptional and histological analyses showed temporal and spatial correlations between transcript levels and protein abundance during grain development. MT3 was present in tissues of both maternal and filial origin throughout grain filling. In contrast, MT4 was confined to the embryo and aleurone layer, where it appeared during tissue specialization and remained until maturity. Using state-of-the-art speciation analysis by size-exclusion chromatography inductively coupled plasma mass spectrometry and electrospray ionization time-of-flight mass spectrometry on recombinant MT3 and MT4, their specificity and capacity for metal ion binding were quantified, showing a strong preferential Zn binding relative to Cu and cadmium (Cd) in MT4, which was not the case for MT3. When complementary DNAs from barley MTs were expressed in Cu- or Cd-sensitive yeast mutants, MT3 provided a much stronger complementation than did MT4. We conclude that MT3 may play a housekeeping role in metal homeostasis, while MT4 may function in Zn storage in developing and mature grains. The localization of MT4 and its discrimination against Cd make it an ideal candidate for future biofortification strategies directed toward increasing food and feed Zn concentrations.

Structural features specific to plant metallothioneins

The metallothionein (MT) superfamily combines a large variety of small cysteine-rich proteins from nearly all phyla of life that have the ability to coordinate various transition metal ions, including Zn(II), Cd(II), and Cu(I). The members of the plant MT family are characterized by great sequence diversity, requiring further subdivision into four subfamilies. Very peculiar and not well understood is the presence of rather long cysteine-free amino acid linkers between the cysteine-rich regions. In light of the distinct differences in sequence to MTs from other families, it seems obvious to assume that these differences will also be manifested on the structural level. This was already impressively demonstrated with the elucidation of the three-dimensional structure of the wheat E(c)-1 MT, which revealed two metal cluster arrangements previously unprecedented for any MT. However, as this structure is so far the only one available for the plant MT family, other sources of information are in high demand. In this review the focus is thus set on any structural features known, deduced, or assumed for the plant MT proteins. This includes the determination of secondary structural elements by circular dichroism, IR, and Raman spectroscopy, the analysis of the influence of the long linker regions, and the evaluation of the spatial arrangement of the sequence separated cysteine-rich regions with the aid of, e.g., limited proteolytic digestion. In addition, special attention is paid to the contents of divalent metal ions as the metal ion to cysteine ratios are important for predicting and understanding possible metal-thiolate cluster structures.

Protein and metal cluster structure of the wheat metallothionein domain γ-E(c)-1: the second part of the puzzle

Metallothioneins (MTs) are small cysteine-rich proteins coordinating various transition metal ions, including Zn(II), Cd(II), and Cu(I). MTs are ubiquitously present in all phyla, indicating a successful molecular concept for metal ion binding in all organisms. The plant MT E(c)-1 from Triticum aestivum, common bread wheat, is a Zn(II)-binding protein that comprises two domains and binds up to six metal ions. The structure of the C-terminal four metal ion binding β(E) domain was recently described. Here we present the structure of the N-terminal second domain, γ-E(c)-1, determined by NMR spectroscopy. The γ-E(c)-1 domain enfolds an M (2) (II) Cys(6) cluster and was characterized as part of the full-length Zn(6)E(c)-1 protein as well as in the form of the separately expressed domain, both in the Zn(II)-containing isoform and the Cd(II)-containing isoform. Extended X-ray absorption fine structure analysis of Zn(2)γ-E(c)-1 clearly shows the presence of a ZnS(4) coordination sphere with average Zn-S distances of 2.33 Å. (113)Cd NMR experiments were used to identify the M(II)-Cys connectivity pattern, and revealed two putative metal cluster conformations. In addition, the general metal ion coordination abilities of γ-E(c)-1 were probed with Cd(II) binding experiments as well as by pH titrations of the Zn(II) and Cd(II) forms, the latter suggesting an interaction of the γ domain and the β(E) domain within the full-length protein.

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+).

Metallothioneins: unparalleled diversity in structures and functions for metal ion homeostasis and more

Metallothioneins have been the subject of intense study for five decades, and have greatly inspired the development of bio-analytical methodologies including multi-dimensional and multi-nuclear NMR.With further advancements in molecular biology, protein science, and instrumental techniques, recent years have seen a renaissance of research into metallothioneins. The current report focuses on in vitro studies of so-called class II metallothioneins from a variety of phyla, highlighting the diversity of metallothioneins in terms of structure, biological functions, and molecular functions such as metal ion specificity, thermodynamic stabilities, and kinetic reactivity. We are still far from being able to predict any of these properties, and further efforts will be required to generate the knowledge that will enable a better understanding of what governs the biological and chemical properties of these unusual and intriguing small proteins.

Metallothionein-III provides neuronal protection through activation of nuclear factor-kappaB via the TrkA/phosphatidylinositol-3 kinase/Akt signaling pathway

Metallothionein (MT)-III is associated with resistance to neuronal injury. However, the underlying mechanism for its effects is unclear. The present study investigated the mechanisms of MT-III protection of neuronal cells from hypoxia or DNA damage-induced cell death. MT-III reduced the hydrogen peroxide- or DNA damage-induced effects on neuronal cells, including the cell death, the activation of caspase-3 and -9, and the release of mitochondrial cytochrome c to the cytoplasm in a dose-dependent manner. MT-III also increased the activation of Akt, the phosphorylation and degradation of IkappaB, the nuclear translocation/accumulation and the transcriptional activity of nuclear factor-kappaB (NF-kappaB) in neuronal cells in a dose-dependent manner. The MT-III-induced antiapoptotic effects and increase in NF-kappaB activity were blocked by specific inhibitors of TrkA, phosphatidylinositol-3 kinase (PI3K), Akt, or NF-kappaB, indicating that MT-III provides neuronal protection by activating NF-kappaB through the TrkA/PI3K/Akt signaling pathway.

The CdII-binding abilities of recombinant Quercus suber metallothionein: bridging the gap between phytochelatins and metallothioneins

In this work, we have analyzed both at stoichiometric and at conformational level the Cd(II)-binding features of a type 2 plant metallothionein (MT) (the cork oak, Quercus suber, QsMT). To this end four peptides, the wild-type QsMT and three constructs previously engineered to characterize its Zn(II)- and Cu(I)-binding behaviour, were heterologously produced in Escherichia coli cultures supplemented with Cd(II), and the corresponding complexes were purified up to homogeneity. The Cd(II)-binding ability of these recombinant peptides was determined through the chemical, spectroscopic and spectrometric characterization of the recovered clusters. Recombinant synthesis of the four QsMT peptides in cadmium-rich media rendered complexes with a higher metal content than those obtained from zinc-supplemented cultures and, consequently, the recovered Cd(II) species are nonisostructural to those of Zn(II). Also of interest is the fact that three out of the four peptides yielded recombinant preparations that included S(2-)-containing Cd(II) complexes as major species. Subsequently, the in vitro Zn(II)/Cd(II) replacement reactions were studied, as well as the in vitro acid denaturation and S(2-) renaturation reactions. Finally, the capacity of the four peptides for preventing cadmium deleterious effects in yeast cells was tested through complementation assays. Consideration of all the results enables us to suggest a hairpin folding model for this typical type 2 plant Cd(II)-MT complex, as well as a nonnegligible role of the spacer in the detoxification function of QsMT towards cadmium.

Metal ion binding properties of Tricium aestivum Ec-1 metallothionein: evidence supporting two separate metal thiolate clusters

Metallothioneins are ubiquitous low molecular mass, cysteine-rich proteins with an extraordinary high metal ion content. In contrast to the situation for the vertebrate forms, information regarding the properties of members of the plant metallothionein family is still scarce. We present the first spectroscopic investigation aiming to elucidate the metal ion binding properties and metal thiolate cluster formation of the Triticum [corrected] aestivum (common wheat) early cysteine-labeled plant metallothionein (Ec-1). For this, the protein was overexpressed recombinantly in Escherichia coli. Recombinant Ec-1 is able to bind a total of six divalent d10 metal ions in a metal thiolate cluster arrangement. The pH stability of the zinc and cadmium clusters investigated is comparable to stabilities found for mammalian metallothioneins. Using cobalt(II) as a paramagnetic probe, we were able to show the onset of cluster formation taking place with the addition of a fourth metal ion equivalent to the apo protein. Limited proteolytic digestion experiments complemented with mass spectrometry and amino acid analysis provide clear evidence for the presence of two separate metal thiolate clusters. One cluster consists of four metal ions and is made up by a part of the protein containing 11 cysteine residues, comparable to the situation found in the mammalian counterparts. The second cluster features two metal ions coordinated by six cysteine residues. The occurrence of the latter cluster is unprecedented in the metallothionein superfamily so far.