Purification and characterisation of recombinant sea urchin metallothionein expressed in Escherichia coli

Supermetalation of Cd-MT3 beyond the two-domain model

The flexibility of mammalian metallothioneins (MTs) has contributed to the difficulty in obtaining structural information for this family of metalloproteins that bind divalent metals with its twenty cysteines. While the two-domain structure for Cd7MT is well-established as a Cd4S11 and Cd3S9, a third structure has been reported when 8 Cd(II) ions bind to MT1. Isoform 3 of the MT family, MT3, has been of interest to the research community since its isolation as a growth inhibitory factor isolated in brain tissue, and has since been noted as a prominent participant in the mediation of neurodegenerative diseases and regular brain development. The differences between MT3 and the other isoforms of MT include an additional hexapeptide insertion of acidic residues in the α domain as well as the introduction of two prolines in the β domain. It is unclear whether these changes impact the metalation properties of MT3. We report the formation of a Cd8MT3 species is characterized by electrospray ionization mass spectrometry and UV-visible absorption spectroscopy. We report that the spectroscopic properties of this supermetalated Cd8MT3 are similar to those of the supermetalated Cd8MT1, with a clear indication of changes in structure from "fully-metalated" Cd7MT3 to supermetalated Cd8MT3 from circular dichroism spectra and both 1D 113Cd and 2D 1H-113Cd HSQC NMR spectra. We conclude that the metalation properties are not impacted significantly due to the amino acid changes in MT3, and that the cysteinyl thiols are the key players in determining the capacity of metal-binding and the structure of metal-thiolate clusters.

An Overlooked Hepcidin-Cadmium Connection

Hepcidin (DTHFPICIFCCGCCHRSKCGMCCKT), an iron-regulatory hormone, is a 25-amino-acid peptide with four intramolecular disulfide bonds circulating in blood. Its hormonal activity is indirect and consists of marking ferroportin-1 (an iron exporter) for degradation. Hepcidin biosynthesis involves the N-terminally extended precursors prepro-hepcidin and pro-hepcidin, processed by peptidases to the final 25-peptide form. A sequence-specific formation of disulfide bonds and export of the oxidized peptide to the bloodstream follows. In this study we considered the fact that prior to export, reduced hepcidin may function as an octathiol ligand bearing some resemblance to the N-terminal part of the α-domain of metallothioneins. Consequently, we studied its ability to bind Zn(II) and Cd(II) ions using the original peptide and a model for prohepcidin extended N-terminally with a stretch of five arginine residues (5R-hepcidin). We found that both form equivalent mononuclear complexes with two Zn(II) or Cd(II) ions saturating all eight Cys residues. The average affinity at pH 7.4, determined from pH-metric spectroscopic titrations, is 10^10.1 M^-1 for Zn(II) ions; Cd(II) ions bind with affinities of 10^15.2 M^-1 and 10^14.1 M^-1. Using mass spectrometry and 5R-hepcidin we demonstrated that hepcidin can compete for Cd(II) ions with metallothionein-2, a cellular cadmium target. This study enabled us to conclude that hepcidin binds Zn(II) and Cd(II) sufficiently strongly to participate in zinc physiology and cadmium toxicity under intracellular conditions.

Metal Exchange in the Interprotein ZnII -Binding Site of the Rad50 Hook Domain: Structural Insights into CdII -Induced DNA-Repair Inhibition

CdII is a major genotoxic agent that readily displaces ZnII in a multitude of zinc proteins, abrogates redox homeostasis, and deregulates cellular metalloproteome. To date, this displacement has been described mostly for cysteine(Cys)-rich intraprotein binding sites in certain zinc finger domains and metallothioneins. To visualize how a ZnII -to-CdII swap can affect the target protein's status and thus understand the molecular basis of CdII -induced genotoxicity an intermolecular ZnII -binding site from the crucial DNA repair protein Rad50 and its zinc hook domain were examined. By using a length-varied peptide base, ZnII -to-CdII displacement in Rad50's hook domain is demonstrated to alter it in a bimodal fashion: 1) CdII induces around a two-orders-of-magnitude stabilization effect (log  K 12 Zn II =20.8 vs. log  K 12 Cd II =22.7), which defines an extremely high affinity of a peptide towards a metal ion, and 2) the displacement disrupts the overall assembly of the domain, as shown by NMR spectroscopic and anisotropy decay data. Based on the results, a new model explaining the molecular mechanism of CdII genotoxicity that underlines CdII 's impact on Rad50's dimer stability and quaternary structure that could potentially result in abrogation of the major DNA damage response pathway is proposed.

Crosstalk of the structural and zinc buffering properties of mammalian metallothionein-2

Structural insights into partially Zn(ii)-depleted MT2 species and their zinc buffering properties are presented and discussed.

Combining chemical labeling, bottom-up and top-down ion-mobility mass spectrometry to identify metal-binding sites of partially metalated metallothionein

Metalation and demetalation of human metallothionein-2A (MT) with Cd(2+) is investigated by using chemical labeling and "bottom-up" and "top-down" proteomics approaches. Both metalation and demetalation of MT-2A by Cd(2+) are shown to be domain specific and occur as two distinct processes. Metalation involves sequential addition of Cd(2+) to the α-domain resulting in formation of an intermediate, Cd4MT. Chemical labeling with N-ethylmaleimide (NEM) and tandem mass spectrometry experiments clearly show that the four metal ions are located in the α-domain. In the presence of excess Cd(2+), the Cd4MT intermediate reacts to add Cd(2+) to the β-domain to yield the fully metalated Cd7MT. Demetalation occurs in the reverse order, i.e., Cd(2+) is removed (by EDTA) first from the β-domain followed by Cd(2+) removal from the α-domain. Metalation of human MT-2A is shown to be metal ion specific by comparing relative metal ion binding constants for Cd(2+) and Zn(2+).

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.

Metallothioneins in Aquatic Organisms: Fish, Crustaceans, Molluscs, and Echinoderms

Metallothioneins (MTs) have been described in a wide range of organisms, from bacteria to mammals, thus representing an interesting example of evolutionary molecular adaptation. If the moderate variability of MTs across phylogenetically distant organisms reflects their highly conserved function, the specific environmental requirements may explain the multiplicity of isoforms also in the same organism. The MT polymorphism is particularly important in invertebrates with respect to vertebrates. This review is an attempt to summarize the knowledge about MTs from aquatic animals, both vertebrates and invertebrates, to gain new insights into the structure-function relationship of this class of proteins. The large and increasing literature on MTs indicates that MTs from aquatic vertebrates are rather similar to mammalian counterparts, whereas a variety of structures have been described in invertebrates. Although the prototypical αβ-domain organization of vertebrate MTs has been observed in most invertebrate isoforms, some invertebrate MTs display alternative structures in which the canonical organization has been modified, such as the ββ-domain, the αββ-domain, and the multiple α-domain structures of oyster MTs, and the inverted βα-domain organization of sea urchin MTs. In this review we emphasize three major taxa of aquatic invertebrates, the molluscs, the crustaceans and the echinoderms, although some data have been reported for other invertebrates.

The metal-binding features of the recombinant mussel Mytilus edulis MT-10-IV metallothionein

In contrast with the paradigmatic mammalian metallothioneins (MTs), mollusc MT systems consist at least of a high-cadmium induced form, possibly involved in detoxification, and another isoform either constitutive or regulated by essential metals and probably associated with housekeeping metabolism. With the aim of providing a deeper characterization of the coordination features of a molluscan MT peptide of the latter kind, we have analyzed here the metal-binding abilities of the recombinant MeMT-10-IV isoform of Mytilus edulis (MeMT). Also, comparison with other MTs of this type has been undertaken. A synthetic complementary DNA was constructed, cloned and expressed into two Escherichia coli systems. Upon zinc coordination, MeMT folds in vivo into highly chiral and stable Zn(7) complexes, with an exceptional reluctance to fully substitute cadmium(II) and/or copper(I) for zinc(II). In vivo cadmium binding leads to homometallic Cd(7) complexes that structurally differ from any of the in vitro prepared Cd(7) complexes. Homometallic Cu-MeMT can only be obtained in vitro from Zn(7)-MeMT after a great molar excess of copper(I) has been added. In vivo, two different heterometallic Zn,Cu-MeMT complexes are recovered, which nicely correspond to two distinct stages of the in vitro zinc/copper replacement. These MeMT metal-binding features are consistent with a physiological role related to basal/housekeeping metal, mainly zinc, metabolism, and confirm the correspondence between the MeMT gene response pattern and the functional properties of the encoded protein.

Effect of α-domain substitution on the structure, property and function of human neuronal growth inhibitory factor

Human metallothionein-3 (hMT3), also named human neuronal growth inhibitory factor (hGIF), is attractive due to its distinct neuronal growth inhibitory activity, which is not shown by other human MT isoforms. It has been reported that the neuronal growth inhibitory activity arises from the N-terminal beta-domain rather than its C-terminal alpha-domain. However, previous bioassay results have shown that the single beta-domain is less effective at inhibiting the neuron growth than that in intact hMT3 on a molar basis, which suggests that the alpha-domain is indispensable to the neuronal growth inhibitory activity of hMT3. In order to confirm this assumption, we constructed two domain-hybrid mutants, the beta(MT3)-beta(MT3) mutant and the beta(MT3)-alpha(MT1) mutant, and investigated their structural and metal binding properties by UV-vis spectroscopy, CD spectroscopy, pH titration, DTNB reaction, EDTA reaction, etc. The results showed that stability of the Cd(3)S(9) cluster of the beta(MT3)-beta(MT3) mutant decreased significantly while the Cd(3)S(9) cluster of the beta(MT3)-alpha(MT1) mutant had a similar stability and solvent accessibility to that of hMT3. Interestingly, the bioassay results showed that the neuronal growth inhibitory activity of the beta(MT3)-beta(MT3) mutant decreased significantly, while the beta(MT3)-alpha(MT1) mutant showed similar inhibitory activity to hMT3. Based on these results, we conclude that the alpha-domain is indispensable and plays an important role in modulating the stability of the metal cluster in the beta-domain by domain-domain interactions, thus influencing the bioactivity of hMT3.

Studies of metal ion binding by apo-metallothioneins attached onto preformed self-assembled monolayers using a highly sensitive surface plasmon resonance spectrometer

The use of a flow-injection surface plasmon resonance (FI-SPR) spectrometer equipped with a bicell detector or a position-sensitive device for determining coordination of heavy metal ions (Cd(2+) and Hg(2+)) by surface-confined apo-metallothionein (apo-MT) molecules is described. To facilitate the formation of a compact MT adsorbate layer with a uniform surface orientation, MT molecules were attached onto a preformed alkanethiol self-assembled monolayer. The method resorts to the generation of apo-MT at the surface by treating the MT-covered sensor chip with glycine-HCl and the measurement of the apo-MT conformation changes upon metal ion incorporation. Domain-specific metal ion binding processes by the apo-MT molecules were observed. Competitive replacement of one metal ion by another can be monitored in real time by FI-SPR. The tandem use of an immobilization scheme for forming a sub-monolayer of MT molecules at the sensor surface and the highly sensitive FI-SPR instrument affords a low concentration detection level. The detection level for Cd(2+) (0.1 μM or 15 ppb) compares favorably with similar studies and the methodology complements to other well-established sensitive analytical techniques. The extent of metal incorporation by apo-MT molecules was also determined.

Cloning, expression and characterization of metallothionein from the Antarctic clam Laternula elliptica

The genes for two apparent subtypes of metallothionein (MT) isoform were isolated from the Antarctic clam Laternula elliptica. Determination of the nucleotide sequence showed that the gene consists of 222 bp that code a 73-amino acid protein. The comparison between MT cDNA sequences of L. elliptica and other bivalves showed strong homologies on positions of cysteine residues, which are important for their metal binding abilities. The gene for the MT was inserted into a pET vector and overexpressed as a carboxyl terminal extension of glutathionein-S-transferase (GST) in Escherichia coli. After the GST fusion proteins had been purified by glutathione-Sepharose affinity chromatography column and digested with enterokinase, the MT was purified with gel filtration and analyzed for its biochemical properties. Recombinant MTs were reconstituted with Cd, Cu, and Zn, and kinetic studies of the reactions with electrophilic disulphide, DTNB, were investigated to explore their metal binding ability. It is revealed that the Cd-MT and Zn-MT react with DTNB biphasically, and that Zn-MT reacts with DTNB more rapidly, and with a significantly greater pseudo-first-order rate constant. Cu-MT reacts monophasically and releases metal slowly from MT.

The properties of the metal-thiolate clusters in recombinant mouse metallothionein-4

Metallothioneins (MTs) are metal-binding proteins with low molecular weight and conservative cysteine residues. Metallothionein-4 (MT-4), one of MT isoforms, is first reported to be distributed in a tissue-specific manner, mainly in stratified squamous epithelia. Here, we compare the properties of metal-thiolate clusters in MT-4 to those in MT-1 and MT-3, including the stabilities toward both pH change and EDTA, as well as the exposure of thiolates to solvent. The metal-thiolate clusters in MT-3 show different property and activity to the reactions compared with MT-4 and MT-1. The structure of metal-thiolate clusters in MT-4 is similar to that of MT-1 from the UV and CD spectra. During pH titration and DTNB reaction, MT-4 and MT-1 exhibit comparable behavior. But while reacting with EDTA, the metal-thiolate clusters in MT-4 are more stable than those of MT-1. We suppose the negative charge of the beta-domain of MT-4 prevents the EDTA attack to MT-4.

Organization and assembly of metal-thiolate clusters in epithelium-specific metallothionein-4

Mammalian metallothionein-4 (MT-4) was found to be specifically expressed in stratified squamous epithelia where it plays an essential but poorly defined role in regulating zinc or copper metabolism. Here we report on the organization, stability, and the pathway of metal-thiolate cluster assembly in MT-4 reconstituted with Cd(2+) and Co(2+) ions. Both the (113)Cd NMR studies of (113)Cd(7)MT-4 and the spectroscopic characterization of Co(7)MT-4 showed that, similar to the classical MT-1 and MT-2 proteins, metal ions are organized in two independent Cd(4)Cys(11) and Cd(3)Cys(9) clusters with each metal ion tetrahedrally coordinated by terminal and bridging cysteine ligands. Moreover, we have demonstrated that the cluster formation in Cd(7)MT-4 is cooperative and sequential, with the Cd(4)Cys(11) cluster being formed first, and that a distinct single-metal nucleation intermediate Cd(1)MT-4 is required in the cluster formation process. Conversely, the absorption and circular dichroism features of metal-thiolate clusters in Cd(7)MT-4 indicate that marked differences in the cluster geometry exist when compared with those in Cd(7)MT-1/2. The biological implication of our studies as to the role of MT-4 in zinc metabolism of stratified epithelia is discussed.

Fish and molluscan metallothioneins

Metallothioneins (MTs) are noncatalytic peptides involved in storage of essential ions, detoxification of nonessential metals, and scavenging of oxyradicals. They exhibit an unusual primary sequence and unique 3D arrangement. Whereas vertebrate MTs are characterized by the well-known dumbbell shape, with a beta domain that binds three bivalent metal ions and an alpha domain that binds four ions, molluscan MT structure is still poorly understood. For this reason we compared two MTs from aquatic organisms that differ markedly in primary structure: MT 10 from the invertebrate Mytilus galloprovincialis and MT A from Oncorhyncus mykiss. Both proteins were overexpressed in Escherichia coli as glutathione S-transferase fusion proteins, and the MT moiety was recovered after protease cleavage. The MTs were analyzed by gel electrophoresis and tested for their differential reactivity with alkylating and reducing agents. Although they show an identical cadmium content and a similar metal-binding ability, spectropolarimetric analysis disclosed significant differences in the Cd7-MT secondary conformation. These structural differences reflect the thermal stability and metal transport of the two proteins. When metal transfer from Cd7-MT to 4-(2-pyridylazo)resorcinol was measured, the mussel MT was more reactive than the fish protein. This confirms that the differences in the primary sequence of MT 10 give rise to peculiar secondary conformation, which in turn reflects its reactivity and stability. The functional differences between the two MTs are due to specific structural properties and may be related to the different lifestyles of the two organisms.

Isolation and characterisation of metallothionein from the clam Ruditapes decussatus

Metallothioneins (MT) were obtained after purification from metal-exposed clams (Ruditapes decussatus) using gel-permeation and ion-exchange chromatography. Four cadmium-metallothioneins (CdMTs) were resolved by ion-exchange chromatography and they all had similar molecular weights, high cadmium content and an absorption spectra indicative of the presence of characteristic Cd-S aggregates. The NH(2)-terminal sequence suggests the presence of at least two class I clam MT isoforms. For the other two putative clam CdMTs isolated, the results of the amino acid determination were inconclusive. One was slightly contaminated and the other one had a blocked NH(2)-terminal. These clam metalothioneins contain glycine, which seems to be a common feature of molluscan MT family and exhibited more similarity to oysters than to mussels. Further investigation on the inducibility of these isoforms will be necessary if clams are to be used as biomarkers of metal exposure.

Advances in the structure and chemistry of metallothioneins

A low molecular weight (6-7 kDa) class of metalloproteins, designated as metallothioneins (MTs), exhibit repeated sequence motifs of either CxC or CxxC through which mono or divalent d(10) metal ions are bound in polymetallic-thiolate clusters. The preservation of metal-thiolate clusters in an increasing number of three-dimensional structures of these proteins signifies the importance of this structural motif. This review focuses on the recent developments regarding the versatile and striking chemical reactivity of MTs as well as on the existence of conformational/configurational dynamics within their structure. Both properties and their interplay are likely to be essential for the still elusive biological function of these proteins.

Spectroscopic characterization of metallothionein from the terrestrial snail, Helix pomatia

The Cd-sequestering metallothionein (MT) isoform isolated from the midgut gland of Roman snails exposed to Cd supplements in the feed was characterized by compositional and spectroscopic analysis. The preparations contained nearly 5 mol of Cd, small amounts of Cu and about 1 mol of Zn per chain mass of 6620 Da, in numerical agreement with the apoprotein's measured capacity of firmly binding a maximum of 6 equivalents of Cd per molecule. As with other Cd-containing MTs the occurrence of a prominent Cd-mercaptide-specific shoulder at 250 nm in its absorption spectrum showed that Cd is complexed in tetrahedral symmetry by the cysteine residues of the protein, and the multiphasic ellipticity profile in the CD spectrum revealed that these complexes are joined to form one or more oligonuclear Cd-mercapto clusters. Both spectral features vanished with the removal of the metal but were reconstituted to maximum amplitudes by readdition of Cd to the metal-free apoprotein, provided precautions were taken to prevent air oxidation of the latter. Quantitative analysis of snail MT reconstituted with Cd established that the 18 cysteine side chains bind the metal in a 3-to-1 ratio; spectroscopic studies on fractionally restored forms demonstrated that the six Cd ions were bound to the apoprotein molecule in succession in two sets of three Cd ions each. Thus, one can infer from the observed stoichiometry and the coordinating preferences of Cd that this gastropod MT, like the Cd-bearing MTs of marine crustaceans, harboured the metal in two separate cyclically constructed Cd3Cys9 clusters. The snail clusters differed, however, from other MTs in their response to acidification. Their protolytic dissociation proceeded through two separate protonation steps with the manifestation of spectroscopically distinguishable intermediate forms. Thus, this snail isoform displays in its metal composition and its chemical and spectroscopic features both similarities and differences to other animal kingdom MTs. Its properties suggest that it serves an important role in the protection of the terrestrial gastropod from Cd.

Effect of the two conserved prolines of human growth inhibitory factor (metallothionein-3) on its biological activity and structure fluctuation: comparison with a mutant protein

Human neuronal growth inhibitory factor, a metalloprotein classified as metallothionein-3 (MT-3), impairs the survival and the neurite formation of cultured neurons. In these studies the double P7S/P9A mutant (mutMT-3) and single mutants P7S and P9A of human Zn(7)-MT-3 were generated, and their effects on the biological activity and the structure of the protein were examined. The biological results clearly established the necessity of both proline residues for the inhibitory activity, as even single mutants were found to be inactive. Using electronic absorption, circular dichroism (CD), magnetic CD (MCD), and (113)Cd NMR spectroscopy, the structural features of the metal-thiolate clusters in the double mutant Cd(7)-mutMT-3 were investigated and compared with those of wild-type Cd(7)-MT-3 [Faller, P., Hasler, D. W., Zerbe, O., Klauser, S., Winge, D. R., and Vasák, M. (1999) Biochemistry 38, 10158] and the well characterized Cd(7)-MT-2a from rabbit liver. Similarly to (113)Cd(7)-MT-3 the (113)Cd NMR spectrum of (113)Cd(7)-mutMT-3 at 298 K revealed four major and three minor resonances (approximately 20% of the major ones) between 590 and 680 ppm, originating from a Cd(4)S(11) cluster in the alpha-domain and a Cd(3)S(9) cluster in the beta-domain, respectively. Due to the presence of dynamic processes in the structure of MT-3 and mutMT-3, all resonances showed the absence of resolved homonuclear [(113)Cd-(113)Cd] couplings and large apparent line widths (between 140 and 350 Hz). However, whereas in (113)Cd(7)-mutMT-3 the temperature rise to 323 K resulted in a major recovery of the originally NMR nondetectable population of the Cd(3)S(9) cluster resonances, no such temperature effect was observed in (113)Cd(7)-MT-3. To account for the observed NMR features, a dynamic structural model for the beta-domain is proposed, which involves a folded and a partially unfolded state. It is suggested that in the partially unfolded state a slow cis/trans isomerization of Cys-Pro(7) or Cys-Pro(9) amide bonds in (113)Cd(7)-MT-3 takes place and that this process represents a rate-limiting step in a correct domain refolding. In addition, closely similar apparent stability constants of human MT-3, mutMT-3, and rabbit MT-2a with Cd(II) and Zn(II) ions were found. These results suggest that specific structural features dictated by the repetitive (Cys-Pro)(2) sequence in the beta-domain of MT-3 and not its altered metal binding affinity compared to MT-1/MT-2 isoforms are responsible for the biological activity of this protein.

The carcinogen Cd(2+) activates InsP(3)-mediated Ca(2+) release through a specific metal ion receptor in Xenopus oocyte

The effects of the carcinogen Cd(2+) on Xenopus oocyte were evaluated by Inositol (1,4,5)-trisphosphate (InsP(3)) assays and electrophysiological experiments. The stimulation of the Ca(2+)-dependent Cl(-) current by Cd(2+) is clearly linked to InsP(3) formation since the effects of the metal are antagonized by neomycin, heparin and caffeine. A similar inhibition of the Cd(2+) effects is observed when the oocytes are pretreated with thapsigargin. Moreover, the use of sulfhydryl groups reductors such as 2-mercaptoethanol or N-ethylmaleimide strongly suggests that the Cd(2+) response is mediated by an extracellular receptor. Finally, measurements of InsP(3) production demonstrate that Cd(2+) superfusion actually leads to a PIP(2) breakdown. We conclude that extracellular Cd(2+) evokes an increase in [Ca(2+)](i) by stimulating the emptying of the InsP(3)-sensitive Ca(2+) stores, and that it may do so by interacting with a specific cell-surface ion receptor. This putative ion receptor may be important in allowing oocytes to respond to heavy metals.

Electrospray ionization mass spectrometry of zinc, cadmium, and copper metallothioneins: evidence for metal-binding cooperativity

Electrospray ionization (ESI) mass spectra of both well-characterized and novel metallothioneins (MTs) from various species were recorded to explore their metal-ion-binding modes and stoichiometries. The ESI mass spectra of the zinc- and cadmium-binding MTs showed a single main peak corresponding to metal-to-protein ratios of 4, 6, or 7. These findings combined with data obtained by other methods suggest that these MTs bind zinc or cadmium in a single predominant form and are consistent with the presence of three- and four-metal clusters. An unstable copper-specific MT isoform from Roman snails (Helix pomatia) could be isolated intact and was shown to preferentially bind 12 copper ions. To obtain additional information on the formation and relative stability of metal-thiolate clusters in MTs, a mass spectrometric titration study was conducted. One to seven molar equivalents of zinc or of cadmium were added to metal-free human MT-2 at neutral pH, and the resulting complexes were measured by ESI mass spectrometry. These experiments revealed that the formation of the four-metal cluster and of the thermodynamically less stable three-metal cluster is sequential and largely cooperative for both zinc and cadmium. Minor intermediate forms between metal-free MT, Me4MT, and fully reconstituted Me7MT were also observed. The addition of increasing amounts of cadmium to metal-free blue crab MT-I resulted in prominent peaks whose masses were consistent with apoMT, Cd3MT, and Cd6MT, reflecting the known structure of this MT with two Me3Cys9 centers. In a similar reconstitution experiment performed with Caenorhabditis elegans MT-II, a series of signals corresponding to apoMT and Cd3MT to Cd6MT species were observed.

Bioremediation of Heavy Metal Pollution Exploiting Constituents, Metabolites and Metabolic Pathways of Livings. A Review

Removal of heavy metals from the soil and water or their remediation from the waste streams "at source" has been a long-term challenge. During the recent era of environmental protection, the use of microorganisms for the recovery of metals from waste streams as well as employment of plants for landfill applications has generated growing attention. Many studies have demonstrated that both prokaryotes and eukaryotes have the ability to remove metals from contaminated water or waste streams. They sequester metals from soils and sediments or solubilize them to aid their extraction. The proposed microbial processes for bioremediation of toxic metals and radionuclides from waste streams employ living cells and non-living biomass or biopolymers as biosorbents. Microbial biotransformation of metals or metalloids results in an alteration of their oxidation state or in their alkylation and subsequent precipitation or volatilization. Specific metabolic pathways leading to precipitation of heavy metals as metal sulfides, phosphates or carbonates possess significance for possible biotechnology application. Moreover, the possibility of altering the properties of living species used in heavy metal remediation or constructing chimeric organisms possessing desirable features using genetic engineering is now under study in many laboratories. The encouraging evidence as to the usefulness of living organisms and their constituents as well as metabolic pathways for the remediation of metal contamination is reviewed here. A review with 243 references.

Purification and characterization of recombinant Caenorhabditis elegans metallothionein

The roundworm Caenorhabditis elegans adapted for survival at high concentrations of Cd(II) expresses two isoforms of metallothionein, CeMT-I and CeMT-II. To characterize one of these proteins CeMT-II was prepared as its Cd containing form by expressing its cDNA heterologously in Escherichia coli. The purified 63-amino-acid protein was identified as the desired product by ion-spray mass spectrometry and was found to resemble in most of its chemical and spectroscopic features the metallothioneins of other animal phyla. The recombinant protein contains a total of 18 cysteine residues and, as documented by electrophoresis and mass spectrometry, binds firmly six Cd ions through the cysteine's side chains. The (113)Cd NMR spectrum features six (113)Cd resonances. Their chemical shift positions between 615 and 675 ppm denote the existence of clusters of tetrahedrally coordinated cadmium thiolate complexes. The metal thiolate coordination dominates also the electronic far-UV absorption spectrum. It is characterized by a massive absorption profile with Cd thiolate shoulders at 255 and 235 nm. Upon replacement of Cd by Zn the profile was blue-shifted by 30 nm.

NMR structure of the sea urchin (Strongylocentrotus purpuratus) metallothionein MTA

The three-dimensional structure of [(113)Cd7]-metallothionein-A (MTA) of the sea urchin Strongylocentrotus purpuratus was determined by homonuclear(1)H NMR experiments and heteronuclear [(1)H, (113)Cd]-correlation spectroscopy. MTA is composed of two globular domains, an N-terminal four-metal domain of the amino acid residues 1 to 36 and a Cd4Cys11cluster, and a C-terminal three-metal domain including the amino acid residues 37 to 65 and a Cd3Cys9cluster. The structure resembles the known mammalian and crustacean metallothioneins, but has a significantly different connectivity pattern of the Cys-metal co-ordination bonds and concomitantly contains novel local folds of some polypeptide backbone segments. These differences can be related to variations of the Cys sequence positions and thus emphasize the special role of the cysteine residues in defining the structure of metallothioneins, both on the level of the domain architecture and the topology of the metal-thiolate clusters.

Enhanced metallosorption of Escherichia coli cells due to surface display of beta- and alpha-domains of mammalian metallothionein as a fusion to LamB protein

The lamB gene was inserted at with DNA fragments encoding N-terminal beta- and C-terminal alpha-domains of human metallothionein 1A (HMT1A). The hybrid LamB proteins were expressed as full-length products. Virtually whole pool of hybrid LamB proteins was found localized in the outer membrane of E. coli to and cells expressing LamB variants retained sensitivity to lambda phage, indicating their correct folding. Expression of hybrid LamB proteins increased natural ability of E. coli accumulate bivalent heavy metals ions with the highest efficiency observed for cadmium. The order of amount of cadmium accumulated is alpha-domain of HMT1A > HMT1A >> beta-domain of HMT1A. This correlates with affinity for cadmium and stability of metallothionein and its individual domains. This confirms suitability of LamB vehicle for surface display of various bioactive molecules and suggests possibility of engineering of cell surface for bioremediation of heavy metals.

Separation and characterization of the metal-thiolate-cluster domains of recombinant sea urchin metallothionein

Partial metal depletion and 113Cd-NMR studies have suggested that the recombinant Cd-containing metallothionein of the sea urchin Strongylocentrotus purpuratus (Cd7-MTA) binds its metal ions in a four-metal (Cd4Cys11) and a three-metal (Cd3Cys9) cluster associated with the N-terminal and C-terminal halves of the protein, respectively [Wang, Y., Mackay, E.A., Zerbe, O., Hess, D., Hunziker, P.E., Vasak, M. & Kägi, J.H.R. (1995) Biochemistry 34,7460-7467]. This partitioning has now been confirmed by bisecting native Cd7-MTA with subtilisin into products bearing only a single metal-thiolate cluster. Their separation by reverse-phase HPLC and on-line electrospray mass spectrometry in combination with sequence analysis revealed selective cleavage of the protein into a set of N-terminal polypeptides containing 37-39 residues with four Cd ions and a set of C-terminal polypeptides containing 24 and 25 residues with three Cd ions. Thus, sea urchin MTA like its mammalian counterparts is made up of two separate cluster-harboring domains. The fragmentation pattern indicated that the sites of cleavage are located in the peptide loop interspaced between the first two metal-bound cysteine residues of the C-terminal domain. Accordingly, with cleavage, one of the putative nine thiolate ligands of the three-metal cluster was lost to the N-terminal fragment. The coordinational consequences of this repartition were reflected in massive chiroptical changes accompanying the cleavage process. While the liberated N-terminal domain retained the CD profile of the four-metal cluster in the parent protein and thereby indicated preservation of its structure, the CD features attributable to the intact three-metal cluster were largely lost on cleavage. The vanished features bear strong resemblance to the large biphasic ellipticity signal at 250 nm which dominates the CD spectrum of native Cd7-MTA, and allow us thus to attribute this signal to excitonic coupling interactions of Cd-thiolate chromophores in the three-metal cluster.

Roles of the conserved serines of metallothionein in cadmium binding

The sequence of six amino acid residues -Ser-Cys-Cys-Ser-Cys-Cys- is present in all mammalian metallothionein sequences and has been highly conserved during evolution, although the metallothioneins have divergent primary sequences. To determine whether two serines in the sequence play a crucial role in metal-binding of metallothioneins, a mutant metallothionein with these two serines replaced by leucines was obtained using an Escherichia coli expression system. The expressed protein was analyzed for its chemical and spectroscopic properties. It was confirmed that the mutant metallothionein (MT) bound cadmium through a metal-thiolate complex and that there was no strong difference between the mutant and the wild-type MTs in retaining the metal-binding cluster. However, the metal-binding cluster of the mutant metallothionein was more unstable than that of the wild-type metallothionein. The two conservative serines could play a role in the stability of metal-binding ligands.