Replacement of terminal cysteine with histidine in the metallothionein alpha and beta domains maintains its binding capacity

Functional characterization of rice metallothionein OsMT-I-Id: Insights into metal binding and heavy metal tolerance mechanisms

Metallothioneins (MTs) are cysteine-rich proteins known for their strong metal-binding capabilities, making them effective in detoxifying heavy metals (HMs). This study focuses on characterizing the functional properties of OsMT-I-Id, a type-I Metallothionein found in rice. Using a HM-responsive yeast cup1Δ (DTY4), ycf1∆ (for cadmium), and acr3∆ mutants (for trivalent arsenic), we assessed the impact of OsMT-I-Id on metal accumulation and cellular resilience. Our results demonstrated that yeast cells expressing OsMT-I-Id showed increased tolerance and accumulated higher levels of copper (Cu), arsenic (As), and cadmium (Cd), compared to control cells. This can be attributed to the protein's ability to chelate and bind HMs. Site-directed mutagenesis was employed to investigate the specific contributions of cysteine residues. The study revealed that yeast cells with a mutated C-domain displayed heightened HM sensitivity, while cells with a mutated N-domain exhibited reduced sensitivity. This underscores the critical role of C-cysteine-rich domains in metal binding and tolerance of type-I rice MTs. Furthermore, the study identified the significance of the 12th cysteine position at the N-domain and the 68th and 72nd cysteine positions at the C-domain in influencing OsMT-I-Id metal-binding capacity. This research provides novel insights into the structure-function relationship and metal binding properties of type-I plant MTs.

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.

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+).

The two Caenorhabditis elegans metallothioneins (CeMT-1 and CeMT-2) discriminate between essential zinc and toxic cadmium

The nematode Caenorhabditis elegans expresses two metallothioneins (MTs), CeMT-1 and CeMT-2, that are believed to be key players in the protection against metal toxicity. In this study, both isoforms were expressed in vitro in the presence of either Zn(II) or Cd(II). Metal binding stoichiometries and affinities were determined by ESI-MS and NMR, respectively. Both isoforms had equal zinc binding ability, but differed in their cadmium binding behaviour, with higher affinity found for CeMT-2. In addition, wild-type C. elegans, single MT knockouts and a double MT knockout allele were exposed to zinc (340 microm) or cadmium (25 microm) to investigate effects in vivo. Zinc levels were significantly increased in all knockout strains, but were most pronounced in the CeMT-1 knockout, mtl-1 (tm1770), while cadmium accumulation was highest in the CeMT-2 knockout, mtl-2 (gk125) and the double knockout mtl-1;mtl-2 (zs1). In addition, metal speciation was assessed by X-ray absorption fine-structure spectroscopy. This showed that O-donating, probably phosphate-rich, ligands play a dominant role in maintaining the physiological concentration of zinc, independently of metallothionein status. In contrast, cadmium was shown to coordinate with thiol groups, and the cadmium speciation of the wild-type and the CeMT-2 knockout strain was distinctly different to the CeMT-1 and double knockouts. Taken together, and supported by a simple model calculation, these findings show for the first time that the two MT isoforms have differential affinities towards Cd(II) and Zn(II) at a cellular level, and this is reflected at the protein level. This suggests that the two MT isoforms have distinct in vivo roles.

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.

Caenorhabditis elegans metallothionein isoform specificity--metal binding abilities and the role of histidine in CeMT1 and CeMT2

Two metallothionein (MT) isoforms have been identified in the model nematode Caenorhabditis elegans: CeMT1 and CeMT2, comprising two polypeptides that are 75 and 63 residues in length, respectively. Both isoforms encompass a conserved cysteine pattern (19 in CeMT1 and 18 in CeMT2) and, most significantly, as a result of their coordinative potential, CeMT1 includes four histidines, whereas CeMT2 has only one. In the present study, we present a comprehensive and comparative analysis of the metal [Zn(II), Cd(II) and Cu(I)] binding abilities of CeMT1 and CeMT2, performed through spectroscopic and spectrometric characterization of the recombinant metal-MT complexes synthesized for wild-type isoforms (CeMT1 and CeMT2), their separate N- and C-terminal moieties (NtCeMT1, CtCeMT1, NtCeMT2 and CtCeMT2) and a DeltaHisCeMT2 mutant. The corresponding in vitro Zn/Cd- and Zn/Cu-replacement and acidification/renaturalization processes have also been studied, as well as protein modification strategies that make it possible to identify and quantify the contribution of the histidine residues to metal coordination. Overall, the data obtained in the present study are consistent with a scenario where both isoforms exhibit a clear preference for divalent metal ion binding, rather than for Cu coordination, although this preference is more pronounced towards cadmium for CeMT2, whereas it is markedly clearer towards Zn for CeMT1. The presence of histidines in these MTs is revealed to be decisive for their coordination performance. In CeMT1, they contribute to the binding of a seventh Zn(II) ion in relation to the M(II)(6)-CeMT2 complexes, both when synthesized in the presence of supplemented Zn(II) or Cd(II). In CeMT2, the unique C-terminal histidine abolishes the Cu-thionein character that this isoform would otherwise exhibit.

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

For many years, paradigms regarding metallothioneins comprised the exclusive metal coordination by thiolates from cysteine residues and the absence of aromatic residues. As more sequence and in vitro data on metallothioneins, in particular from non-vertebrate organisms, has become available, both the occurrence of and metal coordination by histidine residues in metallothioneins is emerging as a more frequent feature than expected. We discuss the general implications of histidines versus cysteines in zinc binding sites, and review some recent results from literature and our own lab. We conclude that histidines can stabilise metallothionein clusters by reducing the overall charge, offering the ability to help with structural organisation by supplying H-bond donor and acceptor properties, reducing the likelihood for disulfide bond formation, whilst maintaining a high affinity towards metal ions, in particular the borderline zinc ion.

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.

Histidine ligands in bacterial metallothionein enhance cluster stability

The cyanobacterial metallothionein (MT) SmtA is the prototype for bacterial MTs and protects against elevated levels of zinc. In contrast to mammalian MTs, bacterial MTs coordinate to metal ions not only via cysteine sulfurs, but unusually for MTs, also via histidine nitrogens. To investigate whether histidine coordination in these metal-sulfur clusters provides advantages over S-coordination only, we mutated the two metal-binding histidine residues in the cyanobacterial MT SmtA from Synechococcus PCC7942 to cysteines. We show that the mutant proteins are still capable of binding up to four zinc ions as is the wild-type protein. However, the mutations perturb protein folding and metal-binding dynamics. Interestingly, several homologues of SmtA also show variations in these two residues. We conclude that histidine residues in Synechococcus PCC7942 SmtA have a stabilising effect due to electrostatic interactions that impact on protein folding and metal cluster charge, and are involved in fine-tuning the reactivity of the bound metal ions.

Structural study of the zinc and cadmium complexes of a type 2 plant (Quercus suber) metallothionein: Insights by vibrational spectroscopy

Zn- and Cd-complexes of Quercus suber metallothionein (QsMT) were obtained by in vivo-synthesis, in order to obtain physiologically representative aggregates, and characterized by spectrometric and spectroscopic methods. The secondary structure elements and the coordination environments of the metal binding sites of the two aggregates were determined, as well as the main metal-containing species formed. The results obtained from the analysis of the Raman and IR spectra reveal that these metal-MT complexes predominantly contain beta-sheet elements (about 60%), whereas they lack alpha-helices. These structural features slightly depend on the divalent metal bound. In particular, Cd(II) binding to QsMT induces a slight increase of the beta-sheet percentage, as well as a decrease in beta-turn elements with respect to Zn(II) binding. Conversely, the in vivo capability of QsMT to inglobe metal and sulfide ions is metal-depending. Spectroscopic vibrational data also confirm the presence of sulfide ligands in the metal clusters of both Zn- and Cd-QsMT, while the participation of the spacer His residue in metal coordination was only found in Cd-QsMT, in agreement with the CD results. Overall data suggest different coordination environments for Zn(II) and Cd(II) ions in QsMT.

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

Crs5 is a Saccharomyces cerevisiae Metallothionein (MT), non-homologous to the paradigmatic Cu-thionein Cup1. Although considered a secondary copper-resistance agent, we show here that it determines survival under zinc overload in a CUP1-null background. Its overexpression prevents the deleterious effects exhibited by CUP1-CRS5-null cells when exposed to combined Zn/Cu, as it does the mouse MT1 Zn-thionein, but not Cup1. The detailed characterization of Crs5 in vivo and in vitro Zn(II)-, Cd(II)- and Cu(I)-binding abilities fully supports its resemblance to mammalian MTs. Hence, Crs5 exhibits a good divalent metal-binding ability, yielding homometallic, highly chiral and stable Zn and Cd complexes when expressed in media enriched with these metal ions. In Cu-supplemented cultures, heterometallic Zn,Cu complexes are recovered, unless aeration is kept to a minimum. These features define a Crs5 dual metal-binding behaviour that is significantly closer to Zn-thioneins than to Cu-thioneins. Protein sequence similarities fully support these findings. Overall, a Crs5 function in global metal cell homeostasis, based on its Zn-binding features, is glimpsed. The comparative evaluation of Crs5 in the framework of MT functional differentiation and evolution allows its consideration as a representative of the primeval eukaryotic forms that progressively evolved to give rise to the Zn-thionein lineage.

Comparative metal binding and genomic analysis of the avian (chicken) and mammalian metallothionein

Chicken metallothionein (ckMT) is the paradigm for the study of metallothioneins (MTs) in the Aves class of vertebrates. Available literature data depict ckMT as a one-copy gene, encoding an MT protein highly similar to mammalian MT1. In contrast, the MT system in mammals consists of a four-member family exhibiting functional differentiation. This scenario prompted us to analyse the apparently distinct evolutionary patterns followed by MTs in birds and mammals, at both the functional and structural levels. Thus, in this work, the ckMT metal binding abilities towards Zn(II), Cd(II) and Cu(I) have been thoroughly revisited and then compared with those of the mammalian MT1 and MT4 isoforms, identified as zinc- and copper-thioneins, respectively. Interestingly, a new mechanism of MT dimerization is reported, on the basis of the coordinating capacity of the ckMT C-terminal histidine. Furthermore, an evolutionary study has been performed by means of in silico analyses of avian MT genes and proteins. The joint consideration of the functional and genomic data obtained questions the two features until now defining the avian MT system. Overall, in vivo and in vitro metal-binding results reveal that the Zn(II), Cd(II) and Cu(I) binding abilities of ckMT lay between those of mammalian MT1 and MT4, being closer to those of MT1 for the divalent metal ions but more similar to those of MT4 for Cu(I). This is consistent with a strong functional constraint operating on low-copy number genes that must cope with differentiating functional limitation. Finally, a second MT gene has been identified in silico in the chicken genome, ckMT2, exhibiting all the features to be considered an active coding region. The results presented here allow a new insight into the metal binding abilities of warm blooded vertebrate MTs and their evolutionary relationships.

Plant metallothionein domains: functional insight into physiological metal binding and protein folding

Plant metallothioneins (MTs) differ from animal MTs by a peculiar sequence organization consisting of two short cysteine-rich terminal domains linked by a long cysteine-devoid spacer. The role of the plant MT domains in the protein structure and functionality is largely unknown. Here, we investigate the separate domain contribution to the in vivo binding of Zn and Cu and to confer metal tolerance to CUP1-null yeast cells of a plant type 2 MT (QsMT). For this purpose, we obtained three recombinant peptides that, respectively, correspond to the single N-terminal (N25) and C-terminal (C18) cysteine-rich domains of QsMT, and a chimera in which the spacer is replaced with a four-glycine bridge (N25-C18). The metal-peptide preparations recovered from Zn- or Cu-enriched cultures were characterized by ESI-MS, ICP-OES and CD and UV-vis spectroscopy and data compared to full length QsMT. Results are consistent with QsMT giving rise to homometallic Zn- or Cu-MT complexes according to a hairpin model in which the two Cys-rich domains interact to form a cluster. In this model the spacer region does not contribute to the metal coordination. However, our data from Zn-QsMT (but not from Cu-QsMT) support a fold of the spacer involving some interaction with the metal core. On the other hand, results from functional complementation assays in endogenous MT-defective yeast cells suggest that the spacer region may play a role in Cu-QsMT stability or subcellular localization. As a whole, our results provide the first insight into the structure/function relationship of plant MTs using the analysis of the separate domain abilities to bind physiological metals.

Solution structure of Cu6 metallothionein from the fungus Neurospora crassa

The 3D-solution structure of Neurospora crassa Cu(6)-metallothionein (NcMT) polypeptide backbone was determined using homonuclear, multidimensional (1)H-NMR spectroscopy. It represents a new metallothionein (MT) fold with a protein chain where the N-terminal half is left-handed and the C-terminal half right-handedly folded around a copper(I)-sulfur cluster. As seen with other MTs, the protein lacks definable secondary structural elements; however, the polypeptide fold is unique. The metal coordination and the cysteine spacing defines this unique fold. NcMT is only the second MT in the copper-bound form to be structurally characterized and the first containing the -CxCxxxxxCxC- motif. This motif is found in a variety of mammalian MTs and metalloregulatory proteins. The in vitro formation of the Cu(6)NcMT identical to the native Cu(6)NcMT was dependent upon the prior formation of the Zn(3)NcMT and its titration with Cu(I). The enhanced sensitivity and resolution of the 800 MHz (1)H-NMR spectral data permitted the 3D structure determination of the polypeptide backbone without the substitution and utilization of the NMR active spin 1/2 metals such as (113)Cd and (109)Ag. These restraints have been necessary to establish specific metal to cysteine restraints in 3D structural studies on this family of proteins when using lower field, less sensitive (1)H-NMR spectral data. The accuracy of the structure calculated without these constraints is, however, supported by the similarities of the 800 MHz structures of the alpha-domain of mouse MT1 compared to the one recalculated without metal-cysteine connectivities.

Construction of alpha-alpha domains structure in recombinant monkey metallothionein-1

The differences in metal-thiolate coordination and reactivity of mammalian metallothionein (MT) domains are closely related to their distinct, highly conservative cysteine number and position. Monkey metallothionein-1, containing a beta-domain with Cd(3)S(9) cluster and an alpha-domain with Cd(4)S(11) cluster, was used to evaluate the role of cysteine residues in the formation of MT's metal-thiolate clusters. The possible influence of cysteine residues on the binding and stability of MT domains has been examined with the metallothionein mutants: N4C, T27C and N4C/T27C, which possess ten or eleven cysteine residues in the re-constructed beta-domain, respectively. Assisted by study of UV, CD and electrospray ionization mass spectroscopy (ESI-MS) and their reactivity with DTNB (5,5'-dithiobis (2-nitrobenzoic acid)), we found that besides the original alpha-domain, some kinds of new domain containing 4-cadmium-thiolate clusters were formed in the N4C and N4C/T27C mutants of mkMT1. These new domains displayed metal binding and kinetic reactivity with DTNB similar to the alpha-domain. However, the thermal stability of the mutants was less stable than that of WT mkMT1. This might result from the disturbance of the inter-domains hydrogen bonds and of the non-cysteine amino acid residue arrangement.

Expression, characterization, and reaction of recombinant monkey metallothionein-1 and its C33M mutant

After we modified the protocol of purification, monkey metallothionein-1 (mkMT-1) and its mutant at position 33 (C33M mutant) were efficiently expressed and purified by using the glutathione-S-transferase fusion protein system. The protein yield has been considerably improved (8 mg/L culture for mkMT-1 and 10 mg/L culture for C33M mutant). The recombinant MT-1 and C33M mutant were characterized by ESI-MS, UV, and CD spectra. The reactions of MI-1 and C33M mutant with 5,5'-dithiobis(2-nitrobenzoic acid) and EDTA also have been carefully studied. The pH titration of MT-1 and C33M mutant has been studied by UV and CD spectra. The mutation of cysteine-to-methionine at position 33 mostly maintains the alpha-domain structure similar to that in wild-type mkMT-1, but the C33M mutant has significant loss of stability and cooperative properties of the domain.

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.

A new insight into metallothionein (MT) classification and evolution. The in vivo and in vitro metal binding features of Homarus americanus recombinant MT

We report the synthesis and characterization of a Homarus americanus MT-cDNA (MTH) through retrotranscription of MTH-mRNA from metal-injected lobsters. Heterologous Escherichia coli expression in zinc- and copper-supplemented medium was achieved for MTH, the two domains betabetaMTH and betaalphaMTH and three site-directed mutants, betabetaC9H, betaalphaC37H, and betaalphaE31C/T34C. The in vivo conformed metal complexes and the in vitro substituted cadmium aggregates were characterized. Major stoichiometries of M(II)6-MTH for the entire MTH and M(II)3-betabetaMTH and M(II)3-betaalphaMTH for the independent domains fully validated our expression system. A low affinity binding site for a seventh Zn(II) in the in vivo synthesized MTH was located in the betaalpha domain. Additionally, minor M(II)4 species were found for each domain. Both single Cys to His mutations exhibited a similar reduction of their in vivo zinc binding ability but differed in their cadmium binding behavior when compared with the wild-type forms. Conversely, the double mutant showed an enhanced zinc and cadmium binding capacity. In vivo synthesis of MTH and of its independent domains in the presence of copper only afforded heterometallic copper-zinc species. These findings allow consideration of MTH as a zinc thionein and question the view of all crustacea MT structures as copper thioneins. Furthermore, a new approach for the evolutionary and functional classification of MT is proposed, based on the stoichiometry of metal-MT species and molecular phylogenetic analysis.

Drosophila MTN: a metazoan copper-thionein related to fungal forms

Two Drosophila metallothioneins (MT) have been reported: MTN, a 40 residue peptide including 10 Cys, and MTO, a 43 residue peptide including 12 Cys. However, neither functional nor evolutionary analyses for either of the Drosophila MT are available. Here, heterologous expression of Mtn in Escherichia coli is reported. The metal binding abilities of the Cu- and Zn-MTN complexes conformed in vivo, as well as the features of the Cd- and Cu-aggregates produced by metal replacement in vitro, have been determined by atomic emission spectrometry, circular dichroism and electrospray ionization mass spectrometry. Primary structure relationships with other MT have been examined. The results indicate a close resemblance of MTN to fungal copper-thioneins.

A new insight into the Ag+ and Cu+ binding sites in the metallothionein beta domain

The copper(I) and silver(I) binding properties of the beta fragment of recombinant mouse metallothionein I have been studied by electronic absorption and circular dichroism spectroscopy. When possible, the stoichiometry of the species formed was confirmed by electrospray mass spectrometry. The behaviour observed differs from that reported for the native protein. Titration of either Zn3-beta MT at pH 7 or apo-beta MT at pH 3 with Cu+ leads to the formation of species having the same stoichiometry and structure: Cu6-beta MT, Cu7-beta MT and Cu10-beta MT. In the first stage of the titration of Zn3-beta MT with Cu+ at pH 7 one additional species of formula Cu4Zn1-beta MT was detected. In contrast, the titration of Zn3-beta MT at pH 7.5 and of apo-beta MT at pH 2.5 with Ag+ proceeds through different reaction pathways, affording ZnxAg3-beta MT, Ag6-beta MT and Ag9-beta MT or Ag3-beta MT, Ag6-beta MT and Ag9-beta MT, respectively. The CD envelope corresponding to species with the same stoichiometric ratio, Ag6-beta MT and Ag9-beta MT, indicates that they have a different structure at each pH value. On the basis of the differences observed, the postulated similarity between copper and silver binding to metallothionein may be questioned.