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

Juggling cadmium detoxification and zinc homeostasis: A division of labour between the two C. elegans metallothioneins

The chemical properties of toxic cadmium and essential zinc are very similar, and organisms require intricate mechanisms that drive selective handling of metals. Previously regarded as unspecific "metal sponges", metallothioneins (MTLs) are emerging as metal selectivity filters. By utilizing C. elegans mtl-1 and mtl-2 knockout strains, metal accumulation in single worms, single copy fluorescent-tagged transgenes, isoform specific qPCR and lifespan studies it was possible to demonstrate that the handling of cadmium and zinc by the two C. elegans metallothioneins differs fundamentally: the MTL-2 protein can handle both zinc and cadmium, but when it becomes unavailable, either via a knockout or by elevated cadmium exposure, MTL-1 takes over zinc handling, leaving MTL-2 to sequester cadmium. This division of labour is reflected in the folding behaviour of the proteins: MTL-1 folded well in presence of zinc but not cadmium, the reverse was the case for MTL-2. These differences are in part mediated by a zinc-specific mononuclear His3Cys site in the C-terminal insertion of MTL-1; its removal affected the entire C-terminal domain and may shift its metal selectivity towards zinc. Overall, we uncover how metallothionein isoform-specific responses and protein properties allow C. elegans to differentiate between toxic cadmium and essential zinc.

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.

The connection of α- and β-domains in mammalian metallothionein-2 differentiates Zn(II) binding affinities, affects folding, and determines zinc buffering properties

Mammalian metallothioneins (MTs) are small Cys-rich proteins involved in Zn(II) and Cu(I) homeostasis. They bind seven Zn(II) ions in two distinct β- and α-domains, forming Zn3Cys9 and Zn4Cys11 clusters, respectively. After six decades of research, their role in cellular buffering of Zn(II) ions has begun to be understood recently. This is because of different affinities of bound ions and the proteins' coexistence in variously Zn(II)-loaded Zn4-7MT species in the cell. To date, it has remained unclear how these mechanisms of action occur and how the affinities are differentiated despite the Zn(S-Cys)4 coordination environment being the same. Here, we dissect the molecular basis of these phenomena by using several MT2 mutants, hybrid protein, and isolated domains. Through a combination of spectroscopic and stability studies, thiol(ate) reactivity, and steered molecular dynamics, we demonstrate that both protein folding and thermodynamics of Zn(II) ion (un)binding significantly differ between isolated domains and the whole protein. Close proximity reduces the degrees of freedom of separated domains, making them less dynamic. It is caused by the formation of intra- and interdomain electrostatic interactions. The energetic consequence of domains connection has a critical impact on the role of MTs in the cellular environment, where they function not only as a zinc sponge but also as a zinc buffering system keeping free Zn(II) in the right concentrations. Any change of that subtle system affects the folding mechanism, zinc site stabilities, and cellular zinc buffer components.

Arsenic binding to human metallothionein-3

Arsenic poisoning is of great concern with respect to its neurological toxicity, which is especially significant for young children. Human exposure to arsenic occurs worldwide from contaminated drinking water. In human physiology, one response to toxic metals is through coordination with the metallochaperone metallothionein (MT). Central nervous system expression of MT isoform 3 (MT3) is thought to be neuroprotective. We report for the first time on the metalation pathways of As3+ binding to apo-MT3 under physiological conditions, yielding the absolute binding constants (log Kn, n = 1-6) for each sequential As3+ binding event: 10.20, 10.02, 9.79, 9.48, 9.06, and 8.31 M-1. We report on the rate of the reaction of As3+ with apo-MT3 at pH 3.5 with rate constants (kn, n = 1-6) determined for each sequential As3+ binding event: 116.9, 101.2, 85.6, 64.0, 43.9, and 21.0 M-1 s-1. We further characterize the As3+ binding pathway to fully metalated Zn7MT3 and partially metalated Zn-MT3. As3+ binds rapidly with high binding constants under physiological conditions in a noncooperative manner, but is unable to replace the Zn2+ in fully-metalated Zn-MT3. As3+ binding to partially metalated Zn-MT3 takes place with a rearrangement of the Zn-binding profile. Our work shows that As 3+ rapidly and efficiently binds to both apo-MT3 and partially metalated Zn-MT3 at physiological pH.

Metallothionein-3 attenuates the effect of Cu2+ ions on actin filaments

Metallothionein 3 (MT-3) is a cysteine-rich metal-binding protein that is expressed in the mammalian central nervous system and kidney. Various reports have posited a role for MT-3 in regulating the actin cytoskeleton by promoting the assembly of actin filaments. We generated purified, recombinant mouse MT-3 of known metal compositions, either with zinc (Zn), lead (Pb), or copper/zinc (Cu/Zn) bound. None of these forms of MT-3 accelerated actin filament polymerization in vitro, either with or without the actin binding protein profilin. Furthermore, using a co-sedimentation assay, we did not observe Zn-bound MT-3 in complex with actin filaments. Cu²⁺ ions on their own induced rapid actin polymerization, an effect that we attribute to filament fragmentation. This effect of Cu²⁺ is reversed by adding either EGTA or Zn-bound MT-3, indicating that either molecule can chelate Cu²⁺ from actin. Altogether, our data indicate that purified recombinant MT-3 does not directly bind actin but it does attenuate the Cu-induced fragmentation of actin filaments.

Apo-metallothionein-3 cooperatively forms tightly compact structures under physiological conditions

Metallothioneins (MTs) are essential mammalian metal chaperones. MT isoform 1 (MT1) is expressed in the kidneys and isoform 3 (MT3) is expressed in nervous tissue. For MTs, the solution-based NMR structure was determined for metal-bound MT1 and MT2, and only one X-ray diffraction structure on a crystallized mixed metal-bound MT2 has been reported. The structure of solution-based metalated MT3 is partially known using NMR methods; however, little is known about the fluxional de novo apo-MT3 because the structure cannot be determined by traditional methods. Here, we used cysteine modification coupled with electrospray ionization mass spectrometry, denaturing reactions with guanidinium chloride, stopped-flow methods measuring cysteine modification and metalation, and ion mobility mass spectrometry to reveal that apo-MT3 adopts a compact structure under physiological conditions and an extended structure under denaturing conditions, with no intermediates. Compared with apo-MT1, we found that this compact apo-MT3 binds to a cysteine modifier more cooperatively at equilibrium and 0.5 times the rate, providing quantitative evidence that many of the 20 cysteines of apo-MT3 are less accessible than those of apo-MT1. In addition, this compact apo-MT3 can be identified as a distinct population using ion mobility mass spectrometry. Furthermore, proposed structural models can be calculated using molecular dynamics methods. Collectively, these findings provide support for MT3 acting as a noninducible regulator of the nervous system compared with MT1 as an inducible scavenger of trace metals and toxic metals in the kidneys.

Metal binding and interdomain thermodynamics of mammalian metallothionein-3: enthalpically favoured Cu+ supplants entropically favoured Zn2+ to form Cu4 + clusters under physiological conditions

Metallothioneins (MTs) are a ubiquitous class of small metal-binding proteins involved in metal homeostasis and detoxification. While known for their high affinity for d¹⁰ metal ions, there is a surprising dearth of thermodynamic data on metals binding to MTs. In this study, Zn²⁺ and Cu⁺ binding to mammalian metallothionein-3 (MT-3) were quantified at pH 7.4 by isothermal titration calorimetry (ITC). Zn²⁺ binding was measured by chelation titrations of Zn₇MT-3, while Cu⁺ binding was measured by Zn²⁺ displacement from Zn₇MT-3 with competition from glutathione (GSH). Titrations in multiple buffers enabled a detailed analysis that yielded condition-independent values for the association constant (K) and the change in enthalpy (ΔH) and entropy (ΔS) for these metal ions binding to MT-3. Zn²⁺ was also chelated from the individual α and β domains of MT-3 to quantify the thermodynamics of inter-domain interactions in metal binding. Comparative titrations of Zn₇MT-2 with Cu⁺ revealed that both MT isoforms have similar Cu⁺ affinities and binding thermodynamics, indicating that ΔH and ΔS are determined primarily by the conserved Cys residues. Inductively coupled plasma mass spectrometry (ICP-MS) analysis and low temperature luminescence measurements of Cu-replete samples showed that both proteins form two Cu₄ ⁺-thiolate clusters when Cu⁺ displaces Zn²⁺ under physiological conditions. Comparison of the Zn²⁺ and Cu⁺ binding thermodynamics reveal that enthalpically-favoured Cu⁺, which forms Cu₄ ⁺-thiolate clusters, displaces the entropically-favoured Zn²⁺. These results provide a detailed thermodynamic analysis of d¹⁰ metal binding to these thiolate-rich proteins and quantitative support for, as well as molecular insight into, the role that MT-3 plays in the neuronal chemistry of copper.

Regulation of zinc-dependent enzymes by metal carrier proteins

Zn²⁺ ions are essential in many physiological processes, including enzyme catalysis, protein structural stabilization, and the regulation of many proteins. The affinities of proteins for Zn²⁺ ions span several orders of magnitude, with catalytic Zn²⁺ ions generally held more tightly than structural or regulatory ones. Metal carrier proteins, most of which are not specific for Zn²⁺, bind these ions with a broad range of affinities that overlap those of catalytic, structural, and regulatory Zn²⁺ ions and are thought to be responsible for distributing the metal through most cells, tissues, and fluid compartments. While little is known about how many proteins obtain or release these ions, there is now considerable experimental evidence suggesting that metal carrier proteins may be responsible for transferring metals to and from some Zn²⁺-dependent proteins, thus serving as a major regulatory factor for them. In this review, the biological roles of Zn²⁺ and structures of Zn²⁺ binding sites are examined, and experimental evidence demonstrating the direct participation of metal carrier proteins in enzyme regulation is discussed. Mechanisms of metal ion transfer are also offered, and the potential physiological significance of this phenomenon is explored.

The Bioinorganic Chemistry of Mammalian Metallothioneins

The functions, purposes, and roles of metallothioneins have been the subject of speculations since the discovery of the protein over 60 years ago. This article guides through the history of investigations and resolves multiple contentions by providing new interpretations of the structure-stability-function relationship. It challenges the dogma that the biologically relevant structure of the mammalian proteins is only the one determined by X-ray diffraction and NMR spectroscopy. The terms metallothionein and thionein are ambiguous and insufficient to understand biological function. The proteins need to be seen in their biological context, which limits and defines the chemistry possible. They exist in multiple forms with different degrees of metalation and types of metal ions. The homoleptic thiolate coordination of mammalian metallothioneins is important for their molecular mechanism. It endows the proteins with redox activity and a specific pH dependence of their metal affinities. The proteins, therefore, also exist in different redox states of the sulfur donor ligands. Their coordination dynamics allows a vast conformational landscape for interactions with other proteins and ligands. Many fundamental signal transduction pathways regulate the expression of the dozen of human metallothionein genes. Recent advances in understanding the control of cellular zinc and copper homeostasis are the foundation for suggesting that mammalian metallothioneins provide a highly dynamic, regulated, and uniquely biological metal buffer to control the availability, fluctuations, and signaling transients of the most competitive Zn(II) and Cu(I) ions in cellular space and time.

Metallothionein-3 as a multifunctional player in the control of cellular processes and diseases

Transition metals, such as iron, copper, and zinc, play a very important role in life as the regulators of various physiochemical reactions in cells. Abnormal distribution and concentration of these metals in the body are closely associated with various diseases including ischemic seizure, Alzheimer's disease, diabetes, and cancer. Iron and copper are known to be mainly involved in in vivo redox reaction. Zinc controls a variety of intracellular metabolism via binding to lots of proteins in cells and altering their structure and function. Metallothionein-3 (MT3) is a representative zinc binding protein predominant in the brain. Although the role of MT3 in other organs still needs to be elucidated, many reports have suggested critical roles for the protein in the control of a variety of cellular homeostasis. Here, we review various biological functions of MT3, focusing on different cellular molecules and diseases involving MT3 in the body.

Metallation pathway of a plant metallothionein: Cicer arietinum MT2

The plant metallothionein 2 protein from Cicer arietinum (cicMT2) is a typical member of the plant MT subfamily p2 that is characterized by an N- and C-terminal cysteine- (Cys-)rich, metal binding sequence connected by a long cysteine-free linker region. cicMT2 coordinates up to five Zn^II or Cd^II ions by its 14 cysteine thiolate groups forming a single metal-thiolate cluster. While MTs from other phyla are considerably well-studied, many details about plant MTs are missing. In this study the metallation pathway of cicMT2 is investigated using mass spectrometry. To evaluate the influence of the linker region as well as the interplay of the two Cys-rich stretches, the full-length cicMT2 protein as well as the individual Cys-rich domains with and without the linker region were analysed. Up to three Cd^II ions can be coordinated by the eight Cys residues of the N-terminal part and up to two Cd^II ions by the six Cys residues of the C-terminal sequence. However, no preferential binding to either of the two sequences is observed, which is in-line with the closely similar apparent binding constants of the individual domains obtained from competition reactions with the chelator 1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacetic acid. The combination of limited proteolytic digestion, mass spectrometry, dynamic light scattering, size-exclusion chromatography, and ¹⁹F NMR spectroscopy enables us to draw conclusions about the overall protein-fold and the cluster formation process.

Impact of naturally occurring serine/cysteine variations on the structure and function of Pseudomonas metallothioneins

Naturally occurring Ser/Cys variations in Pseudomonas metallothioneinss affect intra-cluster dynamics rather than binding capacity.

Non-coordinative metal selectivity bias in human metallothioneins metal-thiolate clusters

Mammalian metallothioneins (MT-1 through MT-4) are a class of metal binding proteins containing two metal-thiolate clusters formed through the preferential coordination of d10 metals, Cu(i) and Zn(ii), by 20 conserved cysteine residues located in two protein domains. MT metalation (homometallic or heterometallic Zn(ii)/Cu(i) species) appears to be isoform specific and controlling zinc and copper concentrations to perform specific and distinct biological functions. Structural and functional relationships, and in vivo metalation studies, identified evolutionary features defining the metal-selectivity nature for MTs. Metallothionein-3 (MT-3) has been shown to possess the most pronounced Cu-thionein character forming Cu(i)-containing species more favorably than metallothionein-2 (MT-2), which possesses the strongest Zn-thionein character. In this work, we identify isoform-specific determinants which control metal binding selectivity bias in different MTs isoforms. By studying the reactivity of Zn7MT-2, Zn7MT-3 and Zn7MT-3 mutants towards Cu(ii) to form Cu(i)4Zn4MTs, we have identified isoform-specific key non-coordinating residues governing folding/outer sphere control of metal selectivity bias in MTs metal clusters. By mutating selected residues and motifs in MT-3 to the corresponding MT-2 amino acids, we dissected key roles in modulating cluster dynamic and metal exchange rates, in increasing the Cu(i)-affinity in MT-3 N-terminal β-domain and/or modulating the higher stability of the Zn(ii)-thiolate cluster in MT-2 β-domain. We thus engineered MT-3 variants in which the copper-thionein character is converted into a zinc-thionein. These results provide new insights into the molecular determinants governing metal selectivity in metal-thiolate clusters.

Metal-dependent interactions of metallothionein-3 β-domain with amyloid-β peptide and related physiological implications

Aberrant interactions of metal ions with amyloid-β peptide (Aβ) can potentiate Alzheimer's disease (AD) by participating in the aggregation process of Aβ and in the generation of reactive oxygen species (ROS). Metallothionein-3 (MT3), which is aberrantly expressed in AD brains, is believed to play an important role in the AD progression due to its ability of maintaining metal homeostasis and scavenging ROS. However, the related molecular mechanism is not clear. In this work, the metal-dependent interactions of MT3 β-domain (βMT3) with amyloid-β peptide (Aβ) were systematically studied. The results showed that Zn₃-βMT3 has a higher affinity to Aβ (K_d: ~0.7 μM) than Cu₄-βMT3 (K_d: ~22 μM). In Zn₃-βMT3, both Pro⁷ and Pro⁹ face outwards with their five-member rings in parallel, favoring their binding with aromatic residues via CH/π interactions. Two aromatic residues (Phe⁴ and Tyr¹⁰) in Aβ were identified as the specific binding sites for βMT3. Based on these, we posit a characteristic in-situ protection role of Zn-MT3 in inhibiting the Cu²⁺-induced Aβ neurotoxicity, in which stable Zn-MT3/Aβ complex forms via the Zn₃-βMT3/Aβ interaction and effectively prevents the formation of Cu-Aβ in high viscosity physiological fluids. Our results provide the mechanistic pathway and the specific roles of βMT3 in its protective bioactivity against AD progression, which means significant for elucidating the function of MT3 in AD neuropathology and for designing a MT3-related therapeutic strategy for AD.

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.

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

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

Thermodynamics of Pb(ii) and Zn(ii) binding to MT-3, a neurologically important metallothionein

Isothermal titration calorimetry (ITC) was used to quantify the thermodynamics of Pb(2+) and Zn(2+) binding to metallothionein-3 (MT-3). Pb(2+) binds to zinc-replete Zn7MT-3 displacing each zinc ion with a similar change in free energy (ΔG) and enthalpy (ΔH). EDTA chelation measurements of Zn7MT-3 and Pb7MT-3 reveal that both metal ions are extracted in a tri-phasic process, indicating that they bind to the protein in three populations with different binding thermodynamics. Metal binding is entropically favoured, with an enthalpic penalty that reflects the enthalpic cost of cysteine deprotonation accompanying thiolate ligation of the metal ions. These data indicate that Pb(2+) binding to both apo MT-3 and Zn7MT-3 is thermodynamically favourable, and implicate MT-3 in neuronal lead biochemistry.

The Functions of Metamorphic Metallothioneins in Zinc and Copper Metabolism

Recent discoveries in zinc biology provide a new platform for discussing the primary physiological functions of mammalian metallothioneins (MTs) and their exquisite zinc-dependent regulation. It is now understood that the control of cellular zinc homeostasis includes buffering of Zn²⁺ ions at picomolar concentrations, extensive subcellular re-distribution of Zn²⁺, the loading of exocytotic vesicles with zinc species, and the control of Zn²⁺ ion signalling. In parallel, characteristic features of human MTs became known: their graded affinities for Zn²⁺ and the redox activity of their thiolate coordination environments. Unlike the single species that structural models of mammalian MTs describe with a set of seven divalent or eight to twelve monovalent metal ions, MTs are metamorphic. In vivo, they exist as many species differing in redox state and load with different metal ions. The functions of mammalian MTs should no longer be considered elusive or enigmatic because it is now evident that the reactivity and coordination dynamics of MTs with Zn²⁺ and Cu⁺ match the biological requirements for controlling—binding and delivering—these cellular metal ions, thus completing a 60-year search for their functions. MT represents a unique biological principle for buffering the most competitive essential metal ions Zn²⁺ and Cu⁺. How this knowledge translates to the function of other families of MTs awaits further insights into the specifics of how their properties relate to zinc and copper metabolism in other organisms.

Function of Metallothionein-3 in Neuronal Cells: Do Metal Ions Alter Expression Levels of MT3?

A study of factors proposed to affect metallothionein-3 (MT3) function was carried out to elucidate the opaque role MT3 plays in human metalloneurochemistry. Gene expression of Mt2 and Mt3 was examined in tissues extracted from the dentate gyrus of mouse brains and in human neuronal cell cultures. The whole-genome gene expression analysis identified significant variations in the mRNA levels of genes associated with zinc homeostasis, including Mt2 and Mt3. Mt3 was found to be the most differentially expressed gene in the identified groups, pointing to the existence of a factor, not yet identified, that differentially controls Mt3 expression. To examine the expression of the human metallothioneins in neurons, mRNA levels of MT3 and MT2 were compared in BE(2)C and SH-SY5Y cell cultures treated with lead, zinc, cobalt, and lithium. MT2 was highly upregulated by Zn²⁺ in both cell cultures, while MT3 was not affected, and no other metal had an effect on either MT2 or MT3.

Apo-Metallothionein Emerging as a Major Player in the Cellular Activities of Metallothionein

Observations of apo-metallothlonein (apo-MT) have been made under a variety of physiologic circumstances, including zinc deficiency in cell culture and in rodents, cellular induction of MT by dexamethasone with concurrent Zn deficiency, a variety of tumors under normal Zn conditions, MT induction by Zn and Bi citrate, induction of hepatic MT after tumor cell Injection into nude mice, and overexpression of cardiac MT in MT transgenic mice. Experiments demonstrating both the lability of Zn and Cu bound to MT and the cellular stability of apo-MT are described to help rationalize the widespread presence of this metal-depleted species. Finally, comparative in vitro and cellular experiments examined the relative reactivity of Zn- and apo-MT with nitric oxide species, showing that apo-MT is much more reactive chemically and that in cells it may be a principal reactive species within the MT pool.

Defining the metal binding pathways of human metallothionein 1a: balancing zinc availability and cadmium seclusion

Metallothioneins (MTs) are cysteine-rich, metal-binding proteins that are found throughout Nature. This ubiquity highlights their importance in essential metal regulation, heavy metal detoxification and cellular redox chemistry. Missing from the current description of MT function is the underlying mechanism by which MTs achieve their proposed biological functions. To date, there have been conflicting reports on the mechanism of metal binding and the structures of the metal binding intermediates formed during metalation of apoMTs. The form of the metal-bound intermediates dictates the metal sequestering and metal-donating properties of the protein. Through a detailed analysis of spectral data from electrospray ionization mass spectromeric and circular dichroism methods we report that Zn(ii) and Cd(ii) metalation of the human MT1a takes place through two distinct pathways. The first pathway involves formation of beaded structures with up to five metals bound terminally to the 20 cysteines of the protein via a noncooperative mechanism. The second pathway is dominated by the formation of the four-metal domain cluster structure M4SCYS11via a cooperative mechanism. We report that there are different pathway preferences for Zn(ii) and Cd(ii) metalation of apo-hMT1a. Cd(ii) binding follows the beaded pathway above pH 7.1 but beginning below pH 7.1 the clustered (Cd4Scys11) pathway begins to dominate. In contrast, Zn(ii) binding follows the terminal, "beaded", pathway at all physiologically relevant pH (pH ≥ 5.2) only following the clustered pathway below pH 5.1. The results presented here allow us to reconcile the conflicting reports concerning the presence of different metalation intermediates of MTs. The conflict regarding cooperative versus noncooperative binding mechanisms is also reconciled with the experimental results described here. These two metal-specific pathways and the presence of radically different intermediate structures provide insight into the multi-functional nature of MT: binding Zn(ii) terminally for donation to metalloenzymes and sequestering toxic Cd(ii) in a cluster structure.

Earthworm Lumbricus rubellus MT-2: Metal Binding and Protein Folding of a True Cadmium-MT

Earthworms express, as most animals, metallothioneins (MTs)—small, cysteine-rich proteins that bind d¹⁰ metal ions (Zn(II), Cd(II), or Cu(I)) in clusters. Three MT homologues are known for Lumbricus rubellus, the common red earthworm, one of which, wMT-2, is strongly induced by exposure of worms to cadmium. This study concerns composition, metal binding affinity and metal-dependent protein folding of wMT-2 expressed recombinantly and purified in the presence of Cd(II) and Zn(II). Crucially, whilst a single Cd₇wMT-2 species was isolated from wMT-2-expressing E. coli cultures supplemented with Cd(II), expressions in the presence of Zn(II) yielded mixtures. The average affinities of wMT-2 determined for either Cd(II) or Zn(II) are both within normal ranges for MTs; hence, differential behaviour cannot be explained on the basis of overall affinity. Therefore, the protein folding properties of Cd- and Zn-wMT-2 were compared by ¹H NMR spectroscopy. This comparison revealed that the protein fold is better defined in the presence of cadmium than in the presence of zinc. These differences in folding and dynamics may be at the root of the differential behaviour of the cadmium- and zinc-bound protein in vitro, and may ultimately also help in distinguishing zinc and cadmium in the earthworm in vivo.

Putting the pieces into place: Properties of intact zinc metallothionein 1A determined from interaction of its isolated domains with carbonic anhydrase

Competitive metallation reactions between the isolated domain fragments and apo-carbonic anhydrase [CA; metal-free CA (apo-CA)] provided the binding affinities for each of the eight sites and showed that CA competed more efficiently for added zinc with the β-domain fragment. The combined effects of the number of sites, chain length and cysteine accessibility modulate the zinc-binding properties of mammalian metallothionein (MT).

Relationship between the architecture of zinc coordination and zinc binding affinity in proteins – insights into zinc regulation

Relationship between the architecture and stability of zinc proteins.

Zinc insufficiency mediates ethanol-induced alveolar macrophage dysfunction in the pregnant female mouse

AIMS

(a) Establish the minimum number of weeks of chronic ethanol ingestion needed to perturb zinc homeostasis, (b) Examine intracellular zinc status in the alveolar macrophages (AMs) when ethanol ingestion is combined with pregnancy, (c) Investigate whether in vitro zinc treatment reverses the effects of ethanol ingestion on the AM.

METHODS

C57BL/6 female mice were fed a liquid diet (±25% ethanol-derived calories) during preconception and pregnancy. The control group was pair-fed to the ethanol group. In the isolated AMs, we measured intracellular AM zinc levels, zinc transporter expression, alternative activation and phagocytic index. Zinc acetate was added to some cells prior to analysis.

RESULTS

Intracellular zinc levels in the AM decreased within 3 weeks of ethanol ingestion. After ethanol ingestion prior to and during pregnancy, zinc transporter expression and intracellular zinc levels were decreased in the AMs when compared with controls. Bacterial clearance was decreased because the AMs were alternatively activated. In vitro additions of zinc reversed these effects of ethanol.

CONCLUSION

Ethanol ingestion prior to and during pregnancy perturbed AM zinc balance resulting in impaired bacterial clearance, but these effects were ameliorated by in vitro zinc treatments.

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

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

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.

Use of (113)Cd NMR to probe the native metal binding sites in metalloproteins: an overview

Our laboratories have actively published in this area for several years and the objective of this chapter is to present as comprehensive an overview as possible. Following a brief review of the basic principles associated with (113)Cd NMR methods, we will present the results from a thorough literature search for (113)Cd chemical shifts from metalloproteins. The updated (113)Cd chemical shift figure in this chapter will further illustrate the excellent correlation of the (113)Cd chemical shift with the nature of the coordinating ligands (N, O, S) and coordination number/geometry, reaffirming how this method can be used not only to identify the nature of the protein ligands in uncharacterized cases but also the dynamics at the metal binding site. Specific examples will be drawn from studies on alkaline phosphatase, Ca(2+) binding proteins, and metallothioneins.In the case of Escherichia coli alkaline phosphatase, a dimeric zinc metalloenzyme where a total of six metal ions (three per monomer) are involved directly or indirectly in providing the enzyme with maximal catalytic activity and structural stability, (113)Cd NMR, in conjunction with (13)C and (31)P NMR methods, were instrumental in separating out the function of each class of metal binding sites. Perhaps most importantly, these studies revealed the chemical basis for negative cooperativity that had been reported for this enzyme under metal deficient conditions. Also noteworthy was the fact that these NMR studies preceded the availability of the X-ray crystal structure.In the case of the calcium binding proteins, we will focus on two proteins: calbindin D(9k) and calmodulin. For calbindin D(9k) and its mutants, (113)Cd NMR has been useful both to follow actual changes in the metal binding sites and the cooperativity in the metal binding. Ligand binding to calmodulin has been studied extensively with (113)Cd NMR showing that the metal binding sites are not directly involved in the ligand binding. The (113)Cd chemical shifts are, however, exquisitely sensitive to minute changes in the metal ion environment.In the case of metallothionein, we will reflect upon how (113)Cd substitution and the establishment of specific Cd to Cys residue connectivity by proton-detected heteronuclear (1)H-(113)Cd multiple-quantum coherence methods (HMQC) was essential for the initial establishment of the 3D structure of metallothioneins, a protein family deficient in the regular secondary structural elements of α-helix and β-sheet and the first native protein identified with bound Cd. The (113)Cd NMR studies also enabled the characterization of the affinity of the individual sites for (113)Cd and, in competition experiments, for other divalent metal ions: Zn, Cu, and Hg.

Characterization of the role of metallothionein-3 in an animal model of Alzheimer's disease

Among the dementias, Alzheimer's disease (AD) is the most commonly diagnosed, but there are still no effective drugs available for its treatment. It has been suggested that metallothionein-3 (MT-3) could be somehow involved in the etiology of AD, and in fact very promising results have been found in in vitro studies, but the role of MT-3 in vivo needs further analysis. In this study, we analyzed the role of MT-3 in a mouse model of AD, Tg2576 mice, which overexpress human Amyloid Precursor Protein (hAPP) with the Swedish mutation. MT-3 deficiency partially rescued the APP-induced mortality of females, and mildly affected APP-induced changes in behavior assessed in the hole-board and plus-maze tests in a gender-dependent manner. Amyloid plaque burden and/or hAPP expression were decreased in the cortex and hippocampus of MT-3-deficient females. Interestingly, exogenously administered Zn(7)MT-3 increased soluble Aβ40 and Aβ42 and amyloid plaques and gliosis, particularly in the cortex, and changed several behavioral traits (increased deambulation and exploration and decreased anxiety). These results highlight that the control of the endogenous production and/or action of MT-3 could represent a powerful therapeutic target in AD.

Type 4 metallothionein genes are involved in regulating Zn ion accumulation in late embryo and in controlling early seedling growth in Arabidopsis

Type 4 metallothionein (MT) genes are recognized for their specific expression in higher plant seeds, but their functions are still unclear. In this study, the functions of two Arabidopsis metallothionein genes, AtMT4a and AtMT4b, are investigated in seed development, germination and early seedling growth. Transcriptional analysis showed that these two genes are specifically expressed in late embryos. Subcellular localization displayed that both AtMT4a and AtMT4b are widespread distributed in cytoplasm, nucleus and membrane. Co-silencing RNAi of AtMT4a and AtMT4b reduced seed weight and influenced the early seedling growth after germination, whereas overexpression of these two genes caused the opposite results. Detailed analysis showed clearly the correlation of AtMT4a and AtMT4b to the accumulation of some important metal ions in late embryos, especially to Zn ion storing in seeds, which then serves as part of early Zn ion resources for post-germinated seedling growth. Furthermore, phytohormone abscisic acid (ABA) and gibberellic acid (GA) may play roles in regulating the expression and function of AtMT4a and AtMT4b during seed development; and this may influence Zn accumulation in seeds and Zn ion nutrient supplementation in the early seedling growth after germination.

Isolation of metallothionein genes and in silico structural characterization of their proteins using molecular modeling from yak (Bos grunniens)

Yak metallothioneins (BgMTs) are cysteine-rich metal-chelating proteins with highly conserved cysteine residues in their amino acid sequences. The 3D structures of the Cd(7)-BgMTs reconstructed by molecular modeling included two domains: the β-domain with M(3)(S(cys))(9) metal-thiolate clusters and the α-domain with M(4)(S(cys))(11) metal-thiolate clusters. An unusual variant was found at position 30 (Cys30→Ser30) in BgMT-III, which is usually conserved in the mammalian MT-I/-II (Cys29) and MT-III (Cys30). The variant residue of BgMT-III may play a key role in yak genetic evolution, metal-binding activity, dynamic conformation, and heavy metal metabolism. BgMT-III contained a Thr insertion at position 5 (T(5)), which may loosen the structure of the β-domain of BgMT-III, and a conserved C(6)PCP(9) motif, which may provide an interacting surface for protein-protein interactions. There is also an acidic hexapeptide insertion (E(55)GAEAE(60)) that could regulate the particular interdomain interactions and lead to the conformational change in the β-domain.

Mammalian metallothionein in toxicology, cancer, and cancer chemotherapy

The present paper centers on mammalian metallothionein 1 and 2 in relationship to cell and tissue injury beginning with its reaction with Cd²⁺ and then considering its role in the toxicology and chemotherapy of both metals and non-metal electrophiles and oxidants. Intertwined is a consideration of MTs role in tumor cell Zn²⁺ metabolism. The paper updates and expands on our recent review by Petering et al. (Met Ions Life Sci 5:353-398, 2009).

Chemistry and biology of mammalian metallothioneins

Metallothioneins (MTs) are a class of ubiquitously occurring low molecular mass, cysteine- and metal-rich proteins containing sulfur-based metal clusters formed with Zn(II), Cd(II), and Cu(I) ions. In mammals, four distinct MT isoforms designated MT-1 through MT-4 exist. The first discovered MT-1/MT-2 are widely expressed isoforms, whose biosynthesis is inducible by a wide range of stimuli, including metals, drugs, and inflammatory mediators. In contrast, MT-3 and MT-4 are noninducible proteins, with their expression primarily confined to the central nervous system and certain squamous epithelia, respectively. MT-1 through MT-3 have been reported to be secreted, suggesting that they may play different biological roles in the intracellular and extracellular space. Recent reports established that these isoforms play an important protective role in brain injury and metal-linked neurodegenerative diseases. In the postgenomic era, it is becoming increasingly clear that MTs fulfill multiple functions, including the involvement in zinc and copper homeostasis, protection against heavy metal toxicity, and oxidative damage. All mammalian MTs are monomeric proteins, containing two metal-thiolate clusters. In this review, after a brief summary of the historical milestones of the MT-1/MT-2 research, the recent advances in the structure, chemistry, and biological function of MT-3 and MT-4 are discussed.

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.

Neuronal growth-inhibitory factor (metallothionein-3): evaluation of the biological function of growth-inhibitory factor in the injured and neurodegenerative brain

Neuronal growth-inhibitory factor, later renamed metallothionein-3, is one of four members of the mammalian metallothionein family. Metallothioneins are a family of ubiquitous, low-molecular-weight, cysteine-rich proteins. Although neuronal growth-inhibitory factor shares metal-binding and reactive oxygen species scavenging properties with the other metallothioneins, it displays several distinct biological properties. In this review, we examine the recent developments regarding the function of neuronal growth-inhibitory factor within the brain, particularly in response to brain injury or during neurodegenerative disease progression.

The Delta33-35 Mutant alpha-Domain Containing beta-Domain-Like M(3)S(9) Cluster Exhibits the Function of alpha-Domain with M(4)S(11) Cluster in Human Growth Inhibitory Factor

Neuronal growth inhibitory factor (GIF), also known as metallothionein (metallothionein-3), impairs the survival and neurite formation of cultured neurons. It is known that the α-β domain-domain interaction of hGIF is crucial to the neuron growth inhibitory bioactivity although the exact mechanism is not clear. Herein, the β(MT3)-β(MT3) mutant and the hGIF-truncated Δ33-35 mutant were constructed, and their biochemical properties were characterized by pH titration, EDTA, and DTNB reactions. Their inhibitory activity toward neuron survival and neurite extension was also examined. We found that the Δ33-35 mutant α-domain containing β-domain-like M₃S₉ cluster exhibits the function of α-domain with M₄S₁₁ cluster in hGIF. These results showed that the stability and solvent accessibility of the metal-thiolate cluster in β-domain is very significant to the neuronal growth inhibitory activity of hGIF and also indicated that the particular primary structure of α-domain is pivotal to domain-domain interaction in hGIF.

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.

Reaction of the zinc sensor FluoZin-3 with Zn(7)-metallothionein: Inquiry into the existence of a proposed weak binding site

It has been reported that Zn(7)-metallothionein (MT), contains one weak binding site for Zn(2+). To test this conclusion, rabbit liver MT isolated at pH 7 was reacted with chelating agents of modest affinity for Zn(2+). Contrary to the previous study, no evidence was found for Zn(2+) stoichiometrically bound to the protein with an apparent stability constant of about 10(8). Indeed, stability constant measurements based upon competition between Zn(7)-MT and ligands of known stability with Zn(2+) showed that all of the protein bound Zn(2+) displayed the same stability constant at pH 7.4 and 25 degrees C of (1.7+/-0.6)x10(11). Brief reaction of Zn(7)-MT with strong acid converted it into MT(*) and upon reneutralization into Zn(7)-MT(*), which demonstrated reactivity of about 1 Zn(2+)/mol MT with competing ligands. Acid titration of Zn(7)-MT to pH 2 or below rapidly resulted in the formation of Zn(7)-MT(*) that displayed biphasic titration with base, revealing the rebinding of lower affinity Zn(2+) between pH 5 and 7. Since MT is commonly acidified during preparation, care must be taken to document which form of the protein is present in subsequent experiments at pH 7.