The sea urchin metallothionein system: Comparative evaluation of the SpMTA and SpMTB metal-binding preferences

Biotransformation of Pb and As from sewage sludge and food waste by black soldier fly larvae: Migration mechanism of bacterial community and metalloregulatory protein scales

The accumulation and transformation of lead (Pb) and arsenic (As) during the digestion of sewage sludge (SS) by black soldier fly larvae (BSFL) remain unclear. In this study, we used 16 s rRNA and metagenomic sequencing techniques to investigate the correlation between the microbial community, metalloregulatory proteins (MRPs), and Pb and As migration and transformation. During the 15-day test period, BSFL were able to absorb 34-48 % of Pb and 32-45 % of As into their body. Changes in bacterial community abundance, upregulation of MRPs, and redundancy analysis (RDA) results confirmed that ZntA, EfeO, CadC, ArsR, ArsB, ArsD, and ArsA play major roles in the adsorption and stabilization of Pb and As, which is mainly due to the high contribution rates of Lactobacillus (48-59 %) and Enterococcus (21-23 %). Owing to the redox reaction, the regulation of the MRPs, and the change in pH, the Pb and As in the BSFL residue were mainly the residual fraction (F4). The RDA results showed that Lactobacillus and L.koreensis could significantly (P < 0.01) reduce the reducible fraction (F2) and F4 of Pb, whereas Firmicutes and L.fermentum can significantly (P < 0.05) promote the transformation of As to F4, thus realizing the passivation Pb and As. This study contributes to the understanding of Pb and As in SS adsorbed by BSFL and provides important insights into the factors that arise during the BSFL-mediated migration of Pb and As.

Pb exposure causes non-linear accumulation of Pb in D. melanogaster controlled by metallothionein B and exerts ecological effects

Pb pollution can harm human health and the ecosystem. Therefore, it is worthwhile to study the metabolic processes of heavy metals in individual bodies and their influence on ecological systems. In this work, we analyzed the genetic responses and physiological changes of D. melanogaster which took diets exposed to different doses of Pb using transcriptomic analysis, ICP-MS, and various other physiological methods. We found that the Pb accumulated in D. melanogaster in a nonlinear pattern with the increase of Pb content in food. Metallothioneins (Mtns), especially the MtnB directly affects the accumulation and excretion of metal Pb in D. melanogaster, and causes the nonlinear accumulation. Metal regulatory transcription factor-1 (MTF-1) is involved in the regulation of Pb-induced high expressions of Mtns. Furthermore, an interaction between the metal metabolism pathway and xenobiotic response pathway leads to the cross-tolerances of Pb-exposed D. melanogaster to insecticides and other toxins. The oxidative stress induced by Pb toxicity may be the bridge between them. Our findings provide a physiological and molecular genetic basis for further study of the accumulation and metabolism of Pb in D. melanogaster.

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

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

Tunicates Illuminate the Enigmatic Evolution of Chordate Metallothioneins by Gene Gains and Losses, Independent Modular Expansions, and Functional Convergences

To investigate novel patterns and processes of protein evolution, we have focused in the metallothioneins (MTs), a singular group of metal-binding, cysteine-rich proteins that, due to their high degree of sequence diversity, still represents a "black hole" in Evolutionary Biology. We have identified and analyzed more than 160 new MTs in nonvertebrate chordates (especially in 37 species of ascidians, 4 thaliaceans, and 3 appendicularians) showing that prototypic tunicate MTs are mono-modular proteins with a pervasive preference for cadmium ions, whereas vertebrate and cephalochordate MTs are bimodular proteins with diverse metal preferences. These structural and functional differences imply a complex evolutionary history of chordate MTs-including de novo emergence of genes and domains, processes of convergent evolution, events of gene gains and losses, and recurrent amplifications of functional domains-that would stand for an unprecedented case in the field of protein evolution.

Modular Evolution and Population Variability of Oikopleura dioica Metallothioneins

Chordate Oikopleura dioica probably is the fastest evolving metazoan reported so far, and thereby, a suitable system in which to explore the limits of evolutionary processes. For this reason, and in order to gain new insights on the evolution of protein modularity, we have investigated the organization, function and evolution of multi-modular metallothionein (MT) proteins in O. dioica. MTs are a heterogeneous group of modular proteins defined by their cysteine (C)-rich domains, which confer the capacity of coordinating different transition metal ions. O. dioica has two MTs, a bi-modular OdiMT1 consisting of two domains (t-12C and 12C), and a multi-modular OdiMT2 with six t-12C/12C repeats. By means of mass spectrometry and spectroscopy of metal-protein complexes, we have shown that the 12C domain is able to autonomously bind four divalent metal ions, although the t-12C/12C pair –as it is found in OdiMT1– is the optimized unit for divalent metal binding. We have also shown a direct relationship between the number of the t-12C/12C repeats and the metal-binding capacity of the MTs, which means a stepwise mode of functional and structural evolution for OdiMT2. Finally, after analyzing four different O. dioica populations worldwide distributed, we have detected several OdiMT2 variants with changes in their number of t-12C/12C domain repeats. This finding reveals that the number of repeats fluctuates between current O. dioica populations, which provides a new perspective on the evolution of domain repeat proteins.

Metal-metal interaction and metal toxicity: a comparison between mammalian and D. melanogaster

1. Non-essential heavy metals such as mercury (Hg), arsenic (As), cadmium (Cd), and aluminium (Al) are useless to organisms and have shown extensive toxic effects. Previous studies show that two main molecular mechanisms of metal toxicity are oxidative stress and metal-metal interaction which can disrupt metal homeostasis.2. In this paper, we mainly illustrate metal toxicity and metal-metal interaction through examples in mammalians and D. melanogaster (fruit fly).3. We describe the interference of metal homeostasis by metal-metal interactions in three aspects including replacement, cellular transporter competition, and disruption of the regulation mechanism, and elaborate the mechanisms of metal toxicity to better deal with the challenges of heavy metal pollution and related health problems.

Interplay between Carbonic Anhydrases and Metallothioneins: Structural Control of Metalation

Carbonic anhydrases (CAs) and metallothioneins (MTs) are both families of zinc metalloproteins central to life, however, they coordinate and interact with their Zn2+ ion cofactors in completely different ways. CAs and MTs are highly sensitive to the cellular environment and play key roles in maintaining cellular homeostasis. In addition, CAs and MTs have multiple isoforms with differentiated regulation. This review discusses current literature regarding these two families of metalloproteins in carcinogenesis, with a dialogue on the association of these two ubiquitous proteins in vitro in the context of metalation. Metalation of CA by Zn-MT and Cd-MT is described. Evidence for protein-protein interactions is introduced from changes in metalation profiles of MT from electrospray ionization mass spectrometry and the metalation rate from stopped-flow kinetics. The implications on cellular control of pH and metal donation is also discussed in the context of diseased states.

Differential reactivity of closely related zinc(II)-binding metallothioneins from the plant Arabidopsis thaliana

The dynamics of metal binding to and transfer from metalloproteins involved in metal homeostasis are important for understanding cellular distribution of metal ions. The dicotyledonous plant Arabidopsis thaliana has two type 4 seed-specific metallothionein homologues, MT4a and MT4b, with likely roles in zinc(II) homeostasis. These two metallothioneins are 84% identical, with full conservation of all metal-binding cysteine and histidine residues. Yet, differences in their spatial and temporal expression patterns suggested divergence in their biological roles. To investigate whether biological functions are reflected in molecular properties, we compare aspects of zinc(II)-binding dynamics of full-length MT4a and MT4b, namely the pH dependence of zinc(II) binding and protein folding, and zinc(II) transfer to the chelator EDTA. UV-Vis and NMR spectroscopies as well as native electrospray ionisation mass spectrometry consistently showed that transfer from Zn6MT4a is considerably faster than from Zn6MT4b, with pseudo-first-order rate constants for the fastest observed step of k obs = 2.8 × 10-4 s-1 (MT4b) and k obs = 7.5 × 10-4 s-1 (MT4a) (5 µM protein, 500 µM EDTA, 25 mM Tris buffer, pH 7.33, 298 K). 2D heteronuclear NMR experiments allowed locating the most labile zinc(II) ions in domain II for both proteins. 3D homology models suggest that reactivity of this domain is governed by the local environment around the mononuclear Cys2His2 site that is unique to type 4 MTs. Non-conservative amino acid substitutions in this region affect local electrostatics as well as whole-domain dynamics, with both effects rendering zinc(II) ions bound to MT4a more reactive in metal transfer reactions. Therefore, domain II of MT4a is well suited to rapidly release its bound zinc(II) ions, in broad agreement with a previously suggested role of MT4a in zinc(II) transport and delivery to other proteins.

The zinc form of carnosine dipeptidase 2 (CN2) has dipeptidase activity but its substrate specificity is different from that of the manganese form

Carnosine dipeptidase II (CN2), a metallopeptidase present in the cytosol of various vertebrate tissues, catalyzes the hydrolysis of carnosine and several other dipeptides in the presence of Mn²⁺. Although the metal-binding center of mouse CN2 is also able to associate with Zn²⁺in vitro, it was not known whether the zinc form of CN2 has any enzymatic activity. In the present study, we show that Zn²⁺ has a higher affinity for binding to CN2 than Mn²⁺, as evidenced by native mass spectrometry. The issue of whether the zinc form of CN2 has enzymatic activity was also examined using various dipeptides as substrates. The findings indicate that the zinc form of CN2 catalyzes the hydrolysis of several different dipeptides including Leu-His, Met-His and Ala-His at a reaction rate comparable to that for its manganese form. On the other hand, the zinc form of CN2 did not catalyze the hydrolysis of carnosine and several other dipeptides that are hydrolyzed by the manganese form of CN2. Substrate specificity was also examined in HEK293T cells expressing CN2, and the findings indicate that Leu-His, Met-His, but not carnosine, were hydrolyzed in the cell culture. These results suggest that the zinc form of CN2 is an active enzyme, but with a different substrate specificity from that of the manganese form.

Cloning and characterization of metallothionein gene (HcMT) from Halostachys caspica and its expression in E. coli

Halostachys caspica is a short shrub distributed in the semi-arid and saline-alkali area, which evolved various mechanisms for modulating salt and metal level. In the present study, a Type 2 metallothionein (HcMT) gene was cloned from the salt induced suppression subtractive hybridization (SSH) cDNA library of H.caspica. Quantitative real time PCR (qRT-PCR) analysis indicated that HcMT gene was up-regulated under the stress of Cu(2+), Zn(2+) and Cd(2+), and the tolerance of E. coli strain harboring with the recombinant HcMT (pET-32a-HcMT) to Cu(2+), Zn(2+) and Cd(2+) was enhanced compared to strain with control vector (pET-32a). Moreover, the purified TrxA-HcMT fusion protein from E. coli cells grown in the presence of 0.3mM CuSO4, 0.3mM ZnSO4, or 0.1mM CdCl2 could bind more metal ions than TrxA alone. The predicted 3D structure showed that HcMT could form a single metal-thiolate cluster, which confers the ability to bind five divalent metal ions through fourteen cysteine residues. These data indicate that HcMT may be involved in processes of metal tolerance in H. caspica and could be employed as a potential candidate for heavy metal phytoremediation.

Cadmium Protection Strategies--A Hidden Trade-Off?

Cadmium (Cd) is a non-essential transition metal which is introduced into the biosphere by various anthropogenic activities. Environmental pollution with Cd poses a major health risk and Cd toxicity has been extensively researched over the past decades. This review aims at changing the perspective by discussing protection mechanisms available to counteract a Cd insult. Antioxidants, induction of antioxidant enzymes, and complexation of Cd to glutathione (GSH) and metallothionein (MT) are the most potent protective measures to cope with Cd-induced oxidative stress. Furthermore, protection mechanisms include prevention of endoplasmic reticulum (ER) stress, mitophagy and metabolic stress, as well as expression of chaperones. Pre-exposure to Cd itself, or co-exposure to other metals or trace elements can improve viability under Cd exposure and cells have means to reduce Cd uptake and improve Cd removal. Finally, environmental factors have negative or positive effects on Cd toxicity. Most protection mechanisms aim at preventing cellular damage. However, this might not be possible without trade-offs like an increased risk of carcinogenesis.

Understanding the 7-Cys module amplification of C. neoformans metallothioneins: how high capacity Cu-binding polypeptides are built to neutralize host nutritional immunity

Cryptococcus neoformans metallothioneins (MTs), CnMT1 and CnMT2, have been identified as essential infectivity and virulence factors of this pathogen. Both MTs are unusually long Cu-thioneins, exhibiting protein architecture and metal-binding abilities compatible with the hypothesis of resulting from three and five tandem repetitions of 7-Cys motives, respectively, each of them folding into Cu5-clusters. Through the study of the Zn(II)- and Cu(I)-binding capabilities of several CnMT1 truncated mutants, we show that a 7-Cys segment of CnMT1 folds into Cu5-species, of additive capacity when joined in tandem. All the obtained Cu-complexes share practically similar architectural features, if judging by their almost equivalent CD fingerprints, and they also share their capacity to restore copper tolerance in MT-devoid yeast cells. Besides the analysis of the modular composition of these long fungal MTs, we evaluate the features of the Cys-rich stretch spacer and flanking sequences that allow the construction of stable metal clusters by adjacent union of binding modules. Overall, our data support a mechanism by which some microbial MTs may have evolved to enlarge their original metal co-ordination capacity under the specific selective pressure of counteracting the Cu-based immunity mechanisms evolved by the infected hosts.

Metallothioneins, unconventional proteins from unconventional animals: a long journey from nematodes to mammals

Metallothioneins (MTs) are ubiquitous low molecular weight cysteine-rich proteins characterized by high affinity for d10 electron configuration metals, including essential (Zn and Cu) and non-essential (Cd and Hg) trace elements. The biological role of these ancient and well-conserved multifunctional proteins has been debated since MTs were first discovered in 1957. Their main hypothesized functions are: (1) homeostasis of Zn and Cu; (2) detoxification of Cd, and Hg; and (3) free radical scavenging. This review will focus on MTs in unconventional animals, those not traditionally studied in veterinary medicine but of increasing interest in this field of research. Living in different environments, these animals represent an incredible source of physiological and biochemical adaptations still partly unexplored. The study of metal-MT interactions is of great interest for clinicians and researchers working in veterinary medicine, food quality and endangered species conservation.

Metallothionein-like peptides involved in sequestration of Zn in the Zn-accumulating ectomycorrhizal fungus Russula atropurpurea

The first evidence of the existence of gene-encoded Zn-binding peptides that sequester a substantial portion of intracellular Zn in ectomycorrhizal fungi under natural conditions.

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

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

BIOLOGICAL SIGNIFICANCE

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