Metallothionein
Metallothionein is a family of cysteine-rich, low molecular weight proteins. They are localized to the membrane of the Golgi apparatus. MTs have the capacity to bind both physiological and xenobiotic heavy metals through the thiol group of its cysteine residues, which represent nearly 30% of its constituent amino acid residues.
MT was discovered in 1957 by Vallee and Margoshe from purification of a cadmium-binding protein from horse renal cortex. MT plays a role in the protection against metal toxicity and oxidative stress, and is involved in zinc and copper regulation. There are four main isoforms expressed in humans : MT1, MT2, MT3, and MT4. In the human body, large quantities are synthesised primarily in the liver and kidneys. Their production is dependent on availability of the dietary minerals such as zinc, copper, and selenium, as well as the amino acids histidine and cysteine.
Metallothioneins are rich in thiols, causing them to bind a number of trace metals. Metallothionein is one of the few eukaryotic proteins playing a substantial role in metal detoxification. Zinc and cadmium are tetrahedrally coordinated to cysteine residues, and each metallothionein protein molecule may bind up to 7 atoms of Zn or Cd. The biosynthesis of metallothionein appears to increase several-fold during periods of oxidative stress to shield the cells against cytotoxicity and DNA damage. Metallothionein biosynthesis can also be induced by certain hormones, pharmaceuticals, alcohols, and other compounds. Metallothionein expression is upregulated during fetal development, particularly in liver tissue.
Structure and classification
Metallothioneins are present in a vast range of organisms, ranging from prokaryotes, protozoa, plants, yeast, invertebrates and vertebrates.The MTs from this diverse taxonomic range represent a high-heterogeneity sequence and do not show general homology; in spite of this, homology is found inside some taxonomic groups.
From their primary structure, MTs have been classified by different methods. The first one dates from 1987, when Fowler et al., established three classes of MTs: Class I, including the MTs which show homology with horse MT, Class II, including the rest of the MTs with no homology with horse MT, and Class III, which includes phytochelatins, Cys-rich enzymatically synthesised peptides.
The second classification was performed by Binz and Kagi in 2001, and takes into account taxonomic parameters and the patterns of distribution of Cys residues along the MT sequence.
It results in a classification of 15 families for proteinaceous MTs. Family 15 contains the plant MTs, which in 2002 have been further classified by Cobbet and Goldsbrough into 4 Types depending on the distribution of their Cys residues and a Cys-devoid regions characteristic of plant MTs.
A table including the principal aspects of the two latter classifications is included.
| Family | Name | Sequence pattern | Example |
| 1 | Vertebrate | K-x-C-C-x-C-C-P-x-C | Mus musculus MT1 MDPNCSCTTGGSCACAGSCKCKECKCTSCKKCCSCCPVGCAKCAQGCVCKGSSEKCRCCA |
| 2 | Molluscan | C-x-C-x-C-T-G-x-C-x-C-x-C-x-C-K | Mytilus edulis 10MTIV MPAPCNCIETNVCICDTGCSGEGCRCGDACKCSGADCKCSGCKVVCKCSGSCACEGGCTGPSTCKCAPGCSCK |
| 3 | Crustacean | P--P-C-C-x-C-x-C | Homarus americanus MTH MPGPCCKDKCECAEGGCKTGCKCTSCRCAPCEKCTSGCKCPSKDECAKTCSKPCKCCP |
| 4 | Echinoderms | P-D-x-K-C-V-C-C-x-C-x-C-x-C-C-x-C-C-x-C-C | Strongylocentrotus purpuratus SpMTA MPDVKCVCCKEGKECACFGQDCCKTGECCKDGTCCGICTNAACKCANGCKCGSGCSCTEGNCAC |
| 5 | Diptera | C-G-x-C-x-C-x-Q-x-C-x-C-xD-C-x-C | Drosophila melanogaster MTNB MVCKGCGTNCQCSAQKCGDNCACNKDCQCVCKNGPKDQCCSNK |
| 6 | Nematoda | K-C-C-x-C-C | Caenorhabditis elegans MT1 MACKCDCKNKQCKCGDKCECSGDKCCEKYCCEEASEKKCCPAGCKGDCKCANCHCAEQKQCGDKTHQHQGTAAAH |
| 7 | Ciliate | x-C-C-C-x ? | Tetrahymena thermophila MTT1 MDKVNSCCCGVNAKPCCTDPNSGCCCVSKTDNCCKSDTKECCTGTGEGCKCVNCKCCKPQANCCCGVNAKPCCFDPNSGCCCVSKTNNCCKSD TKECCTGTGEGCKCTSCQCCKPVQQGCCCGDKAKACCTDPNSGCCCSNKANKCCDATSKQECQTCQCCK |
| 8 | Fungal 1 | C-G-C-S-x-C-x-C-x-C-x-C-S-x-C | Neurospora crassa MT MGDCGCSGASSCNCGSGCSCSNCGSK |
| 9 | Fungal 2 | --- | Candida glabrata MT2 MANDCKCPNGCSCPNCANGGCQCGDKCECKKQSCHGCGEQCKCGSHGSSCHGSCGCGDKCECK |
| 10 | Fungal 3 | --- | Candida glabrata MT2 MPEQVNCQYDCHCSNCACENTCNCCAKPACACTNSASNECSCQTCKCQTCKC |
| 11 | Fungal 4 | C-X-K-C-x-C-x-C-K-C | Yarrowia lipolytica MT3 MEFTTAMLGASLISTTSTQSKHNLVNNCCCSSSTSESSMPASCACTKCGCKTCKC |
| 12 | Fungal 5 | --- | Saccharomyces cerevisiae CUP1 MFSELINFQNEGHECQCQCGSCKNNEQCQKSCSCPTGCNSDDKCPCGNKSEETKKSCCSGK |
| 13 | Fungal 6 | --- | Saccharomyces cerevisiae CRS5 TVKICDCEGECCKDSCHCGSTCLPSCSGGEKCKCDHSTGSPQCKSCGEKCKCETTCTCEKSKCNCEKC |
| 14 | Procaryota | K-C-A-C-x-C-L-C | Synechococcus sp SmtA MTTVTQMKCACPHCLCIVSLNDAIMVDGKPYCSEVCANGTCKENSGCGHAGCGCGSA |
| 15 | Plant | -x-C--C---x-C-x- | |
| 15.1 | Plant MTs Type 1 | C-X-C-X- C-X-C-X- C-X-C-X-spacer-C-X-C-X- C-X-C-X- C-X-C-X | Pisum sativum MT MSGCGCGSSCNCGDSCKCNKRSSGLSYSEMETTETVILGVGPAKIQFEGAEMSAASEDGGCKCGDNCTCDPCNCK |
| 15.2 | Plant MTs Type 2 | C-C-X-C-X-C-X- C-X-C-X- C-X-C-X-spacer- C-X-C-X- C-X-C-X- C-X-C-X | Lycopersicon esculentum MT MSCCGGNCGCGSSCKCGNGCGGCKMYPDMSYTESSTTTETLVLGVGPEKTSFGAMEMGESPVAENGCKCGSDCKCNPCTCSK |
| 15.3 | Plant MTs Type 3 | --- | Arabidopsis thaliana MT3 MSSNCGSCDCADKTQCVKKGTSYTFDIVETQESYKEAMIMDVGAEENNANCKCKCGSSCSCVNCTCCPN |
| 15.4 | Plant MTs Type 4 or Ec | C-x-C-X-C-X-C-X-C-X-C-X-HTTCGCGEHC- X-C-X-CSCGAXCNCASC-X | Triticum aestivum MT MGCNDKCGCAVPCPGGTGCRCTSARSDAAAGEHTTCGCGEHCGCNPCACGREGTPSGRANRRANCSCGAACNCASCGSTTA |
| 99 | Phytochelatins and other non-proteinaceous MT-like polypeptides | --- | Schizosaccharomyces pombe γEC-γEC-γECG |
More data on this classification are discoverable at the Expasy metallothionein page.Secondary structure elements have been observed in several MTs SmtA from Syneccochoccus, mammalian MT3, echinoderm SpMTA, fish Notothenia coriiceps MT, crustacean MTH, but until this moment, the content of such structures is considered to be poor in MTs, and its functional influence is not considered.
Tertiary structure of MTs is also highly heterogeneous. While vertebrate, echinoderm and crustacean MTs show a bidominial structure with divalent metals as Zn or Cd, yeast and prokaryotic MTs show a monodominial structure a bidominial structure similar to that of vertebrate MTs; 2) a codominial structure, in which two Cys-rich domains interact to form a single metallic cluster.
Quaternary structure has not been broadly considered for MTs. Dimerization and oligomerization processes have been observed and attributed to several molecular mechanisms, including intermolecular disulfide formation, bridging through metals bound by either Cys or His residues on different MTs, or inorganic phosphate-mediated interactions. Dimeric and polymeric MTs have been shown to acquire novel properties upon metal detoxification, but the physiological significance of these processes has been demonstrated only in the case of prokaryotic Synechococcus SmtA. The MT dimer produced by this organism forms structures similar to zinc fingers and has Zn-regulatory activity.
Metallothioneins have diverse metal-binding preferences, which have been associated with functional specificity. As an example, the mouse MT1 preferentially binds divalent metal ions, Cd, while yeast CUP1 is selective for monovalent metal ions, Ag. Strictly metal-selective MTs with metal-specific physiological functions were discovered by Dallinger et al. in pulmonate snails. The Roman snail, for example, possesses a Cd-selective and a Cu-selective isoform involved in Cd detoxification and Cu regulation, respectively. While both isoforms contain unvaried numbers and positions of Cys residues responsible for metal ligation, metal selectivity is apparently achieved by sequence modulation of amino acid residues not directly involved in metal binding.
A novel functional classification of MTs as Zn- or Cu-thioneins is currently being developed based on these functional preferences.
Function
The main biological function of metallothioneins is to maintain homeostasis of the essential metals zinc and copper, but metallothioneins also protect against metal toxicity and oxidative stress.Metal binding
Metallothionein has been documented to bind a wide range of metals including cadmium, lead, zinc, mercury, copper, arsenic, silver, etc. Metalation of MT was previously reported to occur cooperatively but recent reports have provided strong evidence that metal-binding occurs via a sequential, noncooperative mechanism. The observation of partially metalated MT suggest that these species are biologically important.Metallothioneins likely participate in the uptake, transport, and regulation of zinc in biological systems. Mammalian MT binds three Zn ions in its beta domain and four in the alpha domain. Cysteine is a sulfur-containing amino acid, hence the name "-thionein". However, the participation of inorganic sulfide and chloride ions has been proposed for some MT forms. In some MTs, mostly bacterial, histidine participates in zinc binding. By binding and releasing zinc, metallothioneins may regulate zinc levels within the body. Zinc, in turn, is a key element for the activation and binding of certain transcription factors through its participation in the zinc finger region of the protein. Metallothionein also carries zinc ions from one part of the cell to another. When zinc enters a cell, it can be picked up by thionein and carried to another part of the cell where it is released to another organelle or protein. In this way thionein and metallothionein becomes a key component of the zinc signaling system in cells. This system is particularly important in the brain, where zinc signaling is prominent both between and within nerve cells. It also seems to be important for the regulation of the tumor suppressor protein p53.