Metallothioneins (MTs) are ubiquitous low molecular weight proteins and polypeptides of extremely high metal and sulfur content. They are thought to play roles both in the intracellular fixation of the essential trace elements zinc and copper, in controlling the concentrations of the free ions of these elements, in regulating their flow to their cellular destinations, in neutralising the harmful influences of exposure to toxic elements such as cadmium and mercury and in the protection from of a variety of stress conditions (Kägi & Schäffer 1988).

MT was discovered in 1957 when Margoshes and Vallee (1957) identified in equine kidney cortex a cadmium-binding protein responsible for the natural accumulation of cadmium in this tissue. MTs are still the only biological compounds known to naturally contain this metal. However, as already shown in the earliest studies, cadmium is the only one of several optional metallic components, the others being most commonly zinc and copper (Kägi & Vallee 1960, Pulido et al. 1966). Thus, MTs are key compounds involved in the intracellular handling of a variety of essential and nonessential post-transition metal ions.

Definition. MTs have received their designation from their prominent metal and sulfur content which, varying with the metal species present, together may contribute to over 20% of their weight. The mammalian forms are characterized by a molecular weight of 6000-7000 Da, containing 60 to 68 amino acid residues, among them 20 Cys, and binding a total of 7 equiv. of bivalent metal ions. Aromatic amino acids are usually absent. All Cys occur in the reduced form and are coordinated to the metal ions through mercaptide bonds, giving rise to spectroscopic features characteristic of metal-thiolate clusters. According to the recommendations made by The Committee on the Nomenclature of MT any protein or polypeptide resembling mammalian MTs in several of these criteria can be classified as an MT (Fowler et al. 1987).

Occurrence. MTs occur throughout the animal kingdom and are also found in higher plants, eukaryotic microorganisms, and in some prokaryotes (see the metallo.txt file associated with SWISS-PROT). In animals, the genetically polymorphous proteins are most abundant in parenchymatous tissues, i.e. liver, kidney, pancreas, and intestines. There are wide variations in concentration in different species and tissues, reflecting effects of age, stage of development, dietary regimen, and other not yet fully identified factors. Although MT is a cytoplasmic protein, it can also accumulate in lysosomes, and during development it is observed in the nucleus.

Spatial structure. Spatial structures of mammalian MTs, crustacean MTs and an echinodermal MT have been derived from 2D NMR spectroscopy and X-ray crystallography. Alhough their amino acid sequences are very different, they have similar spatial structures. They all have a dumbell-like shape with two separate protein domains containing in their core mineral-like clusters built up of several tetrahedral Me(II)-Cys units. All Cys are involved in the binding of the metals. MTs contain almost no regular secondary structure elements.

Molecular evolution. Phylogenetic relationships of the various MT families have recently been established by different methods and approaches both from protein and gene MT sequences. The results of the analyses lead to the diferentiation into a variety of phylogenetically related subfamilies and subgroups and have allowed to derive an evolutionary pedigree for the highly complex vertebrate family (Binz 1996, Binz & Kägi poster, Binz 1999)

Molecular genetics. MTs are genetically polymorphous protein families with subfamilies, subgroups and isoforms. In vertebrates all MT genes are divided into a 5' flanking region (5'UT), a 5' untranslated region (5'UTR), 3 coding exons separated by 2 introns and a 3' flanking end. Mammals possess genes for four subfamilies, the ubiquitous MT-1 and MT-2, the brain specific MT-3 and the squamous epithelium specific MT-4. All are located on a single chromosome, i.e. chromosome 8 in mouse and chromosome 16 in human (Quaife et al. 1994). The 5'UT contains regulatory elements among them one or more copies of the metal responsive element (MRE) (Stuart et al. 1985) which acts as a binding target for the transcription activating protein factor (MTF-1) (Brugnera et al. 1994) regulating MT-gene expression.

Functional aspects. The most conspicious biological feature of the MTs is their inducibility by a variety of agents and conditions. Thus, their biosynthesis of many MTs is greatly enhanced both in vivo and in cultured cells by transition and d10 metal ions and by certain hormones, cytokines, growth factors, tumor promoters and many other chemicals. A massive accretion of MT is also observed in the livers of animals submitted to physical stress. Physiological MT-synthesis and MT concentrations are increased transiently several folds during cell proliferation. MT interchanges its zinc with zinc-finger proteins in vitro and hence, may imply a contributory role of MT to zinc-dependent processes involved in gene expression.

References:
Binz P.-A. (1996) PhD thesis, University Zürich, Zürich, Switzerland
Binz P.-A. & Kägi J.H.R. (1996) poster presentation, Protein Science 6, suppl. 1, p 68
Binz, P.-A., Kägi, J.H.R. Metallothionein: Molecular evolution and classification. (1999) In: Metallothionein IV, C. Klaassen (ed.), Birkhäuser Verlag Basel, 7-13 Brugnera E., Georgiev O., Radtke F., Heuchel R., Baker E., Sutherland G.R. & Schaffner W. (1994) Nucleic Acids Res 22, 3167-3173
Fowler B.A., Hildebrand C.E., Kojima Y. & Webb M. (1987) Experientia Suppl. 52, 19-22
Kägi J.H.R. & Schäffer A. (1988) Biochemistry 27, 8509-8515
Kägi J.H.R. & Vallee B.L. (1960) J. Biol. Chem. 235, 3460-3465
Margoshes M. & Vallee B.L. (1957) J. Am. Chem. Soc. 79, 4813-4814
Pulido P., Kägi J.H.R. & Vallee B.L. (1966) Biochemistry 5, 1768-1777
Quaife C.J., Findley S.D., Erickson J.C., Froelick G.J., Kelly E.J., Zambrovic B.P. & Palmiter R.D. (1994) Biochemistry 33, 7250-7259
Stuart G.W., Searle P.F. & Palmiter R.D. (1985) Nature 317, 828-831