The properties of the metal-thiolate clusters in recombinant mouse metallothionein-4

Molecular cloning and expression of novel metallothionein (MT) gene in the polychaete Perinereis nuntia exposed to metals

To report a novel metallothionein (MT) gene and evaluate its potency as a biomarker, we clone this MT gene and measured the expression levels in the metal-exposed polychaete Perinereis nuntia. Accumulated metal contents and metallothionein-like proteins (MTLPs), which have been recognized as potential biomarkers, were compared with the relative mRNA expressions of the MT gene of P. nuntia (Pn-MT). In addition, the metal-binding affinity was estimated by recombinant Pn-MT protein. Pn-MT having high cysteine residues with three metal response elements in the promoter region closely clusters with those of other invertebrates. The accumulation patterns of metals were dependent on the exposure times in lead (Pb), cadmium (Cd), and copper (Cu) exposure. Particularly, both MTLP levels and relative mRNA expressions of MT were increased with accumulated metal contents and exposure time in P. nuntia exposed to Pb and Cd. There was no significant modulation of the Pn-MT gene in polychaetes exposed to Zn and As. However, the metal-binding ability of the recombinant Pn-MT protein provides a clear evidence for a high affinity of MT to several metal elements. These results suggest that Pn-MT would play an important role in the detoxification and/or sequestration of specific metals (e.g., Pb and Cd) in P. nuntia and have potential as a molecular biomarker in the monitoring of the marine environment using a polychaete.

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.

Gains, losses and changes of function after gene duplication: study of the metallothionein family

Metallothioneins (MT) are small proteins involved in heavy metal detoxification and protection against oxidative stress and cancer. The mammalian MT family originated through a series of duplication events which generated four major genes (MT1 to MT4). MT1 and MT2 encode for ubiquitous proteins, while MT3 and MT4 evolved to accomplish specific roles in brain and epithelium, respectively. Herein, phylogenetic, transcriptional and polymorphic analyses are carried out to expose gains, losses and diversification of functions that characterize the evolutionary history of the MT family. The phylogenetic analyses show that all four major genes originated through a single duplication event prior to the radiation of mammals. Further expansion of the MT1 gene has occurred in the primate lineage reaching in humans a total of 13 paralogs, five of which are pseudogenes. In humans, the reading frame of all five MT1 pseudogenes is reconstructed by sequence homology with a functional duplicate revealing that loss of invariant cysteines is the most frequent event accounting for pseudogeneisation. Expression analyses based on EST counts and RT-PCR experiments show that, as for MT1 and MT2, human MT3 is also ubiquitously expressed while MT4 transcripts are present in brain, testes, esophagus and mainly in thymus. Polymorphic variation reveals two deleterious mutations (Cys30Tyr and Arg31Trp) in MT4 with frequencies reaching about 30% in African and Asian populations suggesting the gene is inactive in some individuals and physiological compensation for its loss must arise from a functional equivalent. Altogether our findings provide novel data on the evolution and diversification of MT gene duplicates, a valuable resource for understanding the vast set of biological processes in which these proteins are involved.

Organization and assembly of metal-thiolate clusters in epithelium-specific metallothionein-4

Mammalian metallothionein-4 (MT-4) was found to be specifically expressed in stratified squamous epithelia where it plays an essential but poorly defined role in regulating zinc or copper metabolism. Here we report on the organization, stability, and the pathway of metal-thiolate cluster assembly in MT-4 reconstituted with Cd(2+) and Co(2+) ions. Both the (113)Cd NMR studies of (113)Cd(7)MT-4 and the spectroscopic characterization of Co(7)MT-4 showed that, similar to the classical MT-1 and MT-2 proteins, metal ions are organized in two independent Cd(4)Cys(11) and Cd(3)Cys(9) clusters with each metal ion tetrahedrally coordinated by terminal and bridging cysteine ligands. Moreover, we have demonstrated that the cluster formation in Cd(7)MT-4 is cooperative and sequential, with the Cd(4)Cys(11) cluster being formed first, and that a distinct single-metal nucleation intermediate Cd(1)MT-4 is required in the cluster formation process. Conversely, the absorption and circular dichroism features of metal-thiolate clusters in Cd(7)MT-4 indicate that marked differences in the cluster geometry exist when compared with those in Cd(7)MT-1/2. The biological implication of our studies as to the role of MT-4 in zinc metabolism of stratified epithelia is discussed.