Effect of mercuric chloride on cultured rat fibroblasts: survival, protein biosynthesis and binding of mercury to chromatin

An Extremely Stable Interprotein Tetrahedral Hg(Cys)4 Core Forms in the Zinc Hook Domain of Rad50 Protein at Physiological pH

In nature, thiolate-based systems are the primary targets of divalent mercury (Hg^II ) toxicity. The formation of Hg(Cys)_x cores in catalytic and structural protein centers mediates mercury's toxic effects and ultimately leads to cellular damage. Multiple studies have revealed distinct Hg^II -thiolate coordination preferences, among which linear Hg^II complexes are the most commonly observed in solution at physiological pH. Trigonal or tetrahedral geometries are formed at basic pH or in tight intraprotein Cys-rich metal sites. So far, no interprotein tetrahedral Hg^II complex formed at neutral pH has been reported. Rad50 protein is a part of the multiprotein MRN complex, a major player in DNA damage-repair processes. Its central region consists of a conserved CXXC motif that enables dimerization of two Rad50 molecules by coordinating Zn^II . Dimerized motifs form a unique interprotein zinc hook domain (Hk) that is critical for the biological activity of the MRN. Using a series of length-differentiated peptide models of the Pyrococcus furiosus zinc hook domain, we investigated its interaction with Hg^II . Using UV-Vis, CD, PAC, and ¹⁹⁹ Hg NMR spectroscopies as well as anisotropy decay, we discovered that all Rad50 fragments preferentially form homodimeric Hg(Hk)₂ species with a distorted tetrahedral HgS₄ coordination environment at physiological pH; this is the first example of an interprotein mercury site displaying tetrahedral geometry in solution. At higher Hg^II content, monomeric HgHk complexes with linear geometry are formed. The Hg(Cys)₄ core of Rad50 is extremely stable and does not compete with cyanides, NAC, or DTT. Applying ITC, we found that the stability constant of the Rad50 Hg(Hk)₂ complex is approximately three orders of magnitude higher than those of the strongest Hg^II complexes known to date.

A zinc-finger like metal binding site in the nucleosome

UV spectroscopy demonstrated that chicken mononucleosomes bind Co(II) and Zn(II) ions at submicromolar concentrations in a tetrahedral mode, at a conserved zinc finger-like site, composed of Cys110 and His113 residues of both H3 molecules. Neither of these metal ions substituted for another, indicating a limited binding reversibility. Molecular modeling indicated that the tetrahedral site is formed by unhindered rotations around Calpha-Cbeta bonds in the side chains of the zinc binding residues. The resulting local rearrangement of the protein structure shields the bound metal ion from the solvent, explaining the observed lack of reversibility of the binding. Consequences of these findings for zinc homeostasis, metal toxicology and nucleosomal regulation are discussed.

Histone H3 variants and their potential role in indexing mammalian genomes: the "H3 barcode hypothesis"

In the history of science, provocative but, at times, controversial ideas have been put forward to explain basic problems that confront and intrigue the scientific community. These hypotheses, although often not correct in every detail, lead to increased discussion that ultimately guides experimental tests of the principal concepts and produce valuable insights into long-standing questions. Here, we present a hypothesis, the "H3 barcode hypothesis." Hopefully, our ideas will evoke critical discussion and new experimental approaches that bear on general topics, such as nuclear architecture, epigenetic memory, and cell-fate choice. Our hypothesis rests on the central concept that mammalian histone H3 variants (H3.1, H3.2, and H3.3), although remarkably similar in amino acid sequence, exhibit distinct posttranslational "signatures" that create different chromosomal domains or territories, which, in turn, influence epigenetic states during cellular differentiation and development. Although we restrict our comments to H3 variants in mammals, we expect that the more general concepts presented here will apply to other histone variant families in organisms that employ them.

Mercuric ion inhibition of eukaryotic transcription factor binding to DNA

Mercury has harmful effects in both rodents and humans. In rodent tissue culture cells exposed to HgCl(2), the metal ions were observed to concentrate in cell nuclei and to associate with chromatin. Thus, transcription factors and other proteins associated with chromatin are possible targets of mercuric ion toxicity. In this study, mercuric ions were found to inhibit the DNA binding activity of the Cys(2)His(2) zinc finger proteins transcription factor IIIA (TFIIIA) and Sp1. These factors are prototypes of the largest eukaryotic protein superfamily. Neither the presence of excess zinc ions nor beta-mercaptoethanol prevented inhibition by mercuric ions. Mercuric ions also inhibited DNA binding by the non-zinc finger protein AP2. Zinc finger-DNA binding was inhibited when both TFIIIA/5S RNA complex and TFIIIA alone were preincubated with concentrations as low as 15 microM mercuric ion. Inhibition occurred in less than 1 min and was not readily reversible. Mercuric ions also inhibited the digestion of DNA by the restriction enzymes BamHI or EcoRI. Inhibition of transcription factors as well as potentially other DNA binding proteins by micromolar concentrations of mercuric ion suggests additional biochemical mechanisms for mercury toxicity in promoting disease via alterations in gene transcription patterns.