Zinc binding by the methylation signaling domain of the Escherichia coli Ada protein.

The Escherichia coli Ada protein repairs O6-methylguanine residues and methyl phosphotriesters in DNA by direct transfer of the methyl group to a cysteine residue located in its C- or N-terminal domain, respectively. Methyl transfer to the N-terminal domain causes it to acquire a sequence-specific DNA binding activity, which directs binding to the regulatory region of several methylation-resistance genes. In this paper we show that the N-terminal domain of Ada contains a high-affinity binding site for a single zinc atom, whereas the C-terminal domain is free of zinc. The metal-binding domain is apparently located within the first 92 amino acids of Ada, which contains four conserved cysteine residues. We propose that these four cysteines serve as the zinc ligand residues, coordinating the metal in a tetrahedral arrangement. One of the putative ligand residues, namely, Cys69, also serves as the acceptor site for a phosphotriester-derived methyl group. This raises the possibility that methylation-dependent ligand reorganization about the metal plays a role in the conformational switching mechanism that converts Ada from a non-sequence-specific to a sequence-specific DNA-binding protein.     

Multiple domains mediate heme control of the yeast activator HAP1

The activity of the yeast activator HAP1in vivo requires heme. A heme responsive domain of HAP1 was identified previously. It is adjacent to the DNA-binding domain, and appears to block DNA binding in the absence of heme. Here we describe a novel genetic selection which yielded mutants of HAP1 that are independent of heme when assayed for activation. These mutants define a second region of HAP1, close to the activation domain, which also controls its response to heme.     

An arginine residue instead of a conserved leucine residue in the recognition helix of the finger 3 of Zif268 stabilizes the domain structure and mediates DNA binding

The Cys(2)His(2)-type zinc finger is a common DNA binding motif that is widely used in the design of artificial zinc finger proteins. In almost all Cys(2)His(2)-type zinc fingers, position 4 of the α-helical DNA-recognition site is occupied by a Leu residue involved in formation of the minimal hydrophobic core. However, the third zinc finger domain of native Zif268 contains an Arg residue instead of the conserved Leu. Our aim in the present study was to clarify the role of this Arg in the formation of a stable domain structure and in DNA binding by substituting it with a Lys, Leu, or Hgn, which have different terminal side-chain structures. Assessed were the metal binding properties, peptide conformations, and DNA-binding abilities of the mutants. All three mutant finger 3 peptides exhibited conformations and thermal stabilities similar to the wild-type peptide. In DNA-binding assays, the Lys mutant bound to target DNA, though its affinity was lower than that of the wild-type peptide. On the other hand, the Leu and Hgn mutants had no ability to bind DNA, despite the similarity in their secondary structures to the wild-type. Our results demonstrate that, as with the Leu residue, the aliphatic carbon side chain of this Arg residue plays a key role in the formation of a stable zinc finger domain, and its terminal guanidinium group appears to be essential for DNA binding mediated through both electrostatic interaction and hydrogen bonding with DNA phosphate backbone.     

Identification of the DNA-binding domains of the switch-activating-protein Sap1 from S.pombe by random point mutations screening in E.coli.

Mating type switching in fission yeast, Schizosaccharomyces pombe, is initiated by a site-specific double-strand break (DSB) at the mat1 locus. The DSB is controlled from a distance by cis- and trans-acting elements. The switch-activating protein, Sap1 binds to the SAS1 cis-acting element which controls the frequency of the DSB at the mat1 locus and, consequently the efficiency of mating type switching. We developed a general method for screening randomly mutagenized expression libraries of DNA-binding protein in E.coli. Sap1 gene was mutagenized by PCR under conditions of reduced Taq polymerase fidelity. The mutated DNA was expressed in E.coli and screened for SAS1-recognition. This method was used to isolated 16 point mutations that abolished SAS1 interaction together with 18 mutations that did not affect binding. The position of these point mutations allowed the identification of three protein domains located in the N-terminal part of Sap1 that are essential for DNA-binding. Deletions and biochemical analysis showed that Sap1 is a dimer both in solution and when bound to SAS1 sequence. The dimerization domain was localized C-terminally to the three domains described above and when used in exess it inhibited DNA binding.     

Zinc coordination scheme for the C-terminal zinc binding site of nuclear hormone receptors.

The DNA-binding domain of the glucocorticoid receptor contains two zinc ions which are important for the structure and function of the protein. The zinc ions are tetrahedrally coordinated by cysteine residues within the DNA-binding domain. The DNA-binding domain of the glucocorticoid receptor, as well as of the other nuclear hormone receptors, contains nine highly conserved cysteine residues. It has not been clearly established which of these nine cysteine residues are involved in the coordination of zinc. Two models have been proposed for the zinc coordination scheme. We present evidence in favour of the model which excludes the most C-terminal cysteine residue (Cys-481 of the human glucocorticoid receptor) from the zinc coordination scheme. Mutation of this residue in the context of the glucocorticoid receptor DNA-binding domain expressed in E. coli does not significantly reduce the structural integrity of the protein or its DNA-binding properties. These in vitro results are also confirmed by in vivo transactivation assays in yeast.     

Role of conserved residues of the WRKY domain in the DNA-binding of tobacco WRKY family proteins.

Four cDNA clones of tobacco that could code for polypeptides with two WRKY domains were isolated. Among four NtWRKYs and other WRKY family proteins, sequence similarity was basically limited to the two WRKY domains. Glutathione S-transferase fusion proteins with the C-terminal WRKY domain of four NtWRKYs bound specifically to the W-box (TTGACC), and the N-terminal WRKY domain showed weaker binding activity with the W-box compared to the C-terminal domain. The DNA-binding activity of the WRKY domain was abolished by o-phenanthroline and this inhibition was recovered specifically by Zn2+. Substitution of the conserved cysteine and histidine residues of the plant-specific C2H2-type zinc finger-like motif in the WRKY domain abolished the DNA binding. In addition, mutations in the invariable WRKYGQK sequence at the N-terminal side of the zinc finger-like motif also significantly reduced the DNA-binding activity, suggesting that these residues are required for proper folding of the DNA-binding zinc finger.     

CCHX zinc finger derivatives retain the ability to bind Zn(II) and mediate protein-DNA interactions

Classical (CCHH) zinc fingers are among the most common protein domains found in eukaryotes. They function as molecular recognition elements that mediate specific contact with DNA, RNA, or other proteins and are composed of a betabetaalpha fold surrounding a single zinc ion that is ligated by two cysteine and two histidine residues. In a number of variant zinc fingers, the final histidine is not conserved, and in other unrelated zinc binding domains, residues such as aspartate can function as zinc ligands. To test whether the final histidine is required for normal folding and the DNA-binding function of classical zinc fingers, we focused on finger 3 of basic Krüppel-like factor. The structure of this domain was determined using NMR spectroscopy and found to constitute a typical classical zinc finger. We generated a panel of substitution mutants at the final histidine in this finger and found that several of the mutants retained some ability to fold in the presence of zinc. Consistent with this result, we showed that mutation of the final histidine had only a modest effect on DNA binding in the context of the full three-finger DNA-binding domain of basic Krüppel-like factor. Further, the zinc binding ability of one of the point mutants was tested and found to be indistinguishable from the wild-type domain. These results suggest that the final zinc chelating histidine is not an essential feature of classical zinc fingers and have implications for zinc finger evolution, regulation, and the design of experiments testing the functional roles of these domains.     

Functional analyses of the Dof domain, a zinc finger DNA-binding domain, in a pumpkin DNA-binding protein AOBP

AOBP, a DNA-binding protein in pumpkin, contains a Dof domain that is composed of 52 amino acid residues and is highly conserved in several DNA-binding proteins of higher plants. The Dof domain has a significant resemblance to Cys2/Cys2 zinc finger DNA-binding domains of steroid hormone receptors and GATA1, but has a longer putative loop where an extra Cys residue is conserved. We show that the Dof domain in AOBP functions as a zinc finger DNA-binding domain and suggest that the Cys residue uniquely conserved in the putative loop might negatively regulate the binding to DNA.