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D H Hamer

Publications and source records attributed to D H Hamer.

At least 55 records · Page 3Linked to original sources
PubMed1988

Effect of mutation of cysteinyl residues in yeast Cu-metallothionein.

Metallothioneins have been isolated from Saccharomyces cerevisiae CUP1 mutants generated by Wright et al. (Wright, C. F., Hamer, D. H., and McKenney, K. (1986) Nucleic Acids Res. 14, 8489-8499). In the mutant metallothioneins, pairs of cysteinyl residues have been converted to seryl residues. The mutant proteins differ only in the positions of the double substitutions; each mutant molecule contains 10 cysteinyl residues. Each mutant protein lacks the first 8 residues at the amino terminus from the decoded gene sequence of the CUP1 locus. Mutant molecules consist of 53 residues analogous to the wild-type metallothionein and are designated 9/11, 24/26, 36/38, and 49/50 (in reference to the sequence positions of the Cys----Ser conversions). The properties of the mutant metallothioneins are vastly different, and host cells harboring the different plasmid-encoded mutant molecules show marked differences in sensitivity to CuSO4. Growth inhibition was observed at CuSO4 concentrations up to mM in cells containing the 9/11, 24/26, and 36/38 molecules, but not for cells containing protein 49/50. A CuSO4 concentration of 5 mM was required to inhibit the growth of yeast containing either 49/50 or the wild-type metallothionein. In the purified proteins the copper binding stoichiometry of each molecule, except protein 24/26, was nearly 8 mol eq. Protein 24/26 bound 5.5 copper ions/molecule. The Cu(I) chelator bathocuproine disulfonate reacted with over 50% of the copper ions in proteins 9/11, 24/26, and 36/38, but less than 10% of the copper ions in proteins 49/50 and wild-type metallothionein were reactive. The thiolates in 9/11, 24/26, and 36/38 were also more reactive in a disulfide exchange reaction with dithiodipyridine compared with the sulfhydryls in 49/50 and the wild-type molecules. The four mutant copper proteins are luminescent and exhibit a similar quantum yield. The cluster structures contributing to the particular electronic transitions are markedly more sensitive to oxygen in proteins 9/11, 24/26, and 36/38 compared with 49/50 and the wild-type molecules. The air-sensitive proteins exhibit a tertiary fold not recognized by polyclonal antibodies directed to a conformational epitope on yeast Cu-metallothionein. Protein 49/50 cross-reacts with the antibody in a concentration-dependent fashion similar to the wild-type protein. Mutation of 2 cysteinyl residues in the carboxyl portion of metallothionein does not significantly alter properties of the molecule, whereas mutation of several cysteines in the amino-terminal portion of the molecule yields a different conformation.

Carrier Proteins↗
PubMed1988

Autoregulation of the yeast copper metallothionein gene depends on metal binding.

The yeast CUP1 gene product, copper metallothionein, acts to repress the basal transcription of its own structural gene. By creating a series of truncation and amino acid substitutions in CUP1, we show that the ability of the protein to autoregulate is directly correlated to its ability to bind and detoxify copper. These results support a model in which metallothionein controls the level of free intracellular copper available to interact with positive transcription factors. In addition, mutations in chemically equivalent cysteine residues were functionally dissimilar, suggesting that partial sites in the molecule are critical for the formation of the sulfur-metal cluster.

Carrier Proteins↗
PubMed1987

Structural and functional studies of the amino terminus of yeast metallothionein.

Purified yeast copper-metallothionein lacks 8 amino-terminal residues that are predicted from the DNA sequence of its gene. The removed sequence is unusual for metallothionein in its high content of hydrophobic and aromatic residues and its similarity to mitochondrial leader sequences. To study the significance of this amino-terminal cleavage, several mutations were introduced into the metallothionein coding gene, CUP1. One mutant, which deletes amino acid residues 2-8, had a minor effect on the ability of the molecule to confer copper resistance to yeast but did not affect CUP1 gene regulation. A second mutation, which changes two amino acids adjacent to the cleavage site, blocked removal of the extension peptide but had no effect on copper detoxification or gene regulation. Immunofluorescence studies showed that both the wild-type and these two mutant proteins are predominantly cytoplasmic with no evidence for mitochondrial localization. The cleavage site mutation allowed isolation and structural characterization of a full length metallothionein polypeptide. The copper content and luminescent properties of this molecule were identical to those of the truncated wild-type protein indicating a homologous cluster structure. Moreover, the amino-terminal peptide was selectively removed by various endopeptidases and an exopeptidase suggesting that it does not participate in the tertiary fold. These results argue that the amino-terminal peptide is not required for either the structural integrity or biological function of yeast metallothionein.

Amino Acid Sequence↗
PubMed1987

Regulation in vitro of metallothionein gene binding factors.

Mouse nuclear factors that bind to an upstream metal regulatory element of the mouse metallothionein-I gene have been identified by DNA footprinting and oligonucleotide band shift assays. The formation of complexes at this site can be activated 20- to 40-fold by the vitro addition of ionic cadmium. The activation reaction is rapid, reversible by a metal chelator, and may involve multiple proteins. These results suggest that the initial step in cadmium detoxification is an interaction between the metal and nuclear DNA-binding factors leading to an increase in metallothionein gene transcription. The ability to observe metal activation in vitro makes this a powerful system to study the biochemistry of eukaryotic gene regulation.

Animals↗
PubMed1987

Function and regulation of yeast copperthionein.

A functional copperthionein (CUP1) gene in Saccharomyces cerevisiae is essential to prevent copper-mediated cytotoxicity, but is dispensable for cell growth in the absence of exogenous copper. The CUP1 gene is negatively autoregulated, as observed by the necessity for a functional CUP1 gene in order to repress basal level transcription from the CUP1 promoter. Both the copper protection and transcriptional autoregulatory functions can be complemented by expression in yeast of either of two monkey metallothionein isoform cDNAs. The expression of the CUP1 gene is induced at the level of transcription by copper via cis-dominant upstream control sequences which are tandemly repeated. Synthetic CUP1 upstream control sequences confer copper inducibility on a heterologous yeast promoter in a manner similar to that observed for the authentic upstream control region.

Animals↗
PubMed1987

Abnormal copper metabolism and regulation of metallothionein gene expression in Menkes' disease.

Menkes' kinky hair disease, a lethal X-linked recessive trait, is characterized by abnormal copper accumulation in several non-hepatic tissues. The level of many copper enzymes is severely reduced, leading to damage of the connective and nervous tissues of the patients. Cultured skin fibroblasts from Menkes' patients retain more copper then normal controls, and the excess metal is bound to metallothionein. Low doses of copper in the media induce MT gene transcription in Menkes' but not in normal cells. Transfection experiments using a plasmid containing the mouse MT-I promoter fused to the enzyme chloramphenicol acetyl transferase show that the activation of the mMTI promoter is in trans. Two other effects are observed in Menkes' cells: (a) two heat-shock like proteins are synthesized in response to low doses of copper in the growth medium, and (b) Menkes' cells are more sensitive then normal fibroblasts to copper toxicity. Our interpretation of these results supports a model for a defect in one or more steps in copper metabolism or transport.

Brain Diseases, Metabolic↗
PubMed1987

Metallothionein gene regulation in Menkes' syndrome.

The characteristic feature of Menkes' disease is a maldistribution of bodily copper; decreased copper levels are present in the serum, brain, and liver, whereas excess levels are present in gut, kidney, and most other nonhepatic tissues. Using cultured fibroblasts, we have shown that low extracellular copper concentrations induce synthesis of metallothionein, a copper-binding protein, in Menkes' cells but not in normal cells. This is due to a defect in a diffusable regulatory factor that is probably involved in copper metabolism. To further understand the role of the defective factor in transcription, assays have been developed to study the metal-dependent binding of nuclear proteins to metallothionein gene control sequences.

Animals↗
PubMed1986

Chromogenic identification of oligonucleotide-directed mutants.

We describe a simple plaque color assay for identifying oligonucleotide-directed mutations in cloned DNA fragments. The basis of the method is to: fuse the sequence of interest in-frame to the E.coli lacZ gene to produce a blue plaque phage, mutate the site of interest to a stop codon to generate a white plaque phage, and revert the stop codon and surrounding nucleotides to give a blue plaque phage containing one or more desired amino acid changes. The advantages of this cyclic method are that it produces truncated as well as amino acid substituted protein molecules, it can be repeated to introduce additional mutations, and it eliminates the need for labor intensive screening. Essentially any piece of DNA can be mutated using this method if the fragment has one open reading frame. If there is an open reading frame between the site and the lacZ gene, ATG codons can be inserted at the target site. We have used this method to produce termination and amino acid substitution mutants in the yeast CUP1 gene.

Base Sequence↗
PubMed1986

Mammalian metallothionein is functional in yeast.

Expression of two monkey metallothioneins in yeast leads to complementation of both known functions of the endogenous yeast copperthionein gene, namely copper detoxification and autoregulation of transcription. The metallothionein-like proteins of higher and lower eukaryotes are therefore functionally analogous despite their dissimilar primary sequences.

Animals↗
PubMed1986

Cell specificity and an effect of ras on human metallothionein gene expression.

The expression of three human metallothionein (hMT) genes has been compared in various established cell lines and primary liver. The single gene for hMT isoform II is ubiquitously expressed in all cell types in response to cadmium. In contrast, two genes encoding hMT-I isoforms are expressed in a highly specific, reciprocal fashion that correlates with the embryonic germ layer origin of the cells. In one cell line that failed to express detectable amounts of hMT-IE, treatment with the demethylating agent 5-azacytidine led to cadmium-inducible expression of this subtype. The genes for both MT-I isoforms are coordinately inducible by heavy metals but differ in their response to glucocorticoids. Surprisingly, cells transformed with the Ha-ras oncogene contain elevated basal levels of both MT-I and MT-II RNA. The implications of these results for growth-related and developmental functions of MT are discussed.

Cell Differentiation↗
PubMed1986

Tandemly duplicated upstream control sequences mediate copper-induced transcription of the Saccharomyces cerevisiae copper-metallothionein gene.

Transcription of the Saccharomyces cerevisiae copper-metallothionein gene, CUP1, inducible by copper. By analyzing deletion and fusion mutants in the CUP1 5'-flanking region, we identified two closely related, tandemly arranged copper regulatory elements. A synthetic version of one of these elements conferred efficient copper induction on a heterologous promoter when present in two tandem copies.

Base Sequence↗
PubMed1986

Metallothionein.

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Animals↗
PubMed1985

Yeast metallothionein. Sequence and metal-binding properties.

The protein product of the CUP1 locus in Cu-resistant Saccharomyces cerevisiae has been purified and characterized. The protein was found to lack the first 8 amino acids predicted by the nucleotide sequence of the gene. The residues removed from the amino-terminal region include 5 hydrophobic residues, two of which are aromatic. The unique amino terminus starting at Gln9 of the putative DNA translation product was observed for metallothionein purified in the presence of various protease inhibitors or from a pep4 mutant yeast strain deficient in vacuolar proteases. The remainder of the primary structure of the protein is equivalent to the decoded DNA sequence, so yeast metallothionein is a 53-residue polypeptide of molecular weight 5655. The isolated protein contained 8 copper ions ligated by 12 cysteines/molecule. Reconstitution studies of the apo-molecule revealed that 8 mol eq of Cu(I) conferred maximal stability against proteolysis and depleted the zinc content of zinc-saturated metallothionein. These assays suggested that the protein has 8 binding sites for Cu(I). Ag(I) ions bound to the protein with the same stoichiometry. Yeast metallothionein was also observed to coordinate Cd(II) and Zn(II) ions in vitro. In studies of direct binding, protection against proteolysis, and metal ion exchange, these divalent ions were found to associate with the protein with a maximal stoichiometry of 4 ions/molecule. Yeast metallothionein thus exhibits two distinct binding configurations for Cu(I) and Cd(II) as does the mammalian protein.

Amino Acid Sequence↗
PubMed1985

Structure and expression of two human metallothionein-I isoform genes and a related pseudogene.

Three members of the human metallothionein-I gene family have been cloned and characterized. Two of the genes encode closely related but distinct metallothionein-I subtypes. Both of these genes are functional as shown by their transcription in cultured hepatoblastoma cells and by their ability to render transfected cells resistant to cadmium toxicity. The cotranscription of these nonallelic genes shows that the previously observed microheterogeneity of metallothionein-I protein preparations is due to the expression of distinct gene products. The third clone is incapable of encoding a typical metallothionein due to an early termination codon and two nonconservative amino acid replacements. This nonfunctional pseudogene retains introns. Evolutionary comparisons reveal conserved DNA sequences in both the coding and regulatory regions of these genes.

Amino Acid Sequence↗
PubMed1985

Function and autoregulation of yeast copperthionein.

The CUP1 gene of yeast encodes a small, metallothionein-like protein that binds to and is inducible by copper. A gene replacement experiment shows that this protein protects cells against copper poisoning but is dispensable for normal cellular growth and development throughout the yeast life cycle. The transcription of CUP1 is negatively autoregulated. This feedback mechanism, which is mediated through upstream control sequences, may play an important role in heavy metal homeostasis.

Carrier Proteins↗
PubMed1985

Menkes' disease: abnormal metallothionein gene regulation in response to copper.

Menkes' disease, an inherited disorder of copper metabolism, is characterized by the accumulation of excess copper-metallothionein in certain tissues and cell types. Using cultured fibroblasts, we show that this is due to the ability of low concentrations of copper to induce metallothionein mRNA synthesis in Menkes' but not normal cells. We also show that copper, which is unusually toxic to Menkes' cells, induces the synthesis of 84 kd and 68 kd polypeptides tentatively identified as heat shock proteins. Transfection experiments with a cloned metallothionein fusion gene show that this is due to a defect in a diffusible factor involved in either metallothionein gene transcriptional regulation or copper metabolism.

Brain Diseases, Metabolic↗
PubMed1985

Stable transcription complex on a class III gene in a minichromosome.

We have constructed recombinant simian virus 40 molecules containing Xenopus 5S RNA and tRNA genes. Recombinant minichromosomes containing these genes were isolated to study the interaction of RNA polymerase III transcription factors with these model chromatin templates. Minichromosomes containing a tRNAMet gene can be isolated in a stable complex with transcription factors (IIIB and IIIC) and are active in vitro templates for purified RNA polymerase III. In contrast, minichromosomes containing a 5S RNA gene are refractory to transcription by purified RNA polymerase III in either the absence or the presence of other factors.

Animals↗