A molecular genetic approach for the identification of essential residues in human glutathione S-transferase function in Escherichia coli

Identification and validation of suitable reference genes for quantitative expression of xylA and xylE genes in Pseudomonas putida mt-2

Reference genes are used to normalize target genes for relative quantification in gene expression studies. However, different experimental conditions may affect the expression of reference genes, which could lead to erroneous quantitative results. In this study, we performed real-time polymerase chain to investigate the expression of eight reference genes (rpoN, rpoD, dbhA, phaF, 16S rRNA, gst, lexA, and atkA) in Pseudomonas putida mt-2 during degradation of p-xylene. According to their expression stability, geNorm software analysis revealed that rpoN, rpoD, 16S rRNA, and atkA were suitable reference genes with highly stable expression, whereas phaF and dbhA were not suitable due to unstable expression. When normalized either to phaF or dbhA, xylA and xylE expression were significantly different compared to the expression levels normalized with the normalization factor (NF(4)) obtained from the four most stable reference genes (rpoN, -rpoD, -16S rRNA, and -atkA). The use of unstably expressing reference genes resulted in an over- or underestimation of target gene expression, a delay in maximal gene expression, and an increase in gene expression in the absence of inducer. While experimental results indicated that the relative maximum expression of xylA and xylE occurred at different times, unstable reference genes indicated that the maximum expression occurred at the same time. Our study indicates that a valid set of reference genes covering a broad expression range is recommended to accurately normalize and quantify the relative expression levels of the target gene(s) transcripts in many microbial processes.

Differential regulation of the antiapoptotic action of B-cell lymphoma 2 (Bcl-2) and B-cell lymphoma extra long (Bcl-xL) by c-Jun N-terminal protein kinase (JNK) 1-involved pathway in neuroglioma cells

Here, we confirmed that stable expression of B-cell lymphoma-xL (Bcl-xL) in N18TG neuroglioma cells could suppress c-Jun N-terminal protein kinase (JNK) activation, nuclear fragmentation, and cell death caused by etoposide treatment. Moreover, additional overexpression of JNK1 led to partially antagonize the antiapoptotic environment attained by Bcl-xL, implying that JNK1-involved pathway may play a role in down-regulation of the antiapoptotic effect of Bcl-xL. However, the antagonistic effect of JNK1 on the antiapoptotic action of Bcl-xL was significantly weaker than that on the action of Bcl-2. Interestingly, we found that overexpression of JNK1 led to increase of Bcl-xL expression. Thus, these results suggest that Bcl-xL and Bcl-2 may induce its antiapoptotic effect in a different mechanism, provoking the possibility of involvement of JNK1-involved pathway in Bcl-xL expression.

Molecular cloning and characterization of three sigma glutathione S-transferases from disk abalone (Haliotis discus discus)

Three novel glutathione S-transferase (GSTs) cDNAs were cloned from a disk abalone (Haliotis dicus discus) cDNA library. Multiple alignment and phylogenetic analysis of three GSTs revealed that their closest relationship is with insect sigma GSTs. Recombinant GSTs were over-expressed in Escherichia coli as soluble fusion proteins. HdGSTS1 and HdGSTS2 were active towards 1-chloro-2,4-dinitrobenzene and ethacrynic acid, whereas HdGSTS3 appeared to be a non-enzymatic GST. Two active GSTs had similar optimum conditions for enzymatic reaction at pH 8.0 and temperature of approximately 30 degrees C. Molecular modeling analysis of three GSTs implicates their diverse active sites as being responsible for their different enzymatic features. Three sigma GSTs had significantly different expression patterns and levels of expression in abalone tissues, indicating their different functions. After 48 h-exposure to three model marine pollutants, only HdGSTS1 exhibited a proper inducibility, exhibiting its good biomarker potential for organic contaminants in marine environment. In contrast, the other two sigma GSTs revealed a minor role in the response of pollutants exposure.

Insights into the Catalytic Mechanism of Glutathione S-Transferase: The Lesson from Schistosoma haematobium

Glutathione S-transferases (GSTs) are involved in detoxification of xenobiotic compounds and in the biosynthesis of important metabolites. All GSTs activate glutathione (GSH) to GS(-); in many GSTs, this is accomplished by a Tyr at H-bonding distance from the sulfur of GSH. The high-resolution structure of GST from Schistosoma haematobium revealed that the catalytic Tyr occupies two alternative positions, one external, involving a pi-cation interaction with the conserved Arg21, and the other inside the GSH binding site. The interaction with Arg21 lowers the pK(a) of the catalytic Tyr10, as required for catalysis. Examination of several other GST structures revealed the presence of an external pocket that may accommodate the catalytic Tyr, and suggested that the change in conformation and acidic properties of the catalytic Tyr may be shared by other GSTs. Arginine and two other residues of the external pocket constitute a conserved structural motif, clearly identified by sequence comparison.

Molecular characterization of a glutathione transferase from Pinus tabulaeformis (Pinaceae)

Glutathione transferases (GSTs) play important roles in stress tolerance and detoxification metabolism in plants. To date, studies on GSTs in higher plants have focused largely on agricultural plants. In contrast, there is virtually no information on the molecular characteristics of GSTs in gymnosperms. The present study reports for the first time the cloning, expression and characteristics of a GST gene (PtGSTU1) from a pine, Pinus tabulaeformis, which is widely distributed from northern to central China covering cold temperate and drought regions. The PtGSTU1 gene encodes a protein of 228 amino acid residues with a calculated molecular mass of 26.37 kDa. Reverse transcription PCR revealed that PtGSTU1 was expressed in different tissues, both above and below ground, of P. tabulaeformis. The over-expressed recombinant PtGSTU1 showed high activity towards the substrates 1-chloro-2,4-dinitrobenzene (CDNB) and 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl). Kinetic analysis with respect to CDNB as substrate revealed a Km of 0.47 mM and Vmax of 169.1 micromol/min per mg of protein. The recombinant PtGSTU1 retained more than 60% of its maximum enzymatic activity from 15 degrees C to 45 degrees C with a broad optimum Tm range of 25 degrees C - 35 degrees C. The enzyme had a maximum activity at approximately pH 8.5 - 9.0. Site-directed mutagenesis revealed that Ser13 in the N-terminal domain is a critical catalytic residue, responsible for stabilisation of the thiolate anion of enzyme-bound glutathione. Based on comparative analyses of its amino acid sequence, phylogeny and predicted three-dimensional structure, the PtGSTU1 should be classified as a tau class GST.

Drosophila glutathione S-transferases

The Drosophila glutathione S-transferases (GSTs; EC2.5.1.18) comprise a host of cytosolic proteins that are encoded by a gene superfamily and a homolog of the human microsomal GST. Biochemical studies of certain recombinant GSTs have linked their enzymatic functions to important substrates such as the pesticide DDT and 4-hydroxynonenal, a reactive lipid metabolite. Moreover, a correspondence has been observed between resistance to insecticide substrates-such as DDT-and elevated enzyme levels in resistant strains. Such significant, recurring connections suggest that these gst genes may feature in a model for the development of insecticide resistance. We have amassed substantial biochemical support for relating the overexpression of a particular gst gene to insecticide resistance but are still short of solid genetic evidence to affirm a causal relationship. With the Drosophila system, we have at our disposal genetic and molecular techniques such as p-element mutagenesis and excision, siRNA technology, and versatile transgenic techniques. We can use these methods to effect loss-of-function and gain-of-function conditions and, in these rendered contexts, study other potentially important functions of the gst gene superfamily. An immediate problem that comes to mind is the possible causal relationship between GST substrate specificity and chemical resistance phenotype(s). In this chapter, we present an analysis of selected strategies and laboratory methods that may be useful in pursuing a variety of interesting problems. We will cover three kinds of approaches-biochemistry, genetics, and genomics-as important instruments in a toolkit for studies of the Drosophila gst superfamily. We make the case that these approaches (biochemistry, genetics, and genomics) have helped us gain important insights and can continue to help the community gain a more complete understanding of the biological functions of GSTs. Such knowledge may be key in addressing questions about the detoxification of pesticides and how oxidative stresses affect life span. We hope that these techniques will prove fruitful in studying a host of other physiologic functions as well.

Negative Regulation of MEKK1-induced Signaling by Glutathione S-Transferase Mu

Mitogen-activated protein kinase/extracellular signal-regulated kinase kinase kinase 1 (MEKK1) is an important component in the stress-activated protein kinase pathway. Glutathione S-transferase Mu 1-1 (GST M1-1) has now been shown to inhibit the stimulation of MEKK1 activity induced by cellular stresses such as UV and hydrogen peroxide. GST M1-1 inhibited MEKK1 activation in a manner independent of its glutathione-conjugating catalytic activity. In vitro binding and kinase assays revealed that GST M1-1 directly bound MEKK1 and inhibited its kinase activity. Co-immunoprecipitation analysis showed a physical association between endogenous GST M1-1 and endogenous MEKK1 in L929 cells. Overexpressed GST M1-1 interfered with the binding of MEKK1 to SEK1 in transfected HEK293 cells. Furthermore, GST M1-1 suppressed MEKK1-mediated apoptosis. Taken together, our results suggest that GST M1-1 functions as a negative regulator of MEKK1.

A novel amphibian Pi-class glutathione transferase isoenzyme from Xenopus laevis: importance of phenylalanine 111 in the H-site

Screening of a liver tumour cDNA library from Xenopus laevis resulted in the isolation of a full-length cDNA clone encoding a novel Pi-class amphibian glutathione transferase (GST) isoenzyme (designated as XlGSTP1-1). The gene encodes a protein of 212 amino acids with a calculated molecular mass of 24428 Da. The product of the gene has been overexpressed in Escherichia coli and characterized. XlGSTP1-1 has one of the highest specific activities towards 1-chloro-2,4-dinitrobenzene (1310 micromol/min per mg of protein) obtained with any GST. A notable feature of XlGSTP1-1 is the presence in the H-site of Phe(111) and Pro(208) in place of tyrosine and glycine residues respectively, present in other mammalian Pi-class GSTs. Site-directed mutagenesis indicate that Phe(111) is involved in substrate specificity of XlGSTP1-1. We provide evidence showing that XlGSTP1-1 is present only in the embryo and its expression might be associated with cellular proliferation.

BphK shows dechlorination activity against 4-chlorobenzoate, an end product of bph-promoted degradation of PCBs

A bphK gene encoding glutathione S-transferase (GST) activity is located in the bph operon in Burkholderia sp. strain LB400 but its role in polychlorinated biphenyl (PCB) metabolism is unknown. This gene was over-expressed in Escherichia coli and an in vivo assay based on growth of E. coli containing GST activity was used to identify potential novel substrates for this enzyme. Using this assay, 4-chlorobenzoate (4-CBA) was identified as a substrate for the BphK enzyme. High pressure liquid chromatography analysis and chloride ion detection showed removal of 4-CBA and an equivalent increase of chloride in cell extracts when incubated with this enzyme. These results would indicate that this BphK enzyme has dechlorination activity in relation to 4-CBA and may have a role in protection of other Bph enzymes against certain chlorinated metabolites of PCB degradation.

Mu-class glutathione transferase from Xenopus laevis: molecular cloning, expression and site-directed mutagenesis

A cDNA encoding a Mu-class glutathione transferase (XlGSTM1-1) has been isolated from a Xenopus laevis liver library, and its nucleotide sequence has been determined. XlGSTM1-1 is composed of 219 amino acid residues with a calculated molecular mass of 25359 Da. Unlike many mammalian Mu-class GSTs, XlGSTM1-1 has a narrow spectrum of substrate specificity and it is also less effective in conjugating 1-chloro-2,4-dinitrobenzene. A notable structural feature of XlGSTM1-1 is the presence of the Cys-139 residue in place of the Glu-139, as well as the absence of the Cys-114 residue, present in other Mu-class GSTs, which is replaced by Ala. Site-directed mutagenesis experiments indicate that Cys-139 is not involved in the catalytic mechanism of XlGSTM1-1 but may be in part responsible for its structural instability, and experiments in vivo confirmed the role of this residue in stability. Evidence indicating that Arg-107 is essential for the 1-chloro-2,4-dinitrobenzene conjugation capacity of XlGSTM1-1 is also presented.

Rat glutathione S-transferase M4-4: an isoenzyme with unique structural features including a redox-reactive cysteine-115 residue that forms mixed disulphides with glutathione

Although the existence of the rat glutathione S-transferase (GST) M4 (rGSTM4) gene has been known for some time, the corresponding protein has not as yet been purified from tissue. A recombinant rGSTM4-4 was thus expressed in Escherichia coli from a chemically synthesized rGSTM4 gene. The catalytic efficiency (k(cat)/K(m)) of rGSTM4-4 for the 1-chloro-2,4-dinitrobenzene (CDNB) conjugation reaction was 50-180-fold less than that of the well-characterized homologous rGSTM1-1, and the pH optimum for the same reaction was 8.5 for rGSTM4-4 as opposed to 6.5 for rGSTM1-1. Molecular-modelling studies predict that key substitutions in the helix alpha4 region of rGSTM4-4 account for this pK(a) difference. A notable structural feature of rGSTM4-4 is the Cys-115 residue in place of the Tyr-115 of other Mu-class GSTs. The thiol group of Cys-115 is redox-reactive and readily forms a mixed disulphide even with GSH; the S-glutathiolated form of the enzyme is catalytically active. A mutated rGSTM4-4 (C115Y) had 6-10-fold greater catalytic efficiency than the wild-type rGSTM4-4. Trp-45, a conserved residue among Mu-class GSTs, is essential in rGSTM4-4 for both enzyme activity and binding to glutathione affinity matrices. Antibodies directed against either the unique C-terminal undecapeptide or tridecapeptide of rGSTM4 reacted with rat and mouse liver GSTs to reveal an orthologous mouse GSTM4-4 present at low basal levels but which is inducible in mouse liver. This subclass of rodent Mu GSTs with redox-active Cys-115 residues could have specialized physiological functions in response to oxidative stress.

Dichloromethane mediated in vivo selection and functional characterization of rat glutathione S-transferase theta 1-1 variants

Methylobacterium dichloromethanicum DM4 is able to grow with dichloromethane as the sole carbon and energy source by using a dichloromethane dehalogenase/glutathione S-transferase (GST) for the conversion of dichloromethane to formaldehyde. Mammalian homologs of this bacterial enzyme are also known to catalyze this reaction. However, the dehalogenation of dichloromethane by GST T1-1 from rat was highly mutagenic and toxic to methylotrophic bacteria. Plasmid-driven expression of rat GST T1-1 in strain DM4-2cr, a mutant of strain DM4 lacking dichloromethane dehalogenase, reduced cell viability 10(5)-fold in the presence of dichloromethane. This effect was exploited to select dichloromethane-resistant transconjugants of strain DM4-2cr carrying a plasmid-encoded rGSTT1 gene. Transconjugants that still expressed the GST T1 protein after dichloromethane treatment included rGSTT1 mutants encoding protein variants with sequence changes from the wild-type ranging from single residue exchanges to large insertions and deletions. A structural model of rat GST T1-1 suggested that sequence variation was clustered around the glutathione activation site and at the protein C-terminus believed to cap the active site. The enzymatic activity of purified His-tagged GST T1-1 variants expressed in Escherichia coli was markedly reduced with both dichloromethane and the alternative substrate 1,2-epoxy-3-(4'-nitrophenoxy)propane. These results provide the first experimental evidence for the involvement of Gln102 and Arg107 in catalysis, and illustrate the potential of in vivo approaches to identify catalytic residues in GSTs whose activity leads to toxic effects.

Molecular cloning, expression and site-directed mutagenesis of glutathione S-transferase from Ochrobactrum anthropi

The gene coding for a novel glutathione S-transferase (GST) has been isolated from the bacterium Ochrobactrum anthropi. A PCR fragment of 230 bp was obtained using oligonucleotide primers deduced from N-terminal and 'internal' sequences of the purified enzyme. The gene was obtained by screening of a genomic DNA partial library from O. anthropi constructed in pBluescript with a PCR fragment probe. The gene encodes a protein (OaGST) of 201 amino acids with a calculated molecular mass of 21738 Da. The product of the gene was expressed and characterized; it showed GST activity with substrates 1-chloro-2, 4-dinitrobenzene (CDNB), p-nitrobenzyl chloride and 4-nitroquinoline 1-oxide, and glutathione-dependent peroxidase activity towards cumene hydroperoxide. The overexpressed product of the gene was also confirmed to have in vivo GST activity towards CDNB. The interaction of the recombinant GST with several antibiotics indicated that the enzyme is involved in the binding of rifamycin and tetracycline. The OaGST amino acid sequence showed the greatest identity (45%) with a GST from Pseudomonas sp. strain LB400. A serine residue in the N-terminal region is conserved in almost all known bacterial GSTs, and it appears to be the counterpart of the catalytic serine residue present in Theta-class GSTs. Substitution of the Ser-11 residue resulted in a mutant OaGST protein lacking CDNB-conjugating activity; moreover the mutant enzyme was not able to bind Sepharose-GSH affinity matrices.

Heterocyst pattern formation controlled by a diffusible peptide

Many filamentous cyanobacteria grow as multicellular organisms that show a developmental pattern of single nitrogen-fixing heterocysts separated by approximately 10 vegetative cells. Overexpression of a 54-base-pair gene, patS, blocked heterocyst differentiation in Anabaena sp. strain PCC 7120. A patS null mutant showed an increased frequency of heterocysts and an abnormal pattern. Expression of a patS-gfp reporter was localized in developing proheterocysts. The addition of a synthetic peptide corresponding to the last five amino acids of PatS inhibited heterocyst development. PatS appears to control heterocyst pattern formation through intercellular signaling mechanisms.

One face of a transmembrane helix is crucial in mechanosensitive channel gating

MscL is a mechanosensitive channel in bacteria that responds directly to membrane tension by opening a large conductance pore. To determine functionally important residues within this molecule, we have randomly mutagenized mscL, expressed the genes in living bacteria, and screened for gain-of-function mutants with hampered growth. Expression of these genes caused leakage of cytoplasmic solutes on little or no hypo-osmotic stress. In excised patches, the mutant channels gated at membrane tensions that are less than that required for the gating of the wild-type MscL. Hence, the data suggest that the slowed or no-growth phenotype is caused by solute loss because of inappropriate gating of the channel. Most of the mutations mapped to the first transmembrane domain. When this domain is modeled as an alpha-helix, the most severe mutations are substitutions of smaller amino acids (three glycines and one valine) on one facet, suggesting an important role for this structure in MS channel gating.

Random mutagenesis reveals a region important for gating of the yeast K+ channel Ykc1

YKC1 (TOK1, DUK1, YORK) encodes the outwardly rectifying K+ channel of the yeast plasma membrane. Non-targeted mutations of YKC1 were isolated by their ability to completely block proliferation when expressed in yeast. All such mutations examined occurred near the cytoplasmic ends of the transmembrane segments following either of the duplicated P loops, which we termed the 'post-P loop' (PP) regions. These PP mutations specifically caused marked defects in the 'C1' states, a set of interrelated closed states that Ykc1 enters and exits at rates of tens to hundreds of milliseconds. These results indicate that the Ykc1 PP region plays a role in determining closed state conformations and that non-targeted mutagenesis and microbial selection can be a valuable tool for probing structure-function relationships of ion channels.

THE FUNCTIONS AND REGULATION OF GLUTATHIONE S-TRANSFERASES IN PLANTS

Glutathione S-transferases (GSTs) play roles in both normal cellular metabolism as well as in the detoxification of a wide variety of xenobiotic compounds, and they have been intensively studied with regard to herbicide detoxification in plants. A newly discovered plant GST subclass has been implicated in numerous stress responses, including those arising from pathogen attack, oxidative stress, and heavy-metal toxicity. In addition, plant GSTs play a role in the cellular response to auxins and during the normal metabolism of plant secondary products like anthocyanins and cinnamic acid. This review presents the current knowledge about the functions of GSTs in regard to both herbicides and endogenous substrates. The catalytic mechanism of GST activity as well as the fate of glutathione S-conjugates are reviewed. Finally, a summary of what is known about the gene structure and regulation of plant GSTs is presented.

The glutathione S-transferase supergene family: regulation of GST and the contribution of the isoenzymes to cancer chemoprotection and drug resistance

The glutathione S-transferases (GST) represent a major group of detoxification enzymes. All eukaryotic species possess multiple cytosolic and membrane-bound GST isoenzymes, each of which displays distinct catalytic as well as noncatalytic binding properties: the cytosolic enzymes are encoded by at least five distantly related gene families (designated class alpha, mu, pi, sigma, and theta GST), whereas the membrane-bound enzymes, microsomal GST and leukotriene C4 synthetase, are encoded by single genes and both have arisen separately from the soluble GST. Evidence suggests that the level of expression of GST is a crucial factor in determining the sensitivity of cells to a broad spectrum of toxic chemicals. In this article the biochemical functions of GST are described to show how individual isoenzymes contribute to resistance to carcinogens, antitumor drugs, environmental pollutants, and products of oxidative stress. A description of the mechanisms of transcriptional and posttranscriptional regulation of GST isoenzymes is provided to allow identification of factors that may modulate resistance to specific noxious chemicals. The most abundant mammalian GST are the class alpha, mu, and pi enzymes and their regulation has been studied in detail. The biological control of these families is complex as they exhibit sex-, age-, tissue-, species-, and tumor-specific patterns of expression. In addition, GST are regulated by a structurally diverse range of xenobiotics and, to date, at least 100 chemicals have been identified that induce GST; a significant number of these chemical inducers occur naturally and, as they are found as nonnutrient components in vegetables and citrus fruits, it is apparent that humans are likely to be exposed regularly to such compounds. Many inducers, but not all, effect transcriptional activation of GST genes through either the antioxidant-responsive element (ARE), the xenobiotic-responsive element (XRE), the GST P enhancer 1(GPE), or the glucocorticoid-responsive element (GRE). Barbiturates may transcriptionally activate GST through a Barbie box element. The involvement of the Ah-receptor, Maf, Nrl, Jun, Fos, and NF-kappa B in GST induction is discussed. Many of the compounds that induce GST are themselves substrates for these enzymes, or are metabolized (by cytochrome P-450 monooxygenases) to compounds that can serve as GST substrates, suggesting that GST induction represents part of an adaptive response mechanism to chemical stress caused by electrophiles. It also appears probable that GST are regulated in vivo by reactive oxygen species (ROS), because not only are some of the most potent inducers capable of generating free radicals by redox-cycling, but H2O2 has been shown to induce GST in plant and mammalian cells: induction of GST by ROS would appear to represent an adaptive response as these enzymes detoxify some of the toxic carbonyl-, peroxide-, and epoxide-containing metabolites produced within the cell by oxidative stress. Class alpha, mu, and pi GST isoenzymes are overexpressed in rat hepatic preneoplastic nodules and the increased levels of these enzymes are believed to contribute to the multidrug-resistant phenotype observed in these lesions. The majority of human tumors and human tumor cell lines express significant amounts of class pi GST. Cell lines selected in vitro for resistance to anticancer drugs frequently overexpress class pi GST, although overexpression of class alpha and mu isoenzymes is also often observed. The mechanisms responsible for overexpression of GST include transcriptional activation, stabilization of either mRNA or protein, and gene amplification. In humans, marked interindividual differences exist in the expression of class alpha, mu, and theta GST. The molecular basis for the variation in class alpha GST is not known. (ABSTRACT TRUNCATED)