Immunogold localization of glutathione transferase B1-1 in Proteus mirabilis

BBE31 from the Lyme disease agent Borrelia burgdorferi, known to play an important role in successful colonization of the mammalian host, shows the ability to bind glutathione

Lyme disease is a tick-borne infection caused by Borrelia burgdorferi sensu lato complex spirochetes. The spirochete is located in the gut of the tick; as the infected tick starts the blood meal, the spirochete must travel through the hemolymph to the salivary glands, where it can spread to and infect the new host organism. In this study, we determined the crystal structures of the key outer surface protein BBE31 from B. burgdorferi and its orthologous protein BSE31 (BSPA14S_RS05060 gene product) from B. spielmanii. BBE31 is known to be important for the transfer of B. burgdorferi from the gut to the hemolymph in the tick after a tick bite. While BBE31 exerts its function by interacting with the Ixodes scapularis tick gut protein TRE31, structural and mass spectrometry data revealed that BBE31 has a glutathione (GSH) covalently attached to Cys142 suggesting that the protein may have acquired some additional functions in contrast to its orthologous protein BSE31, which lacks any interactions with GSH. In the current study, in addition to analyzing the potential reasons for GSH binding, the three-dimensional structure of BBE31 provides new insights into the molecular details of the transmission process as the protein plays an important role in the initial phase before the spirochete is physically transferred to the new host. This knowledge will be potentially used for the development of new strategies to fight against Lyme disease.

Glutathione transferases in bacteria

Bacterial glutathione transferases (GSTs) are part of a superfamily of enzymes that play a key role in cellular detoxification. GSTs are widely distributed in prokaryotes and are grouped into several classes. Bacterial GSTs are implicated in a variety of distinct processes such as the biodegradation of xenobiotics, protection against chemical and oxidative stresses and antimicrobial drug resistance. In addition to their role in detoxification, bacterial GSTs are also involved in a variety of distinct metabolic processes such as the biotransformation of dichloromethane, the degradation of lignin and atrazine, and the reductive dechlorination of pentachlorophenol. This review article summarizes the current status of knowledge regarding the functional and structural properties of bacterial GSTs.

Expression of glutathioneS-transferase and peptide methionine sulphoxide reductase inOchrobactrum anthropiis correlated to the production of reactive oxygen species caused by aromatic substrates

Peptide methionine sulphoxide reductase (MsrA) and glutathione S-transferases (GSTs) are considered as detoxification enzymes. In the xenobiotics-degrading bacterium Ochrobactrum anthropi the two enzymes are co-induced by toxic concentrations of aromatic substrates such as phenol and 4-chlorophenol. In aerobic organisms, degradation of aromatic substrates by mono- and dioxygenases leads to a generation of oxidative stress that causes the occurrence of reactive oxygen species (ROS). A capillary electrophoretic method, using the intracellular conversion of dihydrorhodamine-123 into rhodamine-123, was developed to measure the content of ROS in the bacteria. The presence of toxic concentrations of the aromatic substrate 4-chlorophenol, an inducer of GST and MsrA, leads to a significant increase in the production of ROS. These results strongly suggest that GST and MsrA enzymes are part of the bacterial defence mechanism against particular oxidative stress conditions. As oxidative stress is known to be present predominantly close to the cytoplasmic membrane, we investigated the subcellular distribution of both MsrA and GST enzymes in this bacterium grown in the presence of 4-chlorophenol. By Western blotting, MsrA and GST was assayed in the cytoplasm as well as in the periplasm. Moreover, immunolocalisation by colloidal gold immunoelectron microscopy identified the two proteins associated with the cell envelope.

Proteus mirabilis glutathione S-transferase B1-1 is involved in protective mechanisms against oxidative and chemical stresses

We investigated the effects of several xenobiotics, including antimicrobial agents and general stress factors such as starvation, heat and osmotic shock, on the modulation of expression of Proteus mirabilis glutathione S-transferase B1-1 (PmGST B1-1). The level of expression of PmGST B1-1 was established by both Western- and Northern-blot experiments. Our results show that several compounds can modulate expression of PmGST B1-1. The level of PmGST B1-1 increased when bacterial cells were exposed to a variety of stresses such as 1-chloro-2,4-dinitrobenzene, H(2)O(2), fosfomycin or tetracycline. A knock-out gst B gene was also constructed using the suicide vector pKNOCKlox-Ap. Successful inactivation of the wild-type gene was confirmed by PCR, DNA sequence analysis and Western blotting. Under normal culture conditions, this mutant was viable and displayed no significant phenotypic differences compared with the wild-type. However, viability tests revealed that the null mutant was more sensitive to oxidative stress in the form of H(2)O(2) and to several antimicrobial drugs when compared with the wild-type. These results suggest that PmGST B1-1 has an active role in the protection against oxidative stress generated by H(2)O(2) and it appears to be involved in the detoxification of antimicrobial agents.

GSTB1-1 from Proteus mirabilis: a snapshot of an enzyme in the evolutionary pathway from a redox enzyme to a conjugating enzyme

The native form of the bacterial glutathione transferase B1-1 (EC ) is characterized by one glutathione (GSH) molecule covalently linked to Cys-10. This peculiar disulfide, only found in the Beta and Omega class glutathione S-transferases (GSTs) but absent in all other GSTs, prompts questions about its role and how GSH can be activated and utilized in the reaction normally performed by GSTs. Stopped-flow and spectroscopic experiments suggest that, in the native enzyme (GSTB1-1ox), a second GSH molecule is present, albeit transiently, in the active site. This second GSH binds to the enzyme through a bimolecular interaction followed by a fast thiol-disulfide exchange with the covalently bound GSH. The apparent pK(a) of the non-covalently bound GSH is lowered from 9.0 to 6.4 +/- 0.2 in similar fashion to other GSTs. The reduced form of GSTB1-1 (GSTB1-1red) binds GSH 100-fold faster and also induces a more active deprotonation of the substrate with an apparent pK(a) of 5.2 +/- 0.1. Apparently, the absence of the mixed disulfide does not affect k(cat) and K(m) values in the GST conjugation activity, which is rate-limited by the chemical step both in GSTB1-1red and in GSTB1-1ox. However, GSTB1-1ox follows a steady-state random sequential mechanism whereas a rapid-equilibrium random sequential mechanism is adopted by GSTB1-1red. Remarkably, GSTB1-1ox and GSTB1-1red are equally able to catalyze a glutaredoxin-like catalysis using cysteine S-sulfate and hydroxyethyl disulfide as substrates. Cys-10 is an essential residue in this redox activity, and its replacement by alanine abolishes this enzymatic activity completely. It appears that GSTB1-1 behaves like an "intermediate enzyme" between the thiol-disulfide oxidoreductase and the GST superfamilies.

Properties and utility of the peculiar mixed disulfide in the bacterial glutathione transferase B1-1

Bacterial glutathione transferases appear to represent an evolutionary link between the thiol:disulfide oxidoreductase and glutathione transferase superfamilies. In particular, the observation of a mixed disulfide in the active site of Proteus mirabilis glutathione transferase B1-1 is a feature that links the two families. This peculiar mixed disulfide between Cys10 and one GSH molecule has been studied by means of ESR spectroscopy, stopped-flow kinetic analysis, radiochemistry, and site-directed mutagenesis. This disulfide can be reduced by dithiothreitol but even a thousand molar excess of GSH is poorly effective due to an unfavorable equilibrium constant of the redox reaction (K(eq) = 2 x 10(-4)). Although Cys10 is partially buried in the crystal structure, in solution it reacts with several thiol reagents at a higher or comparable rate than that shown by the free cysteine. Kinetics of the reaction of Cys10 with 4,4'-dithiodipyridine at variable pH values is consistent with a pK(a) of 8.0 +/- 0.1 for this residue, a value about 1 unit lower than that of the free cysteine. The 4,4'-dithiodipyridine-modified enzyme reacts with GSH in a two-step mechanism involving a fast precomplex formation, followed by a slower chemical step. The natural Cys10-GSH mixed disulfide exchanges rapidly with free [3H]GSH in a futile redox cycle in which the bound GSH is continuously replaced by the external GSH. Our data suggest that the active site of the bacterial enzyme has intermediate properties between those of the recently evolved glutathione transferases and those of the thiol:disulfide oxidoreductase superfamily.

Functional analysis of the evolutionarily conserved proline 53 residue in Proteus mirabilis glutathione transferase B1-1

The role of the evolutionarily conserved residue Pro-53 in Proteus mirabilis glutathione transferase B1-1 has been examined by replacing it with a serine residue using site-directed mutagenesis. The effect of the replacement on the activity, thermal stability and antibiotic binding capacity of the enzyme was examined. The results presented support the view that Pro-53 participates in the maintenance of the proper conformation of the enzyme fold rather than playing a direct role in the catalytic reaction. Furthermore, this residue appears to be an important determinant of the antibiotic binding to the enzyme. Experiments with wild type and mutated enzymes provide evidence that glutathione transferases may play an important role in antibiotic resistance exhibited by bacteria.

A mixed disulfide bond in bacterial glutathione transferase: functional and evolutionary implications

BACKGROUND

Glutathione S-transferases (GSTs) are a multifunctional group of enzymes, widely distributed in aerobic organisms, that have a critical role in the cellular detoxification process. Unlike their mammalian counterparts, bacterial GSTs often catalyze quite specific reactions, suggesting that their roles in bacteria might be different. The GST from Proteus mirabilis (PmGST B1-1) is known to bind certain antibiotics tightly and reduce the antimicrobial activity of beta-lactam drugs. Hence, bacterial GSTs may play a part in bacterial resistance towards antibiotics and are the subject of intense interest.

RESULTS

Here we present the structure of a bacterial GST, PmGST B1-1, which has been determined from two different crystal forms. The enzyme adopts the canonical GST fold although it shares less than 20% sequence identity with GSTs from higher organisms. The most surprising aspect of the structure is the observation that the substrate, glutathione, is covalently bound to Cys 10 of the enzyme. In addition, the highly structurally conserved N-terminal domain is found to have an additional beta strand.

CONCLUSIONS

The crystal structure of PmGST B1-1 has highlighted the importance of a cysteine residue in the catalytic cycle. Sequence analyses suggest that a number of other GSTs share this property, leading us to propose a new class of GSTs - the beta class. The data suggest that the in vivo role of the beta class GSTs could be as metabolic or redox enzymes rather than conjugating enzymes. Compelling evidence is presented that the theta class of GSTs evolved from an ancestral member of the thioredoxin superfamily.

Effect of anaerobic environment on the glutathione transferase isoenzymatic pattern in Proteus mirabilis

When Proteus mirabilis was cultured anaerobically in the presence of nitrate as terminal electron acceptor, a dramatic reduction of glutathione transferase production occurred. The analysis of the glutathione affinity purified materials in terms of substrate specificity, SDS-PAGE pattern, IEF pattern and immunoblotting revealed that a significantly different glutathione transferase pattern also occurred: two new glutathione transferase forms with an isoelectric point at pH 4.8 and 5.0 appeared. Their N-terminal amino acid sequence analysis as well as the ability to bind to a glutathione affinity column indicate that major differences between anaerobic and aerobic glutathione transferase forms are mainly located in the C-terminal region of the primary structure. In contrast, no significant changes occurred in the production of glutathione transferase isoenzymes when P. mirabilis was grown anaerobically in the absence of a terminal electron acceptor. These results support the idea that bacterial glutathione transferase expression is not strictly related to the absence of oxygen stress.

Analysis of a Rhizobium leguminosarum gene encoding a protein homologous to glutathione S-transferases

A novel Rhizobium leguminosarum gene, gstA, the sequence of which indicated that it was a member of the gene family of glutathione S-transferases (GSTs), was identified. The homology was greatest to the GST enzymes of higher plants. The Rhizobium gstA gene was normally expressed at a very low level. The product of gstA was over-expressed and purified from Escherichia coli. It was shown to bind to the affinity matrix glutathione-Sepharose, but no enzymic GST activity with 1-chloro-2,4-dinitrobenzene as substrate was detected. gstA encoded a polypeptide of 203 amino acid residues with a calculated molecular mass of 21990 Da. Transcribed divergently from gstA is another gene, gstR, which was similar in sequence to the LysR family of bacterial transcriptional regulators. A mutation in gstR had no effect on the transcription of itself or gstA under the growth conditions used here. Mutations in gstA and gstR caused no obvious phenotypic defect and the biological functions of these genes remain to be determined.

Molecular cloning and overexpression of a glutathione transferase gene from Proteus mirabilis

The structural gene of the Proteus mirabilis glutathione transferase GSTB1-1 (gstB) has been isolated from genomic DNA. A nucleotide sequence determination of gstB predicted a translational product of 203 amino acid residues, perfectly matching the sequence of the previously purified protein [Mignogna, Allocati, Aceto, Piccolomini, Di Ilio, Barra and Martini (1993) Eur. J. Biochem. 211, 421-425]. The P. mirabilis GST sequence revealed 56% identity with the Escherichia coli GST at DNA level and 54% amino acid identity. Similarity has been revealed also with the translation products of the recently cloned gene bphH from Haemophilus influenzae (28% identity) and ORF3 of Burkholderia cepacia (27% identity). Putative promoter sequences with high similarity to the E. coli sigma 70 consensus promoter and to promoters of P. mirabilis cat and glnA genes preceded the ATG of the gstB open reading frame (ORF). gstB was brought under control of the tac promoter and overexpressed in E. coli by induction with isopropyl-beta-D-thiogalactopyranoside and growth at 37 degrees C. The physicochemical and catalytic properties of overexpressed protein were indistinguishable from those of the enzyme purified from P. mirabilis extract. Unlike the GST belonging to Mu and Theta classes, GSTB1-1 was unable to metabolize dichloromethane. The study of the interaction of cloned GSTB1-1 with a number of antibiotics indicates that this enzyme actively participates in the binding of tetracyclines and rifamycin.