Structural evidence for Arabidopsis glutathione transferase AtGSTF2 functioning as a transporter of small organic ligands

Plasticity of the Arabidopsis leaf lipidome and proteome in response to pathogen infection and heat stress

Plants must cope with a variety of stressors during their life cycle, and the adaptive responses to these environmental cues involve all cellular organelles. Among them, comparatively little is known about the contribution of cytosolic lipid droplets (LDs) and their core set of neutral lipids and associated surface proteins to the rewiring of cellular processes in response to stress. Here, we analyzed the changes that occur in the lipidome and proteome of Arabidopsis (Arabidopsis thaliana) leaves after pathogen infection with Botrytis cinerea or Pseudomonas syringae, or after heat stress. Analyses were carried out in wild-type plants and the oil-rich double mutant trigalactosyldiacylglycerol1-1 sugar dependent 1-4 (tgd1-1 sdp1-4) that allowed for an allied study of the LD proteome in stressed leaves. Using liquid chromatography-tandem mass spectrometry-based methods, we showed that a hyperaccumulation of the primary LD core lipid TAG is a general response to stress and that acyl chain and sterol composition are remodeled during cellular adaptation. Likewise, comparative analysis of the LD protein composition in stress-treated leaves highlighted the plasticity of the LD proteome as part of the general stress response. We further identified at least two additional LD-associated proteins, whose localization to LDs in leaves was confirmed by confocal microscopy of fluorescent protein fusions. Taken together, these results highlight LDs as dynamic contributors to the cellular adaptation processes that underlie how plants respond to environmental stress.

Theoretical investigations of some isolated compounds from Calophyllum flavoramulum as potential antioxidant agents and inhibitors of AGEs

In this paper, we have attempted a theoretical calculation of some plant-isolated compounds as potential inhibitors of oxidative stress and Advanced Glycation Endproducts (AGEs). Herein, theoretical reactivity indices based on the CDFT theory were computed to explore the reactivity of five isolated products from Calophyllum flavoramulum. Global reactivity indices based on HOMO and LUMO energy such as electronic chemical potential, hardness, electrophilicity and the local reactivity descriptors Parr function, molecular electrostatic potentials(MEP), electrostatic potential (ESP) and thermodynamic parameters for the studied compounds are computed and discussed using DFT method and two functionals B3LYP and CAM-B3LYP with 6-31 G(d,p) basis set. The free radical scavenging activity mechanisms (HAT, SET-PT, and SPLET) of some of the isolated products with DPPH are also presented in this work. SET-PT mechanism of the antiradical activity is found to be thermodynamically favorable. Furthermore, a molecular docking study with RAGE receptor and AtGSTF2 enzyme was conducted, in which flavonoids 4 and 5 show a low binding affinity with -8.42 and -10.49 kcal/mol for RAGE, -8.67 and -9.00 kcal/mol for AtGSTF2. After the encouraging outcomes from the molecular docking study, the 4-AtGSTF2 and 5-RAGE complex were subjected to 200 ns molecular dynamics simulation using Desmond, where both studied systems exhibited remarkable stability throughout the 200 ns simulations. Also, the MM-GBSA method was measured by calculating the binding free energy using the individual energy components. Finally, the ADMET predictions were assessed to anticipate the behavior of a drug candidate within the human body.

Overlooked and misunderstood: can glutathione conjugates be clues to understanding plant glutathione transferases?

Plant glutathione transferases (GSTs) constitute a large and diverse family of enzymes that are involved in plant stress response, metabolism and defence, yet their physiological functions remain largely elusive. Consistent with the traditional view on GSTs across organisms as detoxification enzymes, in vitro most plant GSTs catalyse glutathionylation, conjugation of the tripeptide glutathione (GSH; γ-Glu-Cys-Gly) onto reactive molecules. However, when it comes to elucidating GST functions, it remains a key challenge that the endogenous plant glutathione conjugates (GS-conjugates) that would result from such glutathionylation reactions are rarely reported. Furthermore, GSTs often display high substrate promiscuity, and their proposed substrates are prone to spontaneous chemical reactions with GSH; hence, single-gene knockouts rarely provide clear chemotypes or phenotypes. In a few cases, GS-conjugates are demonstrated to be biosynthetic intermediates that are rapidly further metabolized towards a pathway end product, explaining their low abundance and rare detection. In this review, we summarize the current knowledge of plant GST functions and how and possibly why evolution has resulted in a broad and extensive expansion of the plant GST family. Finally, we demonstrate that endogenous GS-conjugates are more prevalent in plants than assumed and suggest they are overlooked as clues towards the identification of plant GST functions. This article is part of the theme issue 'The evolution of plant metabolism'.

Regulation of the trade-off between cold stress and growth by glutathione S-transferase phi class 10 (BcGSTF10) in non-heading Chinese cabbage

Cold stress is a serious threat to global crop production and food security, but plant cold resistance is accompanied by reductions in growth and yield. In this study, we determined that the novel gene BcGSTF10 in non-heading Chinese cabbage [NHCC; Brassica campestris (syn. Brassica rapa) ssp. chinensis] is implicated in resistance to cold stress. Biochemical and genetic analyses demonstrated that BcGSTF10 interacts with BcICE1 to induce C-REPEAT BINDING FACTOR (CBF) genes that enhance freezing tolerance in NHCC and in Arabidopsis. However, BcCBF2 represses BcGSTF10 and the latter promotes growth in NHCC and Arabidopsis. This dual function of BcGSTF10 indicates its pivotal role in balancing cold stress and growth, and this important understanding has the potential to inform the future development of strategies to breed crops that are both climate-resilient and high-yielding.

The potential link between Covid-19 and multiple myeloma: A new saga

BACKGROUND

Covid-19 is considered a primary respiratory disease-causing viral pneumonia and, in severe cases, leads to acute lung injury and acute respiratory distress syndrome (ARDS). In addition, though, extra-pulmonary manifestations of Covid-19 have been shown. Furthermore, severe acute respiratory distress syndrome coronavirus type 2 (SARS-CoV-2) infection may coexist with several malignancies, including multiple myeloma (MM).

METHODS

This critical literature review aimed to find the potential association between SARS-CoV-2 infection and MM in Covid-19 patients with underlying MM. Narrative literature and databases search revealed that ARDS is developed in both MM and Covid-19 due to hypercalcemia and proteasome dysfunction.

RESULTS

Notably, the expression of angiogenic factors and glutamine deficiency could link Covid-19 severity and MM in the pathogenesis of cardiovascular complications. MM and Covid-19 share thrombosis as a typical complication; unlike thrombosis in Covid-19, which reflects disease severity, thrombosis does not reflect disease severity in MM. In both conditions, thromboprophylaxis is essential to prevent pulmonary thrombosis and other thromboembolic disorders. Moreover, Covid-19 may exacerbate the development of acute kidney injury and neurological complications in MM patients.

CONCLUSION

These findings highlighted that MM patients might be a risk group for Covid-19 severity due to underlying immunosuppression and most of those patients need specific management in the Covid-19 era.

Biochemical and Structural Insights on the Poplar Tau Glutathione Transferase GSTU19 and 20 Paralogs Binding Flavonoids

Glutathione transferases (GSTs) constitute a widespread superfamily of enzymes notably involved in xenobiotic detoxification and/or in specialized metabolism. Populus trichocarpa genome (V4.1 assembly, Phytozome 13) consists of 74 genes coding for full-length GSTs and ten likely pseudogenes. These GSTs are divided into 11 classes, in which the tau class (GSTU) is the most abundant with 54 isoforms. PtGSTU19 and 20, two paralogs sharing more than 91% sequence identity (95% of sequence similarity), would have diverged from a common ancestor of P. trichocarpa and P. yatungensis species. These enzymes display the distinctive glutathione (GSH)-conjugation and peroxidase activities against model substrates. The resolution of the crystal structures of these proteins revealed significant structural differences despite their high sequence identity. PtGSTU20 has a well-defined deep pocket in the active site whereas the bottom of this pocket is disordered in PtGSTU19. In a screen of potential ligands, we were able to identify an interaction with flavonoids. Some of them, previously identified in poplar (chrysin, galangin, and pinocembrin), inhibited GSH-conjugation activity of both enzymes with a more pronounced effect on PtGSTU20. The crystal structures of PtGSTU20 complexed with these molecules provide evidence for their potential involvement in flavonoid transport in P. trichocarpa.

Molecular cloning and oxidative-stress responses of a novel Phi class glutathione S-transferase (GSTF) gene in the freshwater algae Closterium ehrenbergii

Glutathione S-transferases (GSTs) belong to a family of enzymes involved in diverse biological processes, including detoxification and protection against oxidative damage. Here, we determined the full-length sequence (915 bp) of a novel Phi class cytosolic glutathione S-transferase (GSTF) gene from the green algae Closterium ehrenbergii. We examined the gene structure and expression patterns in response to metals and endocrine disrupting chemicals (EDCs). It was significantly upregulated by metals, but responded differently to EDCs. The highest up-regulation of CeGSTF was registered under 0.1 mg/L CuCl₂ and 0.01 mg/L CuSO₄ treatments. In a 72-h course experiment with treatment of 0.1 mg/L CuCl₂ , CeGSTF was dramatically induced at 6 h, and then gradually decreased with increasing exposure time. This was consistent with the increase in both GST activity and ROS production in copper-treated cells. These results suggest that CeGSTF may be involved in detoxification mechanisms associated with oxidative stress in green algae.

Directed Evolution of a Glutathione Transferase for the Development of a Biosensor for Alachlor Determination

In the present work, DNA recombination of three homologous tau class glutathione transferases (GSTUs) allowed the creation of a library of tau class GmGSTUs. The library was activity screened for the identification of glutathione transferase (GST) variants with enhanced catalytic activity towards the herbicide alachlor (2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide). One enzyme variant (GmGSTsf) with improved catalytic activity and binding affinity for alachlor was identified and explored for the development of an optical biosensor for alachlor determination. Kinetics analysis and molecular modeling studies revealed a key mutation (Ile69Val) at the subunit interface (helix α3) that appeared to be responsible for the altered catalytic properties. The enzyme was immobilized directly on polyvinylidenefluoride membrane by crosslinking with glutaraldehyde and was placed on the inner surface of a plastic cuvette. The rate of pH changes observed as a result of the enzyme reaction was followed optometrically using a pH indicator. A calibration curve indicated that the linear concentration range for alachlor was 30–300 μM. The approach used in the present study can provide tools for the generation of novel enzymes for eco-efficient and environment-friendly analytical technologies. In addition, the outcome of this study gives an example for harnessing protein symmetry for enzyme design.

Diversity of Omega Glutathione Transferases in mushroom-forming fungi revealed by phylogenetic, transcriptomic, biochemical and structural approaches

The Omega class of glutathione transferases (GSTs) forms a distinct class within the cytosolic GST superfamily because most of them possess a catalytic cysteine residue. The human GST Omega 1 isoform was first characterized twenty years ago, but it took years of work to clarify the roles of the human isoforms. Concerning the kingdom of fungi, little is known about the cellular functions of Omega glutathione transferases (GSTOs), although they are widely represented in some of these organisms. In this study, we re-assess the phylogeny and the classification of GSTOs based on 240 genomes of mushroom-forming fungi (Agaricomycetes). We observe that the number of GSTOs is not only extended in the order of Polyporales but also in other orders such as Boletales. Our analysis leads to a new classification in which the fungal GSTOs are divided into two Types A and B. The catalytic residue of Type-A is either cysteine or serine, while that of Type-B is cysteine. The present study focuses on Trametes versicolor GSTO isoforms that possess a catalytic cysteine residue. Transcriptomic data show that Type-A GSTOs are constitutive enzymes while Type-B are inducible ones. The crystallographic analysis reveals substantial structural differences between the two types while they have similar biochemical profiles in the tested conditions. Additionally, these enzymes have the ability to bind antioxidant molecules such as wood polyphenols in two possible binding sites as observed from X-ray structures. The multiplication of GSTOs could allow fungal organisms to adapt more easily to new environments.

Phi class glutathione transferases as molecular targets towards multiple-herbicide resistance: Inhibition analysis and pharmacophore design

Multiple-herbicide resistance (MHR) is a global threat to weed control in cereal crops. MHR weeds express a specific phi class glutathione transferase (MHR-GSTF) that confers resistance against multiple herbicides and therefore represents a promising target against MHR weeds. Kinetics inhibition analysis of MHR-GSTFs from grass weeds Lolium rigidum (LrGSTF) Alopecurus myosuroides (AmGSTF) and crops Hordeum vulgare (HvGSTF) and Triticum aestivum (TaGSTF) allowed the identification of the acetanilide herbicide butachlor as a potent and selective inhibitor towards MHR-GSTFs. Also, butachlor is a stronger inhibitor for LrGSTF and AmGSTF compared to HvGSTF and TaGSTF from crops. The crystal structure of LrGSTF was determined at 1.90 Å resolution in complex with the inhibitor S-(4-nitrobenzyl)glutathione. A specific 3D pharmacophore targeting the MHR-GSTFs was designed and used to identify structural elements important for potent and selective inhibition. Structural analysis of GSTFs revealed a decisive role of conserved Tyr118 in ligand binding and pharmacophore design. Its positioning is dependent on an outer patch of adjacent residues that span from position 132 to 134 which are similar for both LrGSTF and AmGSTF but different in HvGSTF and TaGSTF. The results presented here provide new knowledge that may be adopted to cope with MHR weeds.

Glutathione S-transferase: a versatile protein family

Glutathione-S transferase (GST) is a most ancient protein superfamily of multipurpose roles and evolved principally from gene duplication of an ancestral GSH binding protein. They have implemented in diverse plant functions such as detoxification of xenobiotic, secondary metabolism, growth and development, and majorly against biotic and abiotic stresses. The vital structural features of GSTs like highly divergent functional topographies, conserved integrated architecture with separate binding pockets for substrates and ligand, the stringent structural fidelity with high Tm values (50º-60º), and stress-responsive cis-regulatory elements in the promoter region offer this protein as most flexible plant protein for plant breeding approaches, biotechnological applications, etc. This review article summarizes the recent information of GST evolution, and their distribution and structural features with emphasis on the assorted roles of Ser and Cys GSTs with the signature motifs in their active sites, alongside their recent biotechnological application in the area of agriculture, environment, and nanotechnology have been highlighted.

Is there a role for tau glutathione transferases in tetrapyrrole metabolism and retrograde signalling in plants?

In plants, tetrapyrrole biosynthesis occurs in chloroplasts, the reactions being catalysed by stromal and membrane-bound enzymes. The tetrapyrrole moiety is a backbone for chlorophylls and cofactors such as sirohaems, haems and phytochromobilins. Owing to this diversity, the potential cytotoxicity of some precursors and the associated synthesis costs, a tight control exists to adjust the demand and the fluxes for each molecule. After synthesis, haems and phytochromobilins are incorporated into proteins found in other subcellular compartments. However, there is only very limited information about the chaperones and membrane transporters involved in the trafficking of these molecules. After summarizing evidence indicating that glutathione transferases (GST) may be part of the transport and/or degradation processes of porphyrin derivatives, we provide experimental data indicating that tau glutathione transferases (GSTU) bind protoporphyrin IX and haem moieties and use structural modelling to identify possible residues responsible for their binding in the active site hydrophobic pocket. Finally, we discuss the possible roles associated with the binding, catalytic transformation (i.e. glutathione conjugation) and/or transport of tetrapyrroles by GSTUs, considering their subcellular localization and capacity to interact with ABC transporters. This article is part of the theme issue 'Retrograde signalling from endosymbiotic organelles'.

Comparative structural and functional analysis of phi class glutathione transferases involved in multiple-herbicide resistance of grass weeds and crops

Multiple-herbicide resistant (MHR) weeds are a global problem and a looming threat to weed control in crops. MHR weeds express a specific phi class glutathione transferase (MHR-GSTF) which seems to contribute to herbicide resistance. The present work aims to investigate the structure and catalytic properties of the MHR-GSTFs from different grass weeds and crops (Alopecurus myosuroides, Lolium rigidum, Hordeum vulgare, Triticum aestivum). Recombinant MHR-GSTFs were expressed in E. coli and purified by affinity chromatography. Kinetic analysis of substrate specificity using a range of thiol substrates and xenobiotic compounds suggested that all enzymes display a broad range of specificity and are capable of detoxifying major stress-induced toxic products. Notably, all tested enzymes exhibited high activity towards organic hydroperoxides. The crystal structure of MHR-GSTF from Alopecurus myosuroides (AmGSTF) was determined by molecular replacement at 1.33 Å resolution. The enzyme was resolved with bound glutathione sulfenic acid (GSOH) at the G-site and succinic acid at the H-site. The enzyme shows conserved structural features compared to other Phi class GSTs. However, some differences were observed at the C-terminal helix H9 that may affect substrate specificity. The structural and functional features of AmGSTF were compared with those of the homologue crop enzymes (HvGSTF and TaGSTF) and discussed in light of their contribution to the MHR mechanism.

Structural basis for the activation and inhibition of Sirtuin 6 by quercetin and its derivatives

Mammalian Sirtuin 6 (Sirt6) is an NAD⁺-dependent protein deacylase regulating metabolism and chromatin homeostasis. Sirt6 activation protects against metabolic and aging-related diseases, and Sirt6 inhibition is considered a cancer therapy. Available Sirt6 modulators show insufficient potency and specificity, and even partially contradictory Sirt6 effects were reported for the plant flavone quercetin. To understand Sirt6 modulation by quercetin-based compounds, we analysed their binding and activity effects on Sirt6 and other Sirtuin isoforms and solved crystal structures of compound complexes with Sirt6 and Sirt2. We find that quercetin activates Sirt6 via the isoform-specific binding site for pyrrolo[1,2-a]quinoxalines. Its inhibitory effect on other isoforms is based on an alternative binding site at the active site entrance. Based on these insights, we identified isoquercetin as a ligand that can discriminate both sites and thus activates Sirt6 with increased specificity. Furthermore, we find that quercetin derivatives that inhibit rather than activate Sirt6 exploit the same general Sirt6 binding site as the activators, identifying it as a versatile allosteric site for Sirt6 modulation. Our results thus provide a structural basis for Sirtuin effects of quercetin-related compounds and helpful insights for Sirt6-targeted drug development.

Evaluation of the AGE/sRAGE Axis in Patients with Multiple Myeloma

Glycative stress influences tumor progression. The aim of the present study was to evaluate the advanced glycation end products/soluble receptor of advanced glycation end products (AGE/sRAGE) axis in patients with multiple myeloma (MM). Blood samples were taken from 19 patients affected by MM and from 16 sex-matched and age-matched healthy subjects. AGE and sRAGE axis were dosed in patients with MM and matched with controls. AGEs were measured by spectrofluorimetric methods. Blood samples for the determination of sRAGE were analyzed by ELISA. AGE levels were significantly reduced in patients with respect to controls. Instead, sRAGE was significantly elevated in patients affected by MM compared to healthy subjects. Moreover, we showed that there was a statistically significant difference in sRAGE according to the heavy and light chain. IgA lambda had significantly higher sRAGE values than IgA kappa, IgG kappa, and IgG Lambda MM patients. From our data emerges the role of the sRAGE/AGE axis in MM. Since AGE is a positive regulator of the activity of RAGE, circulating sRAGE concentrations may reflect RAGE expression and may be raised in parallel with serum AGE concentrations as a counter-system against AGE-caused tissue damage. Serum concentrations of AGE and sRAGE could therefore become potential therapeutic targets.

Functional, Structural and Biochemical Features of Plant Serinyl-Glutathione Transferases

Glutathione transferases (GSTs) belong to a ubiquitous multigenic family of enzymes involved in diverse biological processes including xenobiotic detoxification and secondary metabolism. A canonical GST is formed by two domains, the N-terminal one adopting a thioredoxin (TRX) fold and the C-terminal one an all-helical structure. The most recent genomic and phylogenetic analysis based on this domain organization allowed the classification of the GST family into 14 classes in terrestrial plants. These GSTs are further distinguished based on the presence of the ancestral cysteine (Cys-GSTs) present in TRX family proteins or on its substitution by a serine (Ser-GSTs). Cys-GSTs catalyze the reduction of dehydroascorbate and deglutathionylation reactions whereas Ser-GSTs catalyze glutathione conjugation reactions and eventually have peroxidase activity, both activities being important for stress tolerance or herbicide detoxification. Through non-catalytic, so-called ligandin properties, numerous plant GSTs also participate in the binding and transport of small heterocyclic ligands such as flavonoids including anthocyanins, and polyphenols. So far, this function has likely been underestimated compared to the other documented roles of GSTs. In this review, we compiled data concerning the known enzymatic and structural properties as well as the biochemical and physiological functions associated to plant GSTs having a conserved serine in their active site.

Probing the interaction of a quercetin bioconjugate with Bcl-2 in living human cancer cells with in-cell NMR spectroscopy

In-cell NMR spectroscopy has emerged as a powerful technique for monitoring biomolecular interactions at an atomic level inside intact cells. However, current methodologies are inadequate at charting intracellular interactions of nonlabeled proteins and require their prior isotopic labeling. Herein, we describe for the first time the monitoring of the quercetin-alanine bioconjugate interaction with the nonlabeled antiapoptotic protein Bcl-2 inside living human cancer cells. STD and Tr-NOESY in-cell NMR methodologies were successfully applied in the investigation of the binding, which was further validated in vitro. In-cell NMR proved a very promising strategy for the real-time probing of the interaction profile of potential drugs with their therapeutic targets in native cellular environments and could, thus, open a new avenue in drug discovery.

Molecular recognition of wood polyphenols by phase II detoxification enzymes of the white rot Trametes versicolor

Wood decay fungi have complex detoxification systems that enable them to cope with secondary metabolites produced by plants. Although the number of genes encoding for glutathione transferases is especially expanded in lignolytic fungi, little is known about their target molecules. In this study, by combining biochemical, enzymatic and structural approaches, interactions between polyphenols and six glutathione transferases from the white-rot fungus Trametes versicolor have been demonstrated. Two isoforms, named TvGSTO3S and TvGSTO6S have been deeply studied at the structural level. Each isoform shows two distinct ligand-binding sites, a narrow L-site at the dimer interface and a peculiar deep hydrophobic H-site. In TvGSTO3S, the latter appears optimized for aromatic ligand binding such as hydroxybenzophenones. Affinity crystallography revealed that this H-site retains the flavonoid dihydrowogonin from a partially purified wild-cherry extract. Besides, TvGSTO6S binds two molecules of the flavonoid naringenin in the L-site. These data suggest that TvGSTO isoforms could interact with plant polyphenols released during wood degradation.

The root transcriptome of Achyranthes bidentata and the identification of the genes involved in the replanting benefit

The transcriptome profiling in replanting roots revealed that expression pattern changes of key genes promoted important metabolism pathways, antioxidant and pathogen defense systems, adjusted phytohormone signaling and inhibited lignin biosynthesis. The yield of the medicinal plant Achyranthes bidentata could be significantly increased when replanted into a field cultivated previously for the same crop, but the biological basis of this so-called "replanting benefit" is unknown. Here, the RNA-seq technique was used to identify candidate genes responsible for the benefit. The analysis of RNA-seq libraries prepared from mRNA extracted from the roots of first year planting (normal growth, NG) and second year replanting (consecutive monoculture, CM) yielded about 40.22 GB sequencing data. After de novo assembly, 87,256 unigenes were generated with an average length of 1060 bp. Among these unigenes, 55,604 were annotated with public databases, and 52,346 encoding sequences and 2881 transcription factors were identified. A contrast between the NG and CM libraries resulted in a set of 3899 differentially transcribed genes (DTGs). The DTGs related to the replanting benefit and their expression profiles were further analyzed by bioinformatics and qRT-PCR approaches. The major differences between the NG and CM transcriptomes included genes encoding products involved in glycolysis/gluconeogenesis, glutathione metabolism and antioxidant defense, in aspects of the plant/pathogen interaction, phytohormone signaling and phenylpropanoid biosynthesis. The indication was that replanting material enjoyed a stronger level of defense systems, a balance regulation of hormone signals and a suppression of lignin formation, thereby promoting root growth and development. The study provides considerable significant insights for a better understanding of the molecular mechanism of the replanting benefit and suggests their possible application in developing methods to reinforce the effects in medicinal plants.

Protein-Ligand Fishing in planta for Biologically Active Natural Products Using Glutathione Transferases

Screening for natural products which bind to proteins in planta has been used to identify ligands of the plant-specific glutathione transferase (GST) tau (U) and phi (F) classes, that are present in large gene families in crops and weeds, but have largely undefined functions. When expressed as recombinant proteins in Escherichia coli these proteins have been found to tightly bind a diverse range of natural product ligands, with fatty acid-and porphyrinogen-derivatives associated with GSTUs and a range of heterocyclic compounds with GSTFs. With an interest in detecting the natural binding partners of these proteins in planta, we have expressed the two best characterized GSTs from Arabidopsis thaliana (At), AtGSTF2 and AtGSTU19, as Strep-tagged fusion proteins in planta. Following transient and stable expression in Nicotiana and Arabidopsis, respectively, the GSTs were recovered using Strep-Tactin affinity chromatography and the bound ligands desorbed and characterized by LC-MS. AtGSTF2 predominantly bound phenolic derivatives including S-glutathionylated lignanamides and methylated variants of the flavonols kaempferol and quercetin. AtGSTU19 captured glutathionylated conjugates of oxylipins, indoles, and lignanamides. Whereas the flavonols and oxylipins appeared to be authentic in vivo ligands, the glutathione conjugates of the lignanamides and indoles were artifacts formed during extraction. When tested for their binding characteristics, the previously undescribed indole conjugates were found to be particularly potent inhibitors of AtGSTU19. Such ligand fishing has the potential to both give new insight into protein function in planta as well as identifying novel classes of natural product inhibitors of enzymes of biotechnological interest such as GSTs.

Recent advances in protein engineering and biotechnological applications of glutathione transferases

Glutathione transferases (GSTs, EC 2.5.1.18) are a widespread family of enzymes that play a central role in the detoxification, metabolism, and transport or sequestration of endogenous or xenobiotic compounds. During the last two decades, delineation of the important structural and catalytic features of GSTs has laid the groundwork for engineering GSTs, involving both rational and random approaches, aiming to create new variants with new or altered properties. These approaches have expanded the usefulness of native GSTs, not only for understanding the fundamentals of molecular detoxification mechanisms, but also for the development medical, analytical, environmental, and agricultural applications. This review article attempts to summarize successful examples and current developments on GST engineering, highlighting in parallel the recent knowledge gained on their phylogenetic relationships, structural/catalytic features, and biotechnological applications.