Characterization of putative hydrophobic substrate binding site residues of a Delta class glutathione transferase from Anopheles dirus

Glutathione S-Transferase Highly Expressed in Holotrichia parallela Antennae Inactivates the Odorant Unsaturated Aldehyde Volatiles

Odorant-degrading enzymes in insects play a vital role in maintaining olfactory sensitivity. However, the role and molecular mechanism of glutathione S-transferases (GSTs) in odorant inactivation has been rarely studied. In the present study, 31 GSTs were identified from the antennal transcriptome of Holotrichia parallela. HpGSTd1 possesses the highest transcriptome expression level. Recombinant HpGSTd1 showed degradation activity toward various unsaturated aldehyde volatiles. Furthermore, the metabolite of cinnamaldehyde was identified by high-resolution mass spectrometry (HRMS). The molecular docking analysis and site-directed mutagenesis revealed the key residues of HpGSTd1 in degrading odorants. In addition, the unsaturated aldehyde volatiles elicited the behavioral and electrophysiological responses of H. parallela. Taken together, our findings suggest that HpGSTd1 may play an essential role in inactivating odorants in H. parallela, which provides new insights for identifying molecular targets and exploring effective olfactory regulators for this underground pest.

Antennal transcriptomic analysis of carboxylesterases and glutathione S-transferases associated with odorant degradation in the tea gray geometrid, Ectropis grisescens (Lepidoptera, Geometridae)

Introduction: Carboxylesterases (CXEs) and glutathione S-transferases (GSTs) can terminate olfactory signals during chemosensation by rapid degradation of odorants in the vicinity of receptors. The tea grey geometrid, Ectropis grisescens (Lepidoptera, Geometridae), one of the most devastating insect herbivores of tea plants in China, relies heavily on plant volatiles to locate the host plants as well as the oviposition sites. However, CXEs and GSTs involved in signal termination and odorant clearance in E. grisescens remains unknown. Methods: In this study, identification and spatial expression profiles of CXEs and GSTs in this major tea pest were investigated by transcriptomics and qRT-PCR, respectively. Results: As a result, we identified 28 CXEs and 16 GSTs from female and male antennal transcriptomes. Phylogenetic analyses clustered these candidates into several clades, among which antennal CXEs, mitochondrial and cytosolic CXEs, and delta group GSTs contained genes commonly associated with odorants degradation. Spatial expression profiles showed that most CXEs (26) were expressed in antennae. In comparison, putative GSTs exhibited a diverse expression pattern across different tissues, with one GST expressed specifically in the male antennae. Disscussion: These combined results suggest that 12 CXEs (EgriCXE1, 2, 4, 6, 8, 18, 20-22, 24, 26, and 29) and 5 GSTs (EgriGST1 and EgriGST delta group) provide a major source of candidate genes for odorants degradation in E. grisescens.

A glutathione-S-transferase (TuGSTd05) associated with acaricide resistance in Tetranychus urticae directly metabolizes the complex II inhibitor cyflumetofen

Cyflumetofen is a recently introduced acaricide with a novel mode of action, acting as an inhibitor of complex II of mitochondrial electron transport chain. It is activated by hydrolysis and the resulting de-esterified metabolite is a much stronger inhibitor. Cyflumetofen represents a great addition for the control of mite species including Tetranychus urticae, a major agricultural pest, which has the ability to develop resistance to most classes of pesticides rapidly. A resistant strain (Tu008R) was recently described and synergism experiments pointed towards the involvement of GSTs. Here, we conducted genome-wide gene expression analysis, comparing Tu008R with its parental susceptible strain, and identified the delta GST TuGSTd05 as the prime resistance-conferring candidate. Docking analysis suggests that both cyflumetofen and its de-esterified metabolite are potential substrates for conjugation by TuGSTd05. Several amino acids were identified that might be involved in the interaction, with Y107 and N103 possibly having an important role. To further investigate interaction as well as the role of Y107 and N103 in vitro, we recombinantly expressed and kinetically characterized the wild type TuGSTd05, TuGSTd05 Y107F and TuGSTd05 N103L mutants. While cyflumetofen was not found to act as a strong inhibitor, the de-esterified metabolite showed strong affinity for TuGSTd05 (IC₅₀ = 4 μM), which could serve as a mechanism of rapid detoxification. Y107 and N103 might contribute to this interaction. HPLC-MS analysis provided solid indications that TuGSTd05 catalyzes the conjugation of ionized glutathione (GS^-) to cyflumetofen and/or its de-esterified metabolite and the resulting metabolite and possible site of attack were identified.

Identification of Putative Carboxylesterase and Glutathione S-transferase Genes from the Antennae of the Chilo suppressalis (Lepidoptera: Pyralidae)

In insects, rapid degradation of odorants in antennae is extremely important for the sensitivity of olfactory receptor neurons. Odorant degradation in insect antennae is mediated by multiple enzymes, especially the carboxylesterases (CXEs) and glutathione S-transferases (GSTs). The Asiatic rice borer, Chilo suppressalis, is an economically important lepidopteran pest which causes great economic damage to cultivated rice crops in many Asian countries. In this study, we identified 19 putative CXE and 16 GST genes by analyzing previously constructed antennal transcriptomes of C. suppressalis. BLASTX best hit results showed that these genes are most homologous to their respective orthologs in other lepidopteran species. Phylogenetic analyses revealed that these CXE and GST genes were clustered into various clades. Reverse-transcription quantitative polymerase chain reaction assays showed that three CXE genes (CsupCXE8, CsupCXE13, and CsupCXE18) are antennae-enriched. These genes are candidates for involvement in odorant degradation. Unexpectedly, none of the GST genes were found to be antennae-specific. Our results pave the way for future researches of the odorant degradation mechanism of C. suppressalis at the molecular level.

Identification and expression profiles of nine glutathione S-transferase genes from the important rice phloem sap-sucker and virus vector Laodelphax striatellus (Fallén) (Hemiptera: Delphacidae)

BACKGROUND

Glutathione S-transferases (GSTs) have received considerable attention in insects for their roles in insecticide resistance. Laodelphax striatellus (Fallén) is a serious rice pest. L. striatellus outbreaks occur frequently throughout eastern Asia. A key problem in controlling this pest is its rapid adaptation to numerous insecticides. In this research, nine cDNAs encoding GSTs in L. striatellus were cloned and characterised.

RESULTS

The cloned GSTs of L. striatellus belonged to six cytosolic classes and a microsomal subgroup. Exposure to sublethal concentrations of each of the six insecticides, DDT, chlorpyrifos, fipronil, imidacloprid, buprofezin and beta-cypermethrin, quickly induced (6 h) up-expression of LsGSTe1. The expression of LsGSTs2 was increased by chlorpyrifos, fipronil and beta-cypermethrin. Furthermore, exposure of L. striatellus to fipronil, imidacloprid, buprofezin and beta-cypermethrin increased the expression of the LsGSTm gene after 24 or 48 h.

CONCLUSION

This work is the first identification of GST genes from different GST groups in Auchenorrhyncha species and their induction characteristics with insecticide types and time. The elevated expression of GST genes induced by insecticides might be related to the enhanced tolerance of this insect to insecticides and xenobiotics.

Structural and catalytic role of two conserved tyrosines in Delta-class glutathione S-transferase from Locusta migratoria

Glutathione S-transferases (GSTs) are an important family of detoxifying enzymes and play a key role in pesticide resistance in the insect. Tyrosine is essential for its detoxification function. In the present study, two conserved tyrosine residues are located at positions 108 and 116 in H-site of LmGSTD1. To elucidate how the two residues participate in the catalytic process and keeping structural stability, four mutants, Y108A, Y108E, Y116A, and Y116E, were generated. It was found that the four mutants affected the specific activity of LmGSTD1 in various degrees, depending on the types of substrate and reaction mechanism. Steady-state kinetics assay revealed that Y108E and Y116E had a significant influence on GSH-binding ability, which indicates the two tyrosine residues of H-site contribute to topology rearrangement of G-site. Both Y116A and Y116E exhibited lower CDNB-binding affinity, suggesting that Y116 takes part in hydrophobic substrate binding. The thermostability assay, intrinsic, and 8-anilino-1-naphthalenesulfonic acid (ANS) florescence results showed that the two tyrosine residues were involved in regulation of active-site conformation. Finally, homology modeling provided evidence that the two tyrosines in H-site participate in hydrophobic substrate binding. Furthermore, Y108 is closer to the S atom of S-hexylglutathione. In conclusion, the two tyrosines in LmGSTD1 are important residues in both the catalytic process and protein stability.

Characterization of the hydrophobic substrate-binding site of the bacterial beta class glutathione transferase from Proteus mirabilis

Since their discovery, bacterial glutathione (GSH)transferases have been characterized in terms of their ability to catalyse a variety of different reactions on a large set of toxic molecules of xenobiotic or endobiotic origin. Furthermore the contribution of different residues in the GSH-binding site to GSH activation has been extensively investigated. Little is known, however, about the contribution to catalysis and overall stability of single residues shaping the hydrophobic co-substrate binding site (H-site). Here we tackle this problem by site-directed mutagenesis of residues facing the H-site in the bacterial beta class GSH transferase from Proteus mirabilis. We investigate the behaviour of these mutants under a variety of conditions and analyse their activity against several co-substrates, representative of the different reactions catalyzed by bacterial GSH transferases. Our work shows that mutations at the H-site can be used to modulate activity at the level of the different catalytic mechanisms operating on the chosen substrates, each mutation showing a different fingerprint. This work paves the way for future studies aimed at improving the catalytic properties of beta class GSH transferases against selected substrates for bioremediation purposes.

Structural contributions of Delta class glutathione transferase active-site residues to catalysis

GST (glutathione transferase) is a dimeric enzyme recognized for biotransformation of xenobiotics and endogenous toxic compounds. In the present study, residues forming the hydrophobic substrate-binding site (H-site) of a Delta class enzyme were investigated in detail for the first time by site-directed mutagenesis and crystallographic studies. Enzyme kinetics reveal that Tyr111 indirectly stabilizes GSH binding, Tyr119 modulates hydrophobic substrate binding and Phe123 indirectly modulates catalysis. Mutations at Tyr111 and Phe123 also showed evidence for positive co-operativity for GSH and 1-chloro-2,4-dinitrobenzene respectively, strongly suggesting a role for these residues in manipulating subunit–subunit communication. In the present paper we report crystal structures of the wild-type enzyme, and two mutants, in complex with S-hexylglutathione. This study has identified an aromatic ‘zipper’ in the H-site contributing a network of aromatic π–π interactions. Several residues of the cluster directly interact with the hydrophobic substrate, whereas others indirectly maintain conformational stability of the dimeric structure through the C-terminal domain (domain II). The Y119E mutant structure shows major main-chain rearrangement of domain II. This reorganization is moderated through the ‘zipper’ that contributes to the H-site remodelling, thus illustrating a role in co-substrate binding modulation. The F123A structure shows molecular rearrangement of the H-site in one subunit, but not the other, explaining weakened hydrophobic substrate binding and kinetic co-operativity effects of Phe123 mutations. The three crystal structures provide comprehensive evidence of the aromatic ‘zipper’ residues having an impact upon protein stability, catalysis and specificity. Consequently, ‘zipper’ residues appear to modulate and co-ordinate substrate processing through permissive flexing.