Structural Insights into Two Representative Conformations of the Complex Formed by Grapholita molesta (Busck) Pheromone Binding Protein 2 and Z-8-Dodecenyl Acetate

Expression Pattern of RpCSP6 from Rhopalosiphum padi and Its Binding Mechanism with Deltamethrin: Insights into Chemosensory Protein-Mediated Insecticide Resistance

The mechanisms of insecticide resistance are complex. Recent studies have revealed a novel mechanism involving the chemosensory system in insecticide resistance. However, the specific binding mechanism between olfactory-related genes and insecticides needs to be clarified. In this study, the binding mechanism between pyrethroid insecticide deltamethrin and RpCSP6 from Rhopalosiphum padi was investigated by using computational and multiple experimental methods. RpCSP6 was expressed in different tissues and developmental stages of R. padi and can be induced by deltamethrin. Knockdown of RpCSP6 significantly increased the susceptibility of R. padi to deltamethrin. The binding affinity of RpCSP6 to 24 commonly used insecticides was measured. Seven key residues were found to steadily interact with deltamethrin, indicating their significance in the binding affinity to the insecticide. Our research provided insights for effectively analyzing the binding mechanism of insect CSPs with insecticides, facilitating the development of new and effective insecticides that target insect CSPs.

Molecular characterization and functional analysis of the ecdysone receptor isoform (EcR) from the oriental fruit moth Grapholita molesta (Lepidoptera: Tortricidae)

20-Hydroxyecdysone (20E) plays a vital role in a series of biological processes, via the nuclear receptors, EcR/USP by activating the ecdysone regulatory cascade. To clarify the role of EcR during the development of Grapholita molesta, the complementary DNA of ecdysone receptor isoform B1 (GmEcR-B1) was obtained from the transcriptome of G. molesta and verified by PCR. Alignment analysis revealed that the deduced protein sequence of GmEcR-B1 was highly homologous to EcR proteins identified in other lepidopteran species, especially the EcR-B1 isoform in Spodoptera litura. Quantitative real-time PCR showed that GmEcRs was expressed at all test developmental stages, and the expression level of GmEcRs was relatively higher during the period of the 3rd day of fifth instar larvae to 2nd of pupa than those in other stages. Moreover, the messenger RNA of GmEcRs was much more strongly expressed in the Malpighian tubule and epidermis than those in other tissues, which suggests that this gene may function in a tissue-specific manner during larval development. Silencing of GmEcRs could significantly downregulate the transcriptional level of ecdysone-inducible genes and result in increased mortality during metamorphosis and prolonged prepupal duration. Taken together, the present results indicate that GmEcRs may directly or indirectly affect the development of G. molesta.

Identifying the Key Role of Plutella xylostella General Odorant Binding Protein 2 in Perceiving a Larval Attractant, (E,E)-2,6-Farnesol

There is currently a lack of effective olfaction-based techniques to control diamondback moth (DBM) larvae. Identifying behaviorally active odorants for DBM larvae and exploring their recognition mechanisms can provide insights into olfaction-based larval control strategies. Through the two-choice assay, (E,E)-2,6-farnesol (farnesol) was identified as a compound exhibiting significant attractant activity toward DBM larvae, achieving an attraction index of 0.48 ± 0.13. PxylGOBP1 and PxylGOBP2, highly expressed in the antennae of DBM larvae, both showed high affinity toward farnesol. RNAi technology was used to knock down PxylGOBP1 and PxylGOBP2, revealing that the attraction of DBM larvae to farnesol nearly vanished following the knockdown of PxylGOBP2, indicating its critical role in recognizing farnesol. Further investigation into the PxylGOBP2-farnesol interaction revealed the importance of residues like Thr9, Trp37, and Phe118 in PxylGOBP2's binding to farnesol. This research is significant for unveiling the olfactory mechanisms of DBM larvae and developing larval behavior regulation techniques.

High-efficiency removal of pyrethroids using a redesigned odorant binding protein

Pyrethroids are ubiquitously present in environmental media and threaten both the ecosystem and human health. To explore effective ways to remove pyrethroids from the environment, an odorant binding protein (OBP) with affinity for various pyrethroids was investigated. Initially, the target OBP, Spodoptera littoralis pheromone binding protein 1 (SlitPBP1), underwent redesign to enhance its affinity for pyrethroids. The modified SlitPBP1E97ND106E demonstrated a substantially increased affinity for deltamethrin (DeltaM), with a dissociation constant of 0.77 ± 0.17 μM. The affinity of SlitPBP1E97ND106E for other pyrethroids also increased to varying extents. Consequently, SlitPBP1E97ND106E displayed a markedly enhanced capability to adsorb and remove pyrethroids. When exposed to free SlitPBP1E97ND106E in solution, the reduction in DeltaM surged from 16.78 ± 0.32% to 97.51 ± 0.56%. SlitPBP1E97ND106E was immobilized by coupling the protein to Ni2+-NTA agarose resin. Liquid chromatography results attested to the superior efficacy of immobilized SlitPBP1E97ND106E in removing pyrethroids, especially DeltaM. No significant differences in pyrethroid removal were detected across various water samples. Our findings introduce a potent tool for pyrethroid removal. A wider range of OBPs can similarly be optimized and applied to remove organic pollutants, including but not limited to pesticides.

Screening of behaviorally active compounds based on the interaction between two chemosensory proteins and mung bean volatiles in Callosobruchus chinensis

Chemosensory proteins (CSPs) are involved in the earliest steps of the olfactory process by binding and transporting odorants and play a crucial role in the insect's search for food and egg-laying sites. In the present study, the tissue expression profiles showed that both CchiCSP3 and CchiCSP5 of Callosobruchus chinensis were highly expressed in the adult antennae. Subsequently, the recombinant CchiCSP3 and CchiCSP5 proteins were analysed using fluorescence competitive binding assays, and both showed binding affinities for the three mung bean volatiles. Molecular docking and site-directed mutagenesis revealed four key amino acid residues in CchiCSP3 (L47, W80, Y81, and L84) and CchiCSP5 (Y28, K46, L49, and I72). Electroantennogram (EAG) and dual-choice biobehavioral assays showed that the antennae of adult C. chinensis were electrophysiologically active in response to stimulation with all three behaviorally active compounds and that octyl 4-methoxycinnamate and β-ionone had a significant luring effect on adult C. chinensis, whereas vanillin had a significant avoidance effect. Our study screened three effective behaviorally active compounds based on the involvement of two CchiCSPs in the recognition of mung bean volatiles, providing an opportunity to develop an alternative control strategy using behavioral disruptors to limit the impact of pests.

The chemosensory protein 1 contributes to indoxacarb resistance in Plutella xylostella (L.)

BACKGROUND

Insecticide resistance continuously poses a threat to agricultural production. Chemosensory protein-mediated resistance is a new mechanism of insecticide resistance discovered in recent years. In-depth research on resistance mediated by chemosensory proteins (CSPs) provides new insight into aid insecticide resistance management.

RESULTS

Chemosensory protein 1 in Plutella xylostella (PxCSP1) was overexpressed in the two indoxacarb-resistant field populations and PxCSP1 has a high affinity with indoxacarb. PxCSP1 was upregulated when exposed to indoxacarb and the knockdown of this gene elevated sensitivity to indoxacarb, which demonstrate that PxCSP1 is involved in the indoxacarb resistance. Considering that CSPs may confer resistance in insects via binding or sequestering, we explored the binding mechanism of indoxacarb in PxCSP1-mediated resistance. Using molecular dynamics simulations and site-directed mutation, we found that indoxacarb forms a solid complex with PxCSP1 mainly through van der Waals interactions and electrostatic interactions. Between these, the electrostatic interaction provided by the Lys100 side chain in PxCSP1, and especially the hydrogen bonding between the NZ atom and the O of the carbamoyl carbonyl group of indoxacarb, are the key factors for the high affinity of PxCSP1 to indoxacarb.

CONCLUSIONS

The overexpression of PxCPS1 and its high affinity to indoxacarb is partially responsible for indoxacarb resistance in P. xylostella. Modification of indoxacarb's carbamoyl group has the potential to alleviate indoxacarb resistance in P. xylostella. These findings will contribute to solving chemosensory protein-mediated indoxacarb resistance and provide a better understanding of the insecticide resistance mechanism. © 2023 Society of Chemical Industry.

The Mythimna separata general odorant binding protein 2 (MsepGOBP2) is involved in the larval detection of the sex pheromone (Z)-11-hexadecenal

BACKGROUND

Mythimna separata is a notorious pest causing crop damages at the larval stage. Gaining insight into larval olfaction mechanisms would provide knowledge for olfaction-based management of M. separata larvae.

RESULTS

In the present research, (Z)-11-hexadecenal (Z11-16: Ald), a major component of M. separata sex pheromone, was found to attract early-instar larvae of M. separata in a food context. Using a fluorescent binding assay, we found that M. separata general odorant binding protein 2 (MsepGOBP2) exhibited high binding affinity to Z11-16: Ald. Further, silencing of MsepGOBP2 resulted in a sharp reduction of the response to Z11-16: Ald, which could not be mitigated by increasing the concentration of Z11-16: Ald. Additionally, we employed molecular dynamics-based approaches to unravel the interaction details between MsepGOBP2 and Z11-16: Ald, specifically the binding of Z11-16: Ald to MsepGOBP2.

CONCLUSION

Z11-16: Ald is attractive to early-instar larvae of M. separata, and MsepGOBP2 is identified to be indispensable in the larval detection of Z11-16: Ald. These results could aid in the development of olfaction-based methods for controlling M. separata in the larval stage. © 2023 Society of Chemical Industry.

Functional differentiation of two general odorant-binding proteins to sex pheromones in Spodoptera frugiperda

A precise chemosensory system can help insects complete various important behavioral responses by accurately identifying different external odorants. Therefore, deeply understanding the mechanism of insect recognition of important odorants will help us develop efficient and environmentally-friendly behavioral inhibitors. Spodoptera frugiperda is a polyphagous pest that feeds on >350 different host plants worldwide and also harms maize production in China. However, the molecular mechanism of the first step for males to use odorant-binding proteins (OBPs) to recognize sex pheromones remains unclear. Here, we obtained 50 OBPs from the S. frugiperda genome, and the expression level of SfruGOBP1 in females was significantly higher than that in males, whereas SfruGOBP2 displayed male-biased expression. Fluorescence competitive binding assays showed that only SfruGOBP2 showed binding affinities for the four sex pheromones of female S. frugiperda. Subsequently, we identified some key amino acid residues that can participate in the interaction between SfruGOBP2 and sex pheromones using molecular docking and site-directed mutagenesis methods. These findings will help us explore the interaction mechanism between GOBPs and sex pheromones in moths, and provide important target genes for developing new mating inhibitors of S. frugiperda in the future.

Odorant-Binding Protein 6 Contributes High Binding Affinity to Insecticides in a Parasitic Wasp Meteorus pulchricornis (Hymenoptera: Braconidae)

Meteorus pulchricornis is a preponderant parasitic wasp of various lepidopteran pests. The extensive application of broad-spectrum insecticides usually causes serious threats to the olfactory recognition of nontarget insects such as parasitoid wasps. However, the binding mechanism of odorant-binding proteins (OBPs) to insecticides in parasitoid wasps remains unknown. Herein, we find that the MpulOBP6 protein had a strong binding affinity to three insecticides (phoxim, chlorpyrifos, and chlorfenapyr). Results of computational simulations revealed that the hydrophobic interaction contributed by a mass of nonpolar amino acid residues was the primary driving force in the formation and stabilization of MpulOBP6-insecticide complexes. Among them, four residues (Met75, Val84, Phe121, and Pro122) and two residues (Val84 and Phe111) play an essential role in the binding of MpulOBP6 to phoxim and chlorfenapyr, respectively. Our findings could be instrumental to elucidate the effects of insecticide application toward the olfactory recognition of nontarget insects in the processes of agricultural production.

Identification and Functional Characterization of General Odorant Binding Proteins in Orthaga achatina

The olfactory system in insects are crucial for recognition of host plants and oviposition sites. General odorant binding proteins (GOBPs) are thought to be involved in detecting odorants released by host plants. Orthaga achatina (Lepidoptera: Pyralidae) is one of the most serious pests of camphor trees, Cinnamomum camphora (L.) Presl, an important urban tree species in southern China. In this study, we study the GOBPs of O. achatina. Firstly, two full-length GOBP genes (OachGOBP1 and OachGOBP2) were successfully cloned according to transcriptome sequencing results, and real-time quantitative PCR measurements showed that both GOBP genes were specifically expressed in the antennae of both sexes, proposing their important roles in olfaction. Then, both GOBP genes were heterologous expressed in Escherichia coli and fluorescence competitive binding assays were conducted. The results showed that OachGOBP1 could bind Farnesol (K_i = 9.49 μM) and Z11-16: OH (K_i = 1.57 μM). OachGOBP2 has a high binding affinity with two camphor plant volatiles (Farnesol, K_i = 7.33 μM; α-Phellandrene, K_i = 8.71 μM) and two sex pheromone components (Z11-16: OAc, K_i = 2.84 μM; Z11-16: OH, K_i = 3.30 μM). These results indicate that OachGOBP1 and OachGOBP2 differ in terms of odorants and other ligands. Furthermore, key amino acid residues that bind to plant volatiles were identified in GOBPs using 3-D structure modeling and ligand molecular docking, predicting the interactions between the GOBPs and the host plant volatiles.

Functional disparity of four pheromone-binding proteins from the plum fruit moth Grapholita funebrana Treitscheke in detection of sex pheromone components

Grapholita funebrana, also known as the plum fruit moth, is an oligophagous pest species that causes enormous economic losses of the fruits of Rosaceae. An eco-friendly method for the control of G. funebrana besides chemical control has not yet been developed. The sex pheromone communication system plays an important role in moth courtship and mating, in which pheromone-binding proteins (PBPs) are critical. In this research, we identified four PBPs, namely, GfunPBP1.1, GfunPBP1.2, GfunPBP2, and GfunPBP3, from the antennae of G. funebrana. The results of real-time quantitative PCR (RT-qPCR) showed that all four GfunPBPs were overwhelmingly expressed in the antennae and that GfunPBP1.2 and GfunPBP2 showed male-biased expression patterns, whereas GfunPBP1.1 and GfunPBP3 were equally expressed between sexes. The results of ligand-binding assays illustrated that although all four recombinant GfunPBPs (rGfunPBPs) had binding activity with the tested sex pheromone compounds, their preferred ligands were significantly different. rGfunPBP2 had the strongest binding affinity to Z8-12:Ac and Z8-12:OH; rGfunPBP1.1 preferred to bind Z8-14:Ac, Z10-14:Ac, and 12:OH more than to the other three GfunPBPs; and rGfunPBP1.2 exhibited stronger binding affinity to E8-12:Ac than to the other rGfunPBPs. Molecular docking results demonstrated that hydrophobic forces, especially van der Waals forces and hydrogen bonds, were the most important forces that maintained GfunPBP-pheromone ligand complexes. This study will improve our understanding of the sex pheromone recognition mechanisms of G. funebrana and promote the development of novel strategies for controlling G. funebrana.

Binding properties of odorant-binding protein 4 from bean bug Riptortus pedestris to soybean volatiles

The bean bug Riptortus pedestris is a notorious insect pest that can damage various crops, especially soybean, in East Asia. In insects, the olfactory system plays a crucial role in host finding and feeding behaviour in which the odorant-binding proteins (OBPs) are believed to be involved in initial step in this system. In this study, we produced the R. pedestris adult antennae-expressed RpedOBP4 protein using a recombinant expression system in E. coli. Fluorescence competitive binding confirmed that RpedOBP4 has binding affinities to 7 of 20 soybean volatiles (ligands), and that a neutral condition is the best environment for it. The binding property of RpedOBP4 to these ligands was further revealed by integrating data from molecular docking, site-directed mutagenesis and ligand binding assays. This demonstrated that five amino acid residues (I30, L33, Y47, I57 and Y121) are involved in the binding process of RpedOBP4 to corresponding ligands. These findings will not only help us to more thoroughly explore the olfactory mechanism of R. pedestris during feeding on soybean, but also lead to the identification of key candidate targets for developing environmental and efficient behaviour inhibitors to prevent population expansion of R. pedestris in the future.

Key Residues Affecting Binding Affinity of Sirex noctilio Fabricius Odorant-Binding Protein (SnocOBP9) to Aggregation Pheromone

Sirex noctilio Fabricius (Hymenoptera Siricidae) is a major quarantine pest responsible for substantial economic losses in the pine industry. To achieve better pest control, (Z)-3-decen-ol was identified as the male pheromone and used as a field chemical trapping agent. However, the interactions between odorant-binding proteins (OBPs) and pheromones are poorly described. In this study, SnocOBP9 had a higher binding affinity with Z3D (Ki = 1.53 ± 0.09 μM) than other chemical ligands. Molecular dynamics simulation and binding mode analysis revealed that several nonpolar residues were the main drivers for hydrophobic interactions between SnocOBP9 and Z3D. Additionally, computational alanine scanning results indicated that five amino acids (MET54, PHE57, PHE71, PHE74, LEU116) in SnocOBP9 could potentially alter the binding affinity to Z3D. Finally, we used single-site-directed mutagenesis to substitute these five residues with alanine. These results imply that the five residues play crucial roles in the SnocOBP9-Z3D complex. Our research confirmed the function of SnocOBP9, uncovered the key residues involved in SnocOBP9-Z3D interactions, and provides an inspiration to improve the effects of pheromone agent traps.

Structural insights into the discrepant synergistic activity of Codlemone and (Z)-8-dodecenol towards Grapholita molesta pheromones

BACKGROUND

Insect pheromone synergists have been widely used to produce potent pheromone products for environment-friendly pest control. Codlemone (Cod) and (Z)-8-dodecenol (Dod) are two major Grapholita molesta pheromone synergists, with Cod having greater synergism and affinity for G. molesta pheromone binding protein 2 (GmolPBP2). Uncovering structural information key to the different binding affinity of Cod and Dod to GmolPBP2 would gain insights into what causes their synergy activity discrepancy.

RESULTS

Binding modes of the two synergists in the binding pocket of GmolPBP2 were analyzed and compared by molecular dynamics-based approaches. Although Cod and Dod were stabilized in a similar hydrophobic pocket, their interaction details with GmolPBP2 were divergent due to the extra double bond (C10═C11) in Cod. The C10═C11 improved the hydrophobic interactions of Cod with around residues. Such hydrophobic interaction improvement was also reflected in the raised importance of Phe11 in the GmolPBP2-Cod interaction. Not only that, the increased hydrophobic forces introduced by the C10═C11 changed the CH2-OH orientation in the GmolPBP2-Cod complex, which improved the H-bond interaction. Electrostatic complementarity analysis further indicated the positive role of C10═C11 in optimizing GmolPBP2-Cod interaction.

CONCLUSION

The C10═C11 is thought to contribute greatly to Cod's stronger synergy as a group key to the higher GmolPBP2-affinity, based on which the improvement directions for Cod and Dod were addressed as well. Our findings will aid in the development and optimization of more effective pheromone synergists, resulting in more effective pheromone-based pest management.

Identification and Characterization of CYP6 Family Genes from the Oriental Fruit Moth (Grapholita molesta) and Their Responses to Insecticides

Simple Summary

Eight CYP6 family genes were identified in Grapholita molesta (Busck). The expression of individual gene members differed between the developmental stages and insect tissues. High expression was found in third/fourth-instar larvae and in the midgut and Malpighian tubules. The response patterns of the genes exhibited diverse response patterns to the three representative insecticides were diverse.

Abstract

Cytochrome P450 (CYP) monooxygenases comprise a superfamily of proteins that detoxify xenobiotics and plant secondary metabolites in insects. The CYP6 family is unique to the class Insecta, and its members participate in the metabolism of exogenous substances. In this study, we sequenced and characterized the full-length cDNAs of eight CYP6 family genes from Grapholita molesta (Busck), a global pest of pome fruits. P450 genes with the exception of CYP6AN35, which was most highly expressed in adults, consistently showed high expression in third- or fourth-instar larvae. The analysis of different tissues of adults showed that most of these genes were predominantly expressed in the midgut, Malpighian tubules, and/or fat body. The expression of these eight CYP6 genes was differentially affected by three representative insecticides: malathion (organophosphate), deltamethrin (pyrethroid), and chlorantraniliprole (carbamate). All eight CYP6 genes responded to malathion treatment. Only three CYP6 genes were highly expressed in deltamethrin-treated individuals. Chlorantraniliprole treatment exerted weak effects on gene expression. Interestingly, CYP6AN35 was a highly expression level in the adult head and its expression was induced by all three insecticides. CYP6AN35 may be a key gene in the metabolism of insecticides. This study provides a fundamental understanding of the functions of the CYP6 gene family in insecticide metabolism in G. molesta.

Glutathione S-Transferase Genes are Involved in Lambda-Cyhalothrin Resistance in Cydia pomonella via Sequestration

Pest management is mostly accomplished by the use of insecticides. However, the overuse of insecticides has led to the development of resistance. Glutathione S-transferases (GSTs) are vital detoxification enzymes involved in insecticide resistance in insects. In this study, we report the involvement of GSTs in insecticide resistance to lambda-cyhalothrin in Cydia pomonella, a globally quarantined fruit pest. A total of 25 GST, including 22 cytosolic genes and 3 microsomal genes, are identified from the genome database of C. pomonella. These cytosolic genes are further classified into six classes, including four in delta, eight in epsilon, three in omega, three in sigma, one in theta, and one in zeta class, as well as two unclassified genes. The real-time quantitative polymerase chain reaction (RT-qPCR) shows that the majority of these genes are mainly expressed throughout the larval stage and in the midgut of the fourth-instar larvae. Exposure to an LD₁₀ dose of lambda-cyhalothrin resulted in the upregulation of 17 GST genes. Moreover, mRNA levels of most GST genes, with the exception of CpGSTe6, CpGSTd2, CpGSTd4, and CpGSTz1, are considerably higher in a lambda-cyhalothrin-resistant population (ZW_R) than those of susceptible strains. Recombinant CpGSTd1, CpGSTd3, CpGSTe3, and CpGSTs2 can bind and metabolize lambda-cyhalothrin, with the highest metabolic rate observed for CpGSTd3 but no metabolite(s) was detected, supporting the role of GSTs in sequestration of lambda-cyhalothrin. Molecular dynamics simulation analysis indicates that key residues of hydrophobic pocket-derived lipophilic energy S(lipo) interactions with a hydrophobic pharmacophore of lambda-cyhalothrin are crucial for metabolism by CpGSTd3 and further lead to resistance. Our study is the first to experimentally confirm the involvement of GSTs in lambda-cyhalothrin resistance via sequestration and provides new insights into resistance management in C. pomonella.

Reverse chemical ecology guides the screening for Grapholita molesta pheromone synergists

BACKGROUND

Pheromone-based management is a leading nonpesticidal strategy among integrated pest management options. Improving the potency of pheromone products by adding synergists would be a practical way to popularize pheromone-based management as well as to reduce pesticide use.

RESULTS

Using reverse chemical ecology, synergists for Grapholita molesta sex pheromone were screened. Combined molecular docking and in vitro binding assay led to the determination of four potentially active odorants showing high affinity to G. molesta pheromone binding protein 2 (GmolPBP2). Thereafter, the high affinity between Codlemone and GmolPBP2 was further verified by exploration of GmolPBP2-Codlemone interactions. As the only sex pheromone synergist validated by both laboratory behavioral tests and field trapping, Codlemone was used to optimize commercial sex attractants currently used in G. molesta control. The recommended formulation [(Z)-8-dodecenyl acetate:(E)-8-dodecenyl acetate:Codlemone = 95:4:10] was found to trap about five to six times more G. molesta adults than the commercial sex attractant [(Z)-8-dodecenyl acetate:(E)-8-dodecenyl acetate: (Z)-8-dodecenol = 95:4:1].

CONCLUSION

Codlemone is an excellent pheromone synergist that can be potentially sensed by GmolPBP2, which can remarkably improve the potency of G. molesta sex attractants. It is believed that the introduction of reverse chemical ecology would increase the chance of discovering pheromone synergists, promoting the development of more efficacious pheromone products that can be used in controlling G. molesta and beyond. © 2021 Society of Chemical Industry.

Ligand-binding properties of odorant-binding protein 6 in Athetis lepigone to sex pheromones and maize volatiles

BACKGROUND

Athetis lepigone, a noctuid moth feeding on more than 30 different crops worldwide, has evolved a sophisticated, sensitive, and specific chemosensory system to detect and discriminate exogenous chemicals. Odorant-binding proteins (OBPs) are the most important agent in insect chemosensory systems to be explored as an alternative target for environmentally friendly approaches to pest management.

RESULTS

To investigate the olfactory function of A. lepigone OBPs (AlepOBPs), AlepOBP6 was identified and expressed in Escherichia coli. The binding affinity of the recombinant OBP to 20 different ligands was then examined using a competitive binding approach. The results revealed that AlepOBP6 can bind to two sex pheromones and ten maize volatiles, and its conformation stability is pH dependent. We also carried out a structure-function study using different molecular approaches, including structure modeling, molecular docking, and a mutation functional assay to identify amino acid residues (M39, V68, W106, Q107, and Y114) involved in the binding of AlepOBP6 to both sex pheromones and maize volatiles in A. lepigone.

CONCLUSION

These results suggest that AlepOBP6 is likely involved in mediating the responses of A. lepigone to sex pheromones and maize volatiles, which may play a pivotal function in mating, feeding, and oviposition behaviors. This study not only provides new insight into the binding mechanism of OBPs to sex pheromones and host volatiles in moths, but also contributes to the discovery of novel target candidates for developing efficient behavior disruptors to control A. lepigone in the future. © 2021 Society of Chemical Industry.

Exploring disordered loops in DprE1 provides a functional site to combat drug-resistance in Mycobacterium strains

As an anti-tuberculosis target, DprE1 contains two flexible loops (Loop I and Loop II) which have never been exploited for developing DprE1 inhibitors. Here Leu317 in Loop II was discovered as a new functional site to combat drug-resistance in Mycobacterium strains. Based on TCA1, LZDT1 was designed to optimize the hydrophobic interaction with Leu317. A subsequent biochemical and cellular assay displayed increased potency of LZDT1 in inhibiting DprE1 and killing drug-sensitive/-resistant Mycobacterium strains. The improved activity of LZDT1 and its analogue LZDT2 against multidrug resistant tuberculosis was particularly highlighted. For LZDT1, its enhanced interaction with Leu317 also impaired the drug-insensitivity of DprE1 caused by Cys387 mutation. A new nonbenzothiazole lead (LZDT10) with reduced Cys387-dependence was further produced by optimizing interactions with Leu317, improvement directions for LZDT10 were discussed as well. Our research underscores the value of potential functional sites in disordered loops, and affords a feasible way to develop these functional sites into opportunities for drug-resistance management.

Comparison of Gut Bacterial Communities of Grapholita molesta (Lepidoptera: Tortricidae) Reared on Different Host Plants

Intestinal symbiotic bacteria have played an important role in the digestion, immunity detoxification, mating, and reproduction of insects during long-term coevolution. The oriental fruit moth, Grapholita molesta, is an important fruit tree pest worldwide. However, the composition of the G. molesta microbial community, especially of the gut microbiome, remains unclear. To explore the differences of gut microbiota of G. molesta when reared on different host plants, we determined the gut bacterial structure when G. molesta was transferred from an artificial diet to different host plants (apples, peaches, nectarines, crisp pears, plums, peach shoots) by amplicon sequencing technology. The results showed that Proteobacteria and Firmicutes are dominant in the gut microbiota of G. molesta. Plum-feeding G. molesta had the highest richness and diversity of gut microbiota, while apple-feeding G. molesta had the lowest. PCoA and PERMANOVA analysis revealed that there were significant differences in the gut microbiota structure of G. molesta on different diets. PICRUSt2 analysis indicated that most of the functional prediction pathways were concentrated in metabolic and cellular processes. Our results confirmed that gut bacterial communities of G. molesta can be influenced by host diets and may play an important role in host adaptation.

Identification of key residues of carboxylesterase PxEst-6 involved in pyrethroid metabolism in Plutella xylostella (L.)

The long-term and excessive use of insecticides has led to severe environmental problems and the evolution of insecticide resistance in insects. Carboxylesterases (CarEs) are important detoxification enzymes conferring insecticide resistance on insects. Herein, the detoxification process of Plutella xylostella (L.) carboxylesterase 6 (PxEst-6), one representative P. xylostella carboxylesterase, is investigated with cypermethrin, bifenthrin, cyfluthrin and λ-cyhalothrin. RT-qPCR shows that PxEst-6 is highly expressed in the midgut and cuticles of the third instar larvae. Exposure to pyrethroid insecticides resulted in PxEst-6 up-regulation in a short time. Metabolic assays indicate that PxEst-6 has the capacity to metabolize these pyrethroid insecticides. The combination of molecular docking, binding mode analyses and alanine mutations demonstrated that His451, Lys458 and Gln431 were key residues of PxEst-6 for metabolizing pyrethroids and the acetate groups derived from pyrethroids were key sites for being metabolized by PxEst-6. H451- and K458-derived hydrogen bond (H-bond) interactions with the pyrethroid acetate groups and the polar interactions with the pyrethroid acetate group provided by the Q431 sidechain were crucial to the pyrethroids' metabolism by PxEst-6. Our study contributes to revealing the reasons for pyrethroid resistance in P. xylostella, and provides a fundamental basis for the development of novel pyrethroid insecticides.

Analyses of structural dynamics revealed flexible binding mechanism for the Agrilus mali odorant binding protein 8 towards plant volatiles

BACKGROUND

Volatiles from host plants are an important source of insect pest attractants and repellents. Insect odorant binding proteins (OBPs) have been widely characterized, but the molecular binding dynamics and underlying mechanisms are still not well understood. Thus, we characterized binding characteristics of AmalOBP8 from the apple buprestid beetle (Agrilus mali Matsumura), an unprecedented serious threat to rare apple germplasm resources and local ecosystems.

RESULTS

Fluorescence studies demonstrated that the quenching mechanism was clearly static. AmalOBP8 was found to bind with both volatiles at single independent sites. Negative thermodynamic parameters suggested that binding interactions between AmalOBP8 and both volatiles could occur spontaneously. Hydrogen bonding was the key force in AmalOBP8's binding to geranyl formate, for which the amino acid residue Trp106 played a critical role in the binding pocket. Multiple Leu residues in AmalOBP8 created a strong hydrophobic environment, and formed the binding pocket for (Z)-3-hexenyl hexanoate. Compared to classic OBPs, in addition to lack of one disulfide bridge, AmalOBP8 had a small α-helix (α7) at the C-terminus, resulting in greater flexibility and adaptability for this protein to bind with different compound molecules.

CONCLUSION

Key residues of AmalOBP8 in binding interactions with plant volatiles were clarified. AmalOPB8 had a large ligand binding spectrum and great flexibility in binding with plant volatiles, providing good molecular targets for screening insect attractants and repellents. Our results can promote understanding of insects' perception of various odorants, and establish a foundation for discovery of new pest control agents. © 2020 Society of Chemical Industry.

Vitexin Inhibits Protein Glycation through Structural Protection, Methylglyoxal Trapping, and Alteration of Glycation Site

In this study, the antiglycation potential and mechanisms of vitexin were explored in vitro by multispectroscopy, microscope imaging, high-resolution mass spectrometry, and computational simulations. Vitexin was found to show much stronger antiglycation effects than aminoguanidine. The inhibition against the fluorescent advanced glycation end products was more than 80% at 500 μM vitexin in both bovine serum albumin (BSA)-fructose and BSA-methylglyoxal (MGO) models. Treated with 100 and 200 μM vitexin for 24 h, the contents of MGO were reduced to 4.97 and 0.2%, respectively, and only one vitexin-mono-MGO adduct was formed. LC-Orbitrap-MS/MS analysis showed that vitexin altered the glycated sites and reduced the glycation degree of some sites. The mechanisms of vitexin against protein glycation were mainly through BSA structural protection, MGO trapping, and alteration of glycation sites induced by interaction with BSA. These findings provided valuable information about the functional development of vitexin as a potential antiglycation agent.

Computational and Experimental Approaches to Decipher the Binding Mechanism of General Odorant-Binding Protein 2 from Athetis lepigone to Chlorpyrifos and Phoxim

Insect resistance to insecticides is an increasingly serious problem, and the resistant mechanisms are complicated. The resistance research based on the chemosensory pathway is one of the hot problems at present, but the specific binding mechanism of chemosensory genes and insecticides remains elusive. The binding mechanism of AlepGOBP2 (belong to insect chemosensory gene) with two insecticides was investigated by computational and experimental approaches. Our calculation results indicated that four key residues (Phe12, Ile52, Ile94, and Phe118) could steadily interact with these two insecticides and be assigned as hotspot sites responsible for their binding affinities. The significant alkyl-π and hydrophobic interactions involved by these four hotspot residues were found to be the driving forces for their binding affinities, especially for two residues (Phe12 and Ile94) that significantly contribute to the binding of chlorpyrifos, which were also validated by our binding assay results. Furthermore, we also found that the AlepGOBP2-chlorpyrifos/phoxim complexes can be more efficiently converged in the residue-specific force field-(RSFF2C) and its higher accuracy and repeatability in protein dynamics simulation, per-residue free energy decomposition, and computational alanine scanning calculations have also been achieved in this paper. These findings provided useful insights for efficient and reliable calculation of the binding mechanism of relevant AlepGOBPs with other insecticides, facilitating to develop new and efficient insecticides targeting the key sites of AlepGOBP2.

Structural evidence for pheromone discrimination by the pheromone binding protein 3 from Plutella xylostella

Insect pheromone binding proteins (PBPs) are believed to have a high degree of pheromone selectivity, acting as the first filter to discriminate specific pheromones from other volatile compounds. Herein, we provide evidence using homology-based model for the pheromone discrimination of Plutella xylostella pheromone binding protein 3 (PxPBP3). Combining molecular dynamics simulations and in vitro binding assays, two dominant sites are determined to be essential for the PxPBP3 to discriminate (Z)-11-hexadecenyl acetate (Hexadecenyl) from (Z)-11-hexadecenal (Hexadecenal). As the first key site for pheromone discrimination, Arg111 is indispensable to the PxPBP3-Hexadecenyl interaction. However, its importance in the binding of Hexadecenal to PxPBP3 is greatly reduced. A second site where pheromone discrimination occurs is a small loop (residues 34-38) in PxPBP3. It is shown that the hydrophobic strength provided by three hydrophobic residues (Phe34, Tyr37, and Trp38) in the small loop is significantly biased in the two complexes formed by PxPBP3 and the two pheromones. The discrimination capacity of PxPBP3 indicates that the P. xylostella pheromones may not share the same peri-receptor pathway, although they both show high affinity to PxPBP3.

The general odorant receptor GmolOR9 from Grapholita molesta (Lepidoptera: Tortricidae) is mainly tuned to eight host-plant volatiles

Grapholita molesta is one of the most destructive fruit pests distributed worldwide. Odorant receptors (ORs) located on the dendritic membrane of chemosensory neurons are deemed to be key molecules for sensing exogenous chemical signals. In this study, GmolOR9, a general OR from G. molesta, was functionally characterized. Quantitative real-time polymerase chain reaction revealed that GmolOR9 was more highly expressed in adults than in other stages, including eggs, larvae, and pupae. GmolOR9 expression was highly significantly more in the antennae of females than in those of males, and the highest level occurred in the antennae of 3-day-old female adults. GmolOR9 was broadly tuned to eight of 47 odorant components tested, including (Z)-3-hexenyl acetate, butyl propionate, ethyl hexanoate, ethyl heptanoate, 1-hexanol, (Z)-3-hexenol, 2-ethyl-1-hexanol, and linalool, by in vitro heterologous expression. Furthermore, electroantennogram responses indicated that the effects of dsOR9-injected females to (Z)-3-hexenyl acetate dramatically decreased. These results suggested that GmolOR9 might be involved in detecting host-plant volatiles. Moreover, (Z)-3-hexenyl acetate might serve as a potential attractant for the biological control of G. molesta.

Structure dynamics reveal key residues essential for the sense of 1-dodecanol by Cydia pomonella pheromone binding protein 2 (CpomPBP2)

BACKGROUND

Cydia pomonella, a worldwide quarantine fruit pest, causes great damage to fruit production every year. Sex pheromone-mediated control of C. pomonella has been widely used. As an indispensable ingredient of commercial sex attractants, 1-dodecanol (Dod) works to synergize the effect of codlemone in attracting male moths of C. pomonella. The interactions between Dod and its transporter protein, C. pomonella pheromone-binding protein 2 (CpomPBP2), provide inspiration for chemical optimizations to improve the synergistic effects of Dod.

RESULTS

In this research, molecular simulations and biological verifications were used in combination to uncover key residues in CpomPBP2 essential for sensing Dod. After performing 150 ns molecular dynamics (MD) simulations, the C1-C12 chain of Dod was found to be locked by the van der Waals energy contributed by the hydrophobic residues Phe12, Leu68, and Ile113, whereas the -OH part of Dod was anchored by the H-bond derived from Glu98 and the salt-bridge derived from Arg109. Because of the importance of these two electrostatic interactions, Glu98 and Arg109 were further verified as key residues in determining the binding affinity between Dod and CpomPBP2. In addition, interactions unfavorable to the binding of Dod were described.

CONCLUSION

The research detailed the discovery of key residues involved in CpomPBP2-Dod interactions. Our results provide guidance and caution for the prospective discovery, optimization, and design of novel chemicals with a similar or stronger synergistic effect to codlemone in controlling C. pomonella.

Organophosphorus insecticide interacts with the pheromone-binding proteins of Athetis lepigone: Implication for olfactory dysfunction

Athetis lepigone is one of the most severe polyphagous pests, and it has developed resistance to different chemical insecticides. Insects primarily rely on the olfactory system to recognize various environmental chemicals, including xenobiotics such as insecticides. Here, we expressed two A. lepigone pheromone-binding proteins (AlepPBP2 and AlepPBP3), and observed they had higher binding affinities to phoxim than other insecticides, with Ki was 3.30 ± 0.38 μM and 3.27 ± 0.10 μM, respectively. Molecular dynamics simulation, binding mode analysis, and computational alanine scanning showed that six residues (Phe15, Phe39, Ile55, Leu65, Ile97, and Phe122) of AlepPBP2 and three residues (Phe12, Ile52, and Ile134) of AlepPBP3 maybe as potential residues that can change protein ability to bind an organophosphorus insecticide phoxim. Then, we used site-directed mutagenesis assay to mutate these residues into alanine, respectively. Subsequently, the binding assays displayed that Phe15, Phe39, and Ile97 of AlepPBP2, Phe12 and Ile134 of AlepPBP3 caused a significant decrease of AlepPBPs binding ability to phoxim, suggesting they should play crucial roles in the AlepPBPs/phoxim interactions. Our findings could further advance in using PBPs as unique targets to design and develop precise and environmentally-friendly pest control agents with high insecticidal potential using a computer-aided drug design (CADD) approach.

Variability of Gut Microbiota Across the Life Cycle of Grapholita molesta (Lepidoptera: Tortricidae)

Grapholita molesta, the oriental fruit moth, is a serious global pest of many Rosaceae fruit trees. Gut microorganisms play important roles in host nutrition, digestion, detoxification, and resistance to pathogens. However, there are few studies on the microbiota of G. molesta, particularly during metamorphosis. Here, the diversity of gut microbiota across the holometabolous life cycle of G. molesta was investigated comprehensively by Illumina high-throughput sequencing technology. The results showed that the microbiota associated with eggs had a high number of operational taxonomic units (OTUs). OTU and species richness in early-instar larvae (first and second instars) were significantly higher than those in late-instar larvae (third to fifth instars). Species richness increased again in male pupae and adults, apparently during the process of metamorphosis, compared to late-instar larvae. Proteobacteria and Firmicutes were the dominant phyla in the gut and underwent notable changes during metamorphosis. At the genus level, gut microbial community shifts from Gluconobacter and Pantoea in early-instar larvae to Enterococcus and Enterobacter in late-instar larvae and to Serratia in pupae were apparent, in concert with host developmental changes. Principal coordinate analysis (PCoA) and linear discriminant analysis effect size (LEfSe) analyses confirmed the differences in the structure of gut microbiota across different developmental stages. In addition, sex-dependent bacterial community differences were observed. Microbial interaction network analysis showed different correlations among intestinal microbes at each developmental stage of G. molesta, which may result from the different abundance and diversity of gut microbiota at different life stages. Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt) analysis indicated that most functional prediction categories of gut microbiota were related to membrane transport, carbohydrate and amino acid metabolism, and DNA replication and repair. Bacteria isolated by conventional culture-dependent methods belonged to Proteobacteria, Firmicutes, and Actinobacteria, which was consistent with high-throughput sequencing results. In conclusion, exploration of gut bacterial community composition in the gut of G. molesta should shed light into deeper understanding about the intricate associations between microbiota and host and might provide clues to the development of novel pest management strategies against fruit borers.

Key Amino Acid Residues Influencing Binding Affinities of Pheromone-Binding Protein from Athetis lepigone to Two Sex Pheromones

Athetis lepigone is a polyphagous pest found around the world that feeds on maize, wheat, and various other important crops. Although it exhibits a degree of resistance to various chemical insecticides, an effective pest-control method has not yet been developed. The sex pheromone communication system plays an essential role in the mating and reproduction of moths, in which pheromone-binding proteins (PBPs) are crucial genes. In this study, we cloned and purified the protein AlepPBP1 using an E. coli expression system and found it had a higher binding affinity to two sex pheromones of A. lepigone, namely, Z7-12:Ac and Z9-14:Ac (with K_i 0.77 ± 0.10 and 1.10 ± 0.20 μM, respectively), than to other plant volatiles. The binding-mode analysis of protein conformation with equilibrium stabilization was obtained using molecular dynamics (MD) simulation and indicated that hydrophobic interactions involving several nonpolar residues were the main driving force for the binding affinity of AlepPBP1 with sex pheromones. Computational alanine scanning (CAS) was performed to further identify key amino acid residues and validate their binding contributions. Each key residue, including Phe36, Trp37, Val52, and Phe118, was subsequently mutated into alanine using site-directed mutagenesis. Binding assays showed that the efficient binding abilities to Z7-12:Ac (F36A, W37A, and F118A) and Z9-14:Ac (F36A, W37A, V52A, and F118A) were almost lost in the mutated proteins. Our results demonstrated that these key amino acid residues are crucial for determining the binding ability of AlepPBP1 to sex pheromones. These findings provide a basis for the use of AlepPBP1 in the studies as a specific target for the development of novel behavioral antagonists with marked inhibition or mating-disruption abilities using computer-aided drug design (CADD).

Detecting Host-Plant Volatiles with Odorant Receptors from Grapholita molesta (Busck) (Lepidoptera: Tortricidae)

Grapholita molesta is a global pest of stone and pome fruits. The sensitive olfactory system plays a crucial role in regulating key behavioral activities of insects and G. molesta relies heavily on general odorant receptors (ORs) to detect host-plant volatiles. In this study, three general OR genes from G. molesta (GmolOR12, GmolOR20, and GmolOR21) were identified. Quantitative polymerase chain reaction revealed that GmolORs expression was considerably higher in adults and adult antennae than in any other life stages and body parts, respectively. Moreover, the expression of GmolORs was significantly higher in the antennae of females than in those of males, with a peak in the antennae of 3-days-old adult females. GmolOR20 and GmolOR21 displayed no responses to any of the odorant compounds tested in the Xenopus oocyte system. GmolOR12 was tuned mainly to 5 of the 47 odorant components tested (including decanol, heptanal, octanal, nonanal, and decanal), and the response to aldehydes among the 5 components was the highest. Additionally, they all elicited female and male antennae electroantennogram responses, and the aldehydes elicited the highest response among the 5 components. These results suggested that GmolOR12 in the G. molesta olfactory system plays an important role in sensing aldehydes and that GmolOR12 is involved in sensing host-plant volatiles. These findings provide insight into the possibility of using host-plant volatiles for the control of G. molesta.

Different binding properties of two general-odorant binding proteins in Athetis lepigone with sex pheromones, host plant volatiles and insecticides

Athetis lepigone (Alep) is a polyphagous pest native to Europe and Asia that has experienced major outbreaks in the summer maize area of China since 2011 and has shown evidence of resistance to some insecticides. Insect olfaction is crucial for recognition of sex pheromones, host plant volatiles and even insecticides, in which two general-odorant binding proteins (GOBPs) play important roles. To elucidate the functions of GOBPs in A. lepigone, we first expressed the two AlepGOBP proteins in the E. coli expression system. Then, the results of fluorescence competitive binding assays demonstrated that the high binding affinity of AlepGOBP2 with sex pheromones [(Z)-7-dodecenyl acetate (Z7-12:Ac), Ki = 0.65 μM; (Z)-9-tetradecenyl acetate (Z9-14:Ac), Ki = 0.83 μM], two maize plant volatiles [Ocimene, Ki = 9.63 μM; (E)-β-Farnesene, Ki = 4.76 μM] and two insecticides (Chlorpyrifos Ki =5.61 μM; Phoxim, Ki = 4.38 μM). However, AlepGOBP1 could only bind Ocimene (Ki = 13.0 μM) and two insecticides (Chlorpyrifos Ki =4.46 μM; Phoxim, Ki = 3.27 μM). These results clearly suggest that AlepGOBP1 and AlepGOBP2 differentiate among odorants and other ligands. The molecular docking results further revealed different key residues involved in the ligand binding of AlepGOBPs. In summary, this study provides a foundation for exploring the olfactory mechanism of A. lepigone and identified two potential target genes for the development of highly effective insecticides in the future.