Binding proteins from the antennae of Bombyx mori

Binding characteristics and structural dynamics of two general odorant-binding proteins with plant volatiles in the olfactory recognition of Glyphodes pyloalis

Glyphodes pyloalis Walker (Lepidoptera: Pyralidae) is the most destructive pest, causing severe damage to mulberry production in China's sericulture industry. The insecticide application in mulberry orchards poses a significant risk of poisoning to Bombyx mori. Shifting from insecticides to odor attractants is a beneficial alternative, but not much data is available on the olfactory system of G. pyloalis. We identified 114 chemosensory genes from the antennal transcriptome database of G. pyloalis, with 18 odorant-binding protein (OBP) and 17 chemosensory protein (CSP) genes significantly expressed in the antennae. Ligand-binding assays for two antennae-biased expressed general odorant-binding proteins (GOBPs) showed high binding affinities of GOBP1 to hexadecanal, β-ionone, and 2-ethylhexyl acrylate, while GOBP2 exhibited binding to 4-tert-octylphenol, benzyl benzoate, β-ionone, and farnesol. Computational simulations indicated that van der Waal forces predominantly contributed to the binding free energy in the binding processes of complexes. Among them, Phe12 of GOBP1 and Phe19 of GOBP2 were demonstrated to play crucial roles in their bindings to plant volatiles using site-directed mutagenesis experiments. Moreover, hexadecanal and β-ionone attracted G. pyloalis male moths in the behavioral assays, while none of the candidate plant volatiles significantly affected female moths. Our findings provide a comprehensive understanding of the molecular mechanisms underlying olfactory recognition in G. pyloalis, setting the groundwork for novel mulberry pests control strategies based on insect olfaction.

Identification of attractants for adult Spodoptera litura based on the interaction between odorant-binding protein 34 and host volatiles

Odorant-binding proteins (OBPs) play key roles in host plant location by insects, and can accordingly serve as important targets for the development of attractants. In this study, we detected the high expression of SlitOBP34 in male antennae of Spodoptera litura. Subsequently, the fluorescence competitive binding experiments displayed that the SlitOBP34 protein has binding affinity for different ligands. Then, protein-ligand interaction analyses found the presence of six amino acid residues may serve as key recognition sites. Further electroantennographic and biobehavioral assessments revealed that the electrophysiological responses of male antennae were evoked in response to stimulation with the six identified host volatiles, and that these volatiles attracted male moths to varying extents. Notably, low concentrations of benzaldehyde, 1-hexanol, and cis-3-hexenyl acetate were found to have significant attractant effects on male moths, thereby identifying these three host volatiles as potential candidates for the development of male attractants. These findings advance our current understanding of the olfactory-encoded mechanisms of host plants selection in S. litura and have enabled us to develop novel adult attractants for controlling the pest in the future.

Essential oil components interacting with insect odorant-binding proteins: a molecular modelling approach

Essential oils (EOs) are natural products currently used to control arthropods, and their interaction with insect odorant-binding proteins (OBPs) is fundamental for the discovery of new repellents. This in silico study aimed to predict the potential of EO components to interact with odorant proteins. A total of 684 EO components from PubChem were docked against 23 odorant binding proteins from Protein Data Bank using AutoDock Vina. The ligands and proteins were optimized using Gaussian 09 and Sybyl-X 2.0, respectively. The nature of the protein-ligand interactions was characterized using LigandScout 4.0, and visualization of the binding mode in selected complexes was carried out by Pymol. Additionally, complexes with the best binding energy in molecular docking were subjected to 500 ns molecular dynamics simulations using Gromacs. The best binding affinity values were obtained for the 1DQE-ferutidine (-11 kcal/mol) and 2WCH-kaurene (-11.2 kcal/mol) complexes. Both are natural ligands that dock onto those proteins at the same binding site as DEET, a well-known insect repellent. This study identifies kaurene and ferutidine as possible candidates for natural insect repellents, offering a potential alternative to synthetic chemicals like DEET.

Different binding properties of odorant-binding protein 8 to insecticides in Orius sauteri

Chemical sensing systems are vital in the growth and development of insects. Orius sauteri (Poppius) (Hemiptera: Anthocoridae) is an important natural enemy of many pests. The molecular mechanism of odorant binding proteins (OBPs) binding with common insecticides is still unknow in O. sauteri. In this study, we expressed in vitro OsauOBP8 and conducted fluorescence competition binding assay to investigate the function of OsauOBP8 to insecticides. The results showed that OsauOBP8 could bind with four common insecticides (phoxim, fenitrothion, chlorpyrifos, deltamethrin). Subsequently, we used molecular docking to predict and obtained candidate six amino acid residues (K4, K6, K13, R31, K49, K55) and then mutated. The result showed that three key residues (K4, K6, R31) play important role in OsauOBP8 bound to insecticides. Our study identified the key binding sites of OsauOBP8 to insecticides and help to better understand the molecular mechanism of OBPs to insecticides in O. sauteri.

Antennal transcriptome analysis of Psyttalia incisi (silvestri) (Hymenoptera: Braconidae): identification and tissue expression profiling of candidate odorant-binding protein genes

BACKGROUND

Olfaction plays an important role in host-seeking by parasitoids, as they can sense chemical signals using sensitive chemosensory systems. Psyttalia incisi (Silvestri) (Hymenoptera: Braconidae) is the dominant parasitoid of Bactrocera dorsalis (Hendel) in fruit-producing regions of southern China. The olfactory behavior of P. incisi has been extensively studied; however, the chemosensory mechanisms of this species are not fully understood.

RESULTS

Bioinformatics analysis of 64,515 unigenes from the antennal transcriptome of both male and female adults P. incisi identified 87 candidate chemosensory genes. These included 13 odorant-binding proteins (OBPs), seven gustatory receptors (GRs), 55 odorant receptors (ORs), 10 ionotropic receptors (IRs), and two sensory neuron membrane proteins (SNMPs). Phylogenetic trees were constructed to predict evolutionary relationships between these chemosensory genes in hymenopterans. Moreover, the tissue expression profiles of 13 OBPs were analyzed by quantitative real-time PCR, revealing high expression of seven OBPs (1, 3, 6, 7, 8, 12, and 13) in the antennae.

CONCLUSION

This study represents the first identification of chemosensory genes and the determination of their expression patterns in different tissues of P. incisi. These results contribute to a better understanding of the function of the chemosensory system of this parasitoid species.

Molecular and Functional Characterization of Three General Odorant-Binding Protein 2 Genes in Cydia pomonella (Lepidoptera: Tortricidae)

General odorant-binding proteins (GOBPs) play a crucial role in the detection of host plant volatiles and pheromones by lepidopterans. Previous studies identified two duplications in the GOBP2 gene in Cydia pomonella. In this study, we employed qRT-PCR, protein purification, and fluorescence competitive binding assays to investigate the functions of three GOBP2 genes in C. pomonella. Our findings reveal that CpomGOBP2a and CpomGOBP2b are specifically highly expressed in antennae, while CpomGOBP2c exhibits high specific expression in wings, suggesting a potential divergence in their functions. Recombinant proteins of CpomGOBP2a, CpomGOBP2b, and CpomGOBP2c were successfully expressed and purified, enabling an in-depth exploration of their functions. Competitive binding assays with 20 host plant volatiles and the sex pheromone (codlemone) demonstrated that CpomGOBP2a exhibits strong binding to four compounds, namely butyl octanoate, ethyl (2E,4Z)-deca-2,4-dienoate (pear ester), codlemone, and geranylacetone, with corresponding dissolution constants (Ki) of 8.59993 μM, 9.14704 μM, 22.66298 μM, and 22.86923 μM, respectively. CpomGOBP2b showed specific binding to pear ester (Ki = 17.37481 μM), while CpomGOBP2c did not exhibit binding to any tested compounds. In conclusion, our results indicate a functional divergence among CpomGOBP2a, CpomGOBP2b, and CpomGOBP2c. These findings contribute valuable insights for the development of novel prevention and control technologies and enhance our understanding of the evolutionary mechanisms of olfactory genes in C. pomonella.

The binding affinity of two general odorant binding proteins in Spodoptera frugiperda to general volatiles and insecticides

Spodoptera frugiperda is a kind of polyphagous pest, and can damage a large number different host plants around the worldwide. The molecular mechanisms of two general odorant binding proteins (GOBPs) binding with general volatiles and insecticides are still blank. In this study, we investigated the function of two GOBPs in S. frugiperda, by expressing two SfruGOBPs and tested the binding affinities by the fluorescence competition binding assays. The results exhibited that SfruGOBP1 has binding affinities to 4 of 38 general volatiles and 3 of 7 insecticides. In contrast, SfruGOBP2 showed a broader ligand-binding spectrum to 21 volatiles and 4 insecticides, suggesting SfruGOBP2 may plays a more important role in perceiving host volatiles than SfruGOBP1. Furthermore, we used molecular docking and site-directed mutagenesis assay to explored the key amino acid residues of two SfruGOBP to insecticides ligand. This study provides some valuable information to exploring the olfactory mechanism of two GOBPs bound the host plant volatiles and insecticides in S. frugiperda.

Expression, affinity, and binding mode analysis of antennal-binding protein X in the variegated cutworm Peridroma saucia (Hübner)

The variegated cutworm Peridroma saucia (Hübner) is a worldwide pest that causes serious damage to many crops. Odorant-binding proteins (OBPs) are small soluble proteins involved in the first step of odorant reception. In moths, antennal-binding protein Xs (ABPXs) represent a main subfamily of classic OBPs. However, their functions remain unclear. Here, we cloned the ABPX gene from the antennae of P. saucia. RT-qPCR and western-blot analyses showed that PsauABPX is antenna-predominant and male-biased. Further temporal expression investigation indicated that the expression of PsauABPX started 1 day before eclosion and reached the highest 3 days after eclosion. Next, fluorescence binding assays revealed that recombinant PsauABPX had high binding affinities with P. saucia female sex pheromone components Z11-16: Ac and Z9-14: Ac. Then, molecular docking, molecular dynamics simulation, and site-directed mutagenesis were employed to identify key amino acid residues involved in the binding of PsauABPX to Z11-16: Ac and Z9-14: Ac. The results demonstrated that Val-32, Gln-107 and Tyr-114 are essential for the binding to both sex pheromones. This study not only give us insight into the function and binding mechanism of ABPXs in moths, but could also be used to explore novel strategies to control P. saucia.

Analysis of the Antennal Transcriptome and Identification of Tissue-specific Expression of Olfactory-related Genes in Micromelalopha troglodyta (Lepidoptera: Notodontidae)

Micromelalopha troglodyta (Graeser) has been one of the most serious pests on poplars in China. We used Illumina HiSeq 2000 sequencing to construct an antennal transcriptome and identify olfactory-related genes. In total, 142 transcripts were identified, including 74 odorant receptors (ORs), 32 odorant-binding proteins (OBPs), 13 chemosensory proteins (CSPs), 20 ionotropic receptors (IRs), and 3 sensory neuron membrane proteins (SNMPs). The genetic relationships were obtained by the phylogenetic tree, and the tissue-specific expression of important olfactory-related genes was determined by quantitative real-time PCR (qRT-PCR). The results showed that most of these genes are abundantly expressed in the antennae and head. In most insects, olfaction plays a key role in foraging, host localization, and searching for mates. Our research lays the foundation for future research on the molecular mechanism of the olfactory system in M. troglodyta. In addition, this study provides a theoretical basis for exploring the relationship between M. troglodyta and their host plants, and for the biological control of M. troglodyta using olfactory receptor as targets.

Ligand-binding specificities of four odorant-binding proteins in Conogethes punctiferalis

Odorant-binding proteins (OBPs) play essential roles in lepidopteran insects' perception of host volatiles by binding and transporting hydrophobic ligands. The yellow peach moth (YPM), Conogethes punctiferalis (Guenée), is a serious agricultural pest, with broad host range and cryptic feeding habits. However, few studies about YPM perceiving pheromones and host plant odorants have been reported. In this study, four OBP genes (CpunOBP8, CpunOBP9, CpunABP, and CpunGOBP2) were cloned from the antennae of YPM. The recombinant proteins were expressed and purified by prokaryotic expression system, with their binding affinities to 26 ligands being tested. Four CpunOBPs all had six conserved cysteine residues, which were typical structural characteristics of classical OBPs. The fluorescence competitive binding assay indicated that CpunOBP8 and CpunABP could not only exhibit high binding affinities to female sex pheromones, but also to host plant odorants. For example, CpunOBP8 bound strongly with cis-10-hexadecenal, hexadecanal, and so forth, whereas CpunABP bound with cis-10-hexadecenal, camphene, and 3-carene. Comparatively, CpunOBP9 and CpunGOBP2 could only bind with host plant odorants, with CpunOBP9 binding strongly to 3-methyl-1-butanol, hexyl acetate, and so forth, while CpunGOBP2 displaying the widest binding spectra and correlating with 3-carene, pentyl acetate, and so forth. The results indicated that on the one hand, each of the four CpunOBPs had its specific binding spectra when binding and transporting olfactory ligands; on the other hand, the same ligand might be bound to more than one CpunOBPs, which would provide information for the potential application of semiochemicals in controlling YPM.

EsigGOBP1: The Key Protein Binding Alpha-Phellandrene in Endoclita signifer Larvae

Endoclita signifer larvae show olfactory recognition towards volatiles of eucalyptus trunks and humus soils. Further, EsigGOBP1 was identified through larval head transcriptome and speculated as the main odorant-binding proteins in E. signifer larvae. In this study, the highest expression of EsigGOBP1 was only expressed in the heads of 3rd instar larvae of E. signifer, compared with the thorax and abdomen; this was consistent with the phenomenon of habitat transfer of 3rd instar larvae, indicating that EsigGOBP1 was a key OBP gene in E. signifer larvae. Results of fluorescence competition binding assays (FCBA) showed that EsigGOBP1 had high binding affinities to eight GC-EAD active ligands. Furthermore, screening of key active odorants for EsigGOBP1 and molecular docking analysis, indicated that EsigGOBP1 showed high binding activity to alpha-phellandrene in 3rd instar larvae of E. signifer. Conformational analysis of the EsigGOBP1-alpha-phellandrene complex, showed that MET49 and GLU38 were the key sites involved in binding. These results demonstrated that EsigGOBP1 is a key odorant-binding protein in E. signifer larvae, which recognizes and transports eight key volatiles from eucalyptus trunk, especially the main eucalyptus trunks volatile, alpha-phellandrene. Taken together, our results showed that EsigGOBP1 is involved in host selection of E. signifer larvae, which would aid in developing EsigGOBP1 as molecular targets for controlling pests at the larval stage.

Genome-Wide Analysis of Odorant-Binding Proteins and Chemosensory Proteins in the Bean bug Riptortus pedestris

Insects have sensitive olfactory systems to interact with environment and respond to the change in host plant conditions. Key genes in the system can be potential targets for developing new and efficient pest behaviour control methods. Riptortus pedestris is an important soybean pest in East Asia and has caused serious damage to the soybean plants in Huang-Huai-Hai region of China. However, the current treatment of pests is dominated by chemical insecticides and lacks efficient sustainable prevention and control technologies. In this study, we identified 49 putative odorant-binding proteins (OBPs) (43 were new genes) and 25 chemosensory proteins (CSPs) (17 were new genes) in R. pedestris genome. These OBP and CSP genes are clustered in highly conserved groups from other hemipteran species in phylogenetic trees. Most RpedOBPs displayed antennal-biased expression. Among the 49 RpedOBPs, 33 were significantly highly expressed in the antennae, including three male-biased and nine female-biased. While many RpedCSPs were detected both in the antennae and in non-antennal tissues, only 11 RpedCSPs displayed antennal-biased expression, in which four RpedCSPs were male-biased and five RpedCSPs were female-biased. Some OBP and CSP genes showed sex-biased expression profiles. Our results not only provide a foundation for future exploration of the functions of RpedOBPs and RpedCSPs but also aid in developing environmentally friendly insecticides in the future.

Candidate Chemosensory Genes Identified in the Adult Antennae of Sympiezomias velatus and Binding Property of Odorant-Binding Protein 15

Chemosensory genes play important roles in insect behaviors and have thus become potential molecular targets for pest control based on the manipulation of chemoreception-driven behaviors. The great gray weevil Sympiezomias velatus (Chevrolat) (Coleoptera: Curculionidae) is an important agricultural pest that causes serious economic losses to many crops in China, but its chemosensory genes have not been reported. Here we assembled the antennal transcriptomes of female and male adult S. velatus and revealed the major chemosensory genes necessary for olfaction. A total of 138 candidate chemosensory genes in six families were identified, including 41 encoding odorant-binding proteins (OBPs), 11 encoding chemosensory proteins (CSPs), 62 encoding odorant receptors (ORs), 15 encoding gustatory receptors (GRs), six encoding ionotropic receptors (IRs), and three encoding sensory neuron membrane proteins (SNMPs). We analyzed their phylogenetic relationship based on the amino acid sequences of these chemosensory-related protein families in S. velatus and other insects, and the expression profiles based on their antennal transcriptomes. Chemosensory genes that show antenna-abundant/specific or sex-biased expression were observed, suggesting that these genes might have functions in olfaction. Furthermore, we chose an antenna-abundant OBP belonging to ABPX subfamily, SvelOBP15, to investigate its binding property. The results showed that among 33 tested compounds, SvelOBP15 displayed high binding affinities (Ki = 7.36-12.94 μmol/L) with farnesol, nerolidol, limonene and diisobutyl phthalate, indicating that SvelOBP15 plays olfactory roles by binding and transporting specific plant volatiles. These findings will help us better understand the olfactory systems of S. velatus, and provide a basis for functional elucidation of these chemosensory genes.

Transcriptome analysis and molecular characterization of soluble chemical communication proteins in the parasitoid wasp Anagrus nilaparvatae (Hymenoptera: Mymaridae)

Anagrus nilaparvatae is an important egg parasitoid wasp of pests such as the rice planthopper. Based on the powerful olfactory system of sensing chemical information in nature, A. nilaparvatae shows complicated life activities and behaviors, such as feeding, mating, and hosting. We constructed a full‐length transcriptome library and used this to identify the characteristics of soluble chemical communication proteins. Through full‐length transcriptome sequencing, splicing, assembly, and data correction by Illumina, we obtained 163.59 Mb of transcriptome data and 501,179 items with annotation information. We then performed Gene Ontology (GO) functional classification of the transcriptome's unigenes. We analyzed the sequence characteristics of soluble chemical communication protein genes and identified eight genes: AnilOBP2, AnilOBP9, AnilOBP23, AnilOBP56, AnilOBP83, AnilCSP5, AnilCSP6, and AnilNPC2. After sequence alignment and conserved domain prediction, the eight proteins encoded by the eight genes above were found to be consistent with the typical characteristics of odorant‐binding proteins (OBPs), chemosensory proteins (CSPs), and Niemann‐pick type C2 proteins (NPC2s) in other insects. Phylogenetic tree analysis showed that the eight genes share low homology with other species of Hymenoptera. Quantitative real‐time polymerase chain reaction (RT‐qPCR) was used to analyze the expression responses of the eight genes in different sexes and upon stimulation by volatile organic compounds. The relative expression levels of AnilOBP9, AnilOBP26, AnilOBP83, AnilCSP5, and AnilNPC2 in males were significantly higher than those in females, while the relative expression level of AnilCSP6 was higher in females. The expression levels of AnilOBP9 and AnilCSP6 were significantly altered by the stimulation of β‐caryophyllene, suggesting that these two genes may be related to host detection. This study provides the first data for A. nilaparvatae's transcriptome and the molecular characteristics of soluble chemical communication proteins, as well as an opportunity for understanding how A. nilaparvatae behaviors are mediated via soluble chemical communication proteins.

Involvement of GOBP2 in the perception of a sex pheromone component in both larval and adult Spodoptera litura revealed using CRISPR/Cas9 mutagenesis

General odorant-binding proteins (GOBPs) are long considered responsible for the perception of plant odorants. In this study with the important noctuid pest Spodoptera litura, we functionally characterized that GOBP2 is also involved in the perception of sex pheromone components using in vivo CRISPR/Cas9 technique. First, the GOBP2 sgRNA and Cas9 protein were injected into the newly laid insect eggs, resulting in a 35.6% target mutagenesis in G0 moths. Then, the homozygous GOBP2 knockout strain (GOBP2^-/-) was obtained after the screening of three generations. The knockout male and female moths displayed a significant reduction in EAG responses to the sex pheromone components, and the knockout females also displayed a significant reduction to plant odorants. In the behavioral assay of food choice, GOBP2^-/- larvae lost the preference to artificial diet added with the major sex pheromone component Z9, E11-tetradecadienyl acetate (Z9, E11-14:Ac), whereas the WT larvae highly preferred the pheromone diet. Y-tube olfactometer assay and direct pheromone stimulation assay showed that GOBP2^-/- male adults reduced significantly than WT males in percentages of choice, hair pencil displaying and mating attempt to Z9, E11-14:Ac. In the oviposition test, GOBP2^-/- females showed significantly reduced preference for the soybean plants compared to the WT females. Our study demonstrated that GOBP2 plays an important role in perceiving sex pheromones in adult and larval stages, providing new insight into sex pheromone perception and a potential target for sex pheromone-based behavioral regulation in the pest.

The Adelphocoris lineolatus OBP4: Support for evolutionary and functional divergence of a mirid pheromone-binding protein from that found in lepidopteran moths

Pheromone-binding proteins (PBPs) have been extensively investigated in lepidopteran moths, but their evolution and function in hemipteran species remain unclear. Our previous study demonstrated that an odorant-binding protein, OBP4, of the mirid bug Adelphocoris lineolatus functions as a candidate hemipteran PBP but clustered with lepidopteran antennae-binding proteins (ABPs) rather than in the PBP/general odorant-binding protein (GOBP) clade. In this study, we hypothesized that origin and function of PBPs in hemipteran bugs may differ from those of lepidopteran moths. To test this hypothesis, we first constructed a phylogenetic tree using insect OBPs from sister hemipteran and holometabolous lineages, and the results indicated that neither OBP4 nor other types of candidate PBPs of mirid bugs clustered with the lepidopteran PBP/GOBP clade. Then, a fluorescence competitive binding assay was employed to determine binding affinities of recombinant OBP4 protein to host plant volatiles, with functional groups different from A. lineolatus sex pheromone components. The results revealed that OBP4 highly bound the female adult attractant 3-hexanone and 15 other mirid bug biologically active plant volatiles. Finally, we examined cellular expression profiles of OBP4 in putative antennal sensilla that are related to female A. lineolatus host plant location. The fluorescence in situ hybridization and immunocytochemical labeling assay showed that the OBP4 gene was highly expressed in the multiporous olfactory sensilla medium-long sensilla basiconica rather than in the short sensilla basiconica or uniporous sensilla chaetica. These results, together with those of our previous studies, indicate that OBP4 not only functions in recognition of bug-produced sex pheromones in males, but is probably involved in detection of host plant volatiles in both A. lineolatus sexes. Our findings support the hypothesis that the origin and function of PBPs in hemipteran bugs differ from those of well-known PBPs in lepidopteran moths, which provides a novel perspective on evolutionary mechanisms of sex pheromone communication across insect orders.

Molecular characterization of sex pheromone binding proteins from Holotrichia oblita (Coleoptera: Scarabaeida)

Pheromone binding proteins (PBPs), a subfamily of the odorant binding proteins (OBPs), capture and transfer sex pheromones across the sensillum lymph to pheromone receptors and initiate insect courtship and mating. In this study, we functionally characterized ten OBPs from the black chafer, Holotrichia oblita (HoblOBPs), among which six HoblOBPs (HoblOBP2, 4, 5, 8, 9 and 24) were shown to recognize sex pheromones using electroantennography assays (EAG) and in vitro fluorescence competitive binding assays. Insect tropism to sex pheromones was significantly reduced after those genes were knocked down in vivo, e.g. HoblOBP24 RNAi reduced the tropism of H. oblita to methyl glycinate by 34%. Furthermore, molecular docking revealed key residues for the binding of the six HoblOBPs with sex pheromones. And hydrogen bonds and hydrophobic forces were shown to be the main forces in the binding of the six HoblOBPs and their sex pheromone ligands. Our study characterized six H. oblita PBPs and their binding abilities to sex pheromone ligands. The results will improve our understanding on the olfactory mechanisms that H. oblita utilizes to recognize sex pheromones, and will promote the development of novel strategies for controlling H. oblita and other insect pests.

An Expanded Survey of the Moth PBP/GOBP Clade in Bombyx mori: New Insight into Expression and Functional Roles

We studied the expression profile and ontogeny (from the egg stage through the larval stages and pupal stages, to the elderly adult age) of four OBPs from the silkworm moth Bombyx mori. We first showed that male responsiveness to female sex pheromone in the silkworm moth B. mori does not depend on age variation; whereas the expression of BmorPBP1, BmorPBP2, BmorGOBP1, and BmorGOBP2 varies with age. The expression profile analysis revealed that the studied OBPs are expressed in non-olfactory tissues at different developmental stages. In addition, we tested the effect of insecticide exposure on the expression of the four OBPs studied. Exposure to a toxic macrolide insecticide endectocide molecule (abamectin) led to the modulated expression of all four genes in different tissues. The higher expression of OBPs was detected in metabolic tissues, such as the thorax, gut, and fat body. All these data strongly suggest some alternative functions for these proteins other than olfaction. Finally, we carried out ligand docking studies and reported that PBP1 and GOBP2 have the capacity of binding vitamin K1 and multiple different vitamins.

Odorant-Binding Protein 1 Plays a Crucial Role in the Olfactory Response of Bemisia tabaci to R-Curcumene

The cultivated tomato Solanum lycopersicum suffered a severe attack by the whitefly Bemisia tabaci (Gennadius), causing damage to leaves by feeding as well as transmitting the tomato yellow leaf curl virus (TYLCV), while the wild tomato S. habrochaites is considerably less appealing to this insect species. It is reported that B. tabaci shows innate avoidance to R-curcumene, which is produced naturally by S. habrochaites. However, the mechanisms involved in the avoidance behavior of B. tabaci in response to this chiral compound are still unclear yet. In this study, the functional and binding characterization of odorant-binding protein 1 of B. tabaci (BtOBP1) were examined in vivo and in vitro against R-curcumene. The obtained results showed that BtOBP1 exhibits specific binding activity to R-curcumene, which acts as repellents to B. tabaci. By using a fluorescence-based binding assay, the difference of binding-affinity for R-curcumene between wild type BtOBP1 and the mutant BtOBP1 to R-curcumene was performed, which resulted in a single amino acid mutation (ASN108 > SER); moreover, BtOBP1-N108 displays significantly decreased binding affinities to R-curcumene. Most interestingly, a knock-down experiment with the BtOBP1 showed that the whitefly responses to R-curcumene are impaired. This study illustrated that BtOBP1 is a crucial protein involved in the perception and discrimination of R-curcumene. Our findings may provide an excellent chance of finding a suitable antagonist of eco-friendly features that can block the perception of chemosensory signals in insects, preventing behaviors like food-finding.

GOBP1 from the Variegated Cutworm Peridroma saucia (Hübner) (Lepidoptera: Noctuidae) Displays High Binding Affinities to the Behavioral Attractant (Z)-3-Hexenyl acetate

The variegated cutworm Peridroma saucia (Hübner) is a worldwide pest that causes serious damage to many crops. To recognize sex pheromones and host plant volatiles, insects depend on olfactory chemoreception involving general odorant-binding proteins (GOBPs). In this study, PsauGOBP1 was cloned from the adult antennae of P. saucia. RT-qPCR and Western-blot analysis showed that PsauGOBP1 was specifically and equally expressed in the adult antennae of both females and males. Fluorescence competitive-binding assays with sex pheromones and host plant volatiles demonstrated that PsauGOBP1 bound to six host plant volatiles: (Z)-3-hexenyl acetate (K_D = 4.0 ± 0.1 μM), citral (K_D = 5.6 ± 0.4 μM), farnesol (K_D = 6.4 ± 0.6 μM), nonanal (K_D = 6.8 ± 0.3 μM), (Z)-3-hexen-1-ol (K_D = 8.5 ± 0.6 μM), and benzaldehyde (K_D = 9.4 ± 0.5 μM). Electroantennogram recordings with the six host plant volatiles indicated that (Z)-3-hexenyl acetate elicited the strongest responses from both male and female antennae. Further bioassays using Y-tube olfactometers showed that (Z)-3-hexenyl acetate was attractive to adult P. saucia of both sexes. These results suggest that PsauGOBP1 might be involved in detecting host plant volatiles and that (Z)-3-hexenyl acetate might serve as a potential attractant for the biological control of P. saucia.

An Overview of Antennal Esterases in Lepidoptera

Lepidoptera are used as a model for the study of insect olfactory proteins. Among them, odorant degrading enzymes (ODEs), that degrade odorant molecules to maintain the sensitivity of antennae, have received less attention. In particular, antennal esterases (AEs; responsible for ester degradation) are crucial for intraspecific communication in Lepidoptera. Currently, transcriptomic and genomic studies have provided AEs in several species. However, efforts in gene annotation, classification, and functional assignment are still lacking. Therefore, we propose to combine evidence at evolutionary, structural, and functional level to update ODEs as well as key information into an easier classification, particularly of AEs. Finally, the kinetic parameters for putative inhibition of ODEs are discussed in terms of its role in future integrated pest management (IPM) strategies.

The 40-Year Mystery of Insect Odorant-Binding Proteins

The survival of insects depends on their ability to detect molecules present in their environment. Odorant-binding proteins (OBPs) form a family of proteins involved in chemoreception. While OBPs were initially found in olfactory appendages, recently these proteins were discovered in other chemosensory and non-chemosensory organs. OBPs can bind, solubilize and transport hydrophobic stimuli to chemoreceptors across the aqueous sensilla lymph. In addition to this broadly accepted "transporter role", OBPs can also buffer sudden changes in odorant levels and are involved in hygro-reception. The physiological roles of OBPs expressed in other body tissues, such as mouthparts, pheromone glands, reproductive organs, digestive tract and venom glands, remain to be investigated. This review provides an updated panorama on the varied structural aspects, binding properties, tissue expression and functional roles of insect OBPs.

Identification and analysis of olfactory genes in Dioryctria abietella based on the antennal transcriptome

The coneworm Dioryctria abietella (Lepidoptera: Pyralidae) is an economy devastating pest that infests many valuable conifer species in the Holarctic regions, such as Pinus koraiensis Siebold and Zucc. The chemosensory system plays a crucial role in the mating, foraging, and ovipositing of this pest, and therefore it is desirable to identify chemosensory molecules for pest control. However, little is known at molecular level about the olfactory mechanisms in D. abietella. In the present study, we first established antennal transcriptomes of D. abietella and identified 132 putative chemosensory genes, including 15 odorant-binding proteins, 18 chemosensory proteins, 65 odorant receptors, 5 sensory neuron membrane proteins, 24 ionotropic receptors, and 5 gustatory receptors. In addition, phylogenetic trees were constructed for chemosensory genes to investigate the orthologs between D. abietella and other species of insects. Furthermore, we also compared the patterns of motifs between OBPs and CSPs using MEME. Additionally, we observed that most of DabiOBPs and DabiCSPs had the antenna-biased expression by quantitative real-time PCR (RT-qPCR), and there was a higher expression of DabiPBP1 and DabiPBP2 in male antennae than in female antennae. The binding sites of DabiPBPs (DabiPBP1, DabiPBP2) and DabiPRs (DabiOR19, DabiOR31) to the sex pheromone were predicted well by three-dimensional docking structure modelling and molecular docking. Our finding supplied a foundation for further research on the binding process of OBPs or CSPs and sensing process of ORs, SNMPs, IRs or GRs in D. abietella.

Identification and Functional Analysis of SlitOBP11 From Spodoptera litura

Odorant binding proteins (OBPs) play a key role in the olfactory recognition of insects, whose functions have been extensively studied in adult insects but rarely in larvae. In this study, one OBP (SlitOBP11) with high expression in larval antenna but low expression in adult antenna of Spodoptera litura was screened by RNA-seq and verified by quantitative real-time PCR. Furthermore, the function of SlitOBP11 was explored by analysis of the expression patterns and prokaryotic expression of proteins as well as assays of competitive binding. Competitive binding assay demonstrated that SlitOBP11 had high binding affinity to all four female sex pheromone components, but exhibited almost no binding affinity to plant volatiles except for a low affinity to Phenylacetaldehyde and Phenethyl acetate. Homology modeling and molecular docking implied that the shape of these four sex pheromones were linear, which were appropriate for the binding channel of SlitOBP11 and the amino acid residue Asn99 of SlitOBP11 might play an important role in binding. Taken together, our results indicate that SlitOBP11 may be involved in the perception of female sex pheromones by S. litura larvae, and OBPs in the larvae of S. litura play an important role in the olfactory perception process.

Isolation, Identification, and Bioinformatic Analysis of Antibacterial Proteins and Peptides from Immunized Hemolymph of Red Palm Weevil Rhynchophorus ferrugineus

Red palm weevil (Rhynchophorus ferrugineus Olivier, 1791, Coleoptera: Curculionidae) is a destructive pest of palms, rapidly extending its native geographical range and causing large economic losses worldwide. The present work describes isolation, identification, and bioinformatic analysis of antibacterial proteins and peptides from the immunized hemolymph of this beetle. In total, 17 different bactericidal or bacteriostatic compounds were isolated via a series of high-pressure liquid chromatography steps, and their partial amino acid sequences were determined by N-terminal sequencing or by mass spectrometry. The bioinformatic analysis of the results facilitated identification and description of corresponding nucleotide coding sequences for each peptide and protein, based on the recently published R. ferrugineus transcriptome database. The identified compounds are represented by several well-known bactericidal factors: two peptides similar to defensins, one cecropin-A1-like peptide, and one attacin-B-like protein. Interestingly, we have also identified some unexpected compounds comprising five isoforms of pheromone-binding proteins as well as seven isoforms of odorant-binding proteins. The particular role of these factors in insect response to bacterial infection needs further investigation.

Interpopulational Variations of Odorant-Binding Protein Expression in the Black Cutworm Moth, Agrotis ipsilon

Simple Summary

Odorant-binding proteins (OBPs) are small soluble transporter proteins that are believed to play a key role in insect olfaction. However, there is an emerging set of data that shows a role in insecticide resistance for similar families of binding proteins. The black cutworm Agrotis ipsilon is a migrant species of moth known to feed on multiple types of crops (polyphagous) worldwide. It is therefore likely that the olfactory system of this species can be modulated to adapt to different environments. We compared gene expression between American and European continental populations of the moth. We found continental-specific expression of antennal binding protein X (ABPX) and general odorant-binding protein 2 (GOBP2), suggesting a function of these proteins in migration, environment recognition, crop change and adaptation that are required for a polyphagous species such as A. ipsilon.

Abstract

A long-range migrant species of moth (Agrotis ipsilon) has served as a model to compare the expression profiles of antennal proteins between different continental populations. Our results showed that the American and French populations of the black cutworm moth, A. ipsilon, expressed the same odorant-binding proteins (OBPs), but apparently in different levels. Electrophoretic analysis of antennal protein profiles and reverse transcription polymerase chain reaction using RNA as a template showed significant differences between the two populations in the expression of antennal binding protein-X (ABPX) and general odorant-binding protein-2 (GOBP2). However, the two A. ipsilon populations showed no differences in RNA levels coding for pheromone binding proteins (PBPs), suggesting that the expression of generalist OBPs is population-specific and could be affected by specific odor and/or chemical changes in external environmental conditions. To support the role of ABPX and GOBP2 with expression, the role of ABPX and GOBP2 is discussed in regard to odor detection, memorization and/or degradation of toxic chemical insecticides.

Comparative genomic and transcriptomic analyses of chemosensory genes in the citrus fruit fly Bactrocera (Tetradacus) minax

The citrus fruit fly Bactrocera (Tetradacus) minax is a major and devastating agricultural pest in Asian subtropical countries. Previous studies have shown that B. minax interacts with plant hosts via the efficient chemosensory system. However, the molecular components of the B. minax chemosensory system have not been well characterized. Herein, we identified a total of 25 putative odorant-binding receptors (OBPs), 4 single-copy chemosensory proteins (CSPs) and 53 candidate odorant receptors (ORs) using a newly generated whole-genome dataset for B. minax. This study significantly extended the chemosensation-related gene profiles (particularly, OBPs and ORs) in six other tephritid species. Comparative transcriptome analysis of adult B. minax and Bactrocera dorsalis showed that there were 14 highly expressed OBPs (FPKM > 100) in B. dorsalis and 7 highly expressed ones in B. minax. The expression level of CSP3 gene and CSP4 gene was higher in B. dorsalis than that in B. minax. Comparative genomic and transcriptomic analyses of chemosensory genes in the citrus fruit fly B. minax provided new insights for preventive control of this agriculture important pest and closely related species.

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.

Silencing of OBP genes: Generation of loss-of-function mutants of PBP by genome editing

Pheromone binding proteins (PBPs) are small soluble proteins (about 15kDa) that play striking roles in the detection of sex pheromones in insects. Many studies including structural analysis, binding simulation, and in vitro assays have been performed to clarify the modes of action of PBPs. Although these studies have provided valuable contributions toward the understanding of which key amino acid components contribute to the correct folding of PBPs and their binding affinities to sex pheromones, the functional characteristics of PBPs in the natural environment is still obscure. Recent developments in genome editing have begun to enable the functional examination of PBPs in in vivo. Among insect PBPs, BmPBP1 is one of the most well-characterized, there being rich understanding of its structure, biochemical analysis, binding affinity, localization, and the relationship between the type of olfactory receptors and its expression. A recent study has shown that BmPBP1 contributes sensitivity, but not selectivity of sex pheromone detection in the silkmoth Bombyx mori. In this chapter, based on a current report of the functional characterization of BmPBP1 using genome editing, we provide one example of a useful analytical method to clarify the functional role of PBP in vivo.

Molecular and functional characterization of three odorant-binding proteins from the wheat blossom midge, Sitodiplosis mosellana

Sitodiplosis mosellana, a periodic but devastating wheat pest, relies on wheat spike volatiles as a cue in selecting hosts for oviposition. Insect odorant-binding proteins (OBPs) are thought to play essential roles in filtering, binding and transporting hydrophobic odorant molecules to specific receptors. To date, the molecular mechanisms underlying S. mosellana olfaction are poorly understood. Here, three S. mosellana antenna-specific OBP genes, SmosOBP11, 16 and 21, were cloned and bacterially expressed. Binding properties of the recombinant proteins to 28 volatiles emitted from wheat spikes were investigated using fluorescence competitive binding assays. Sequence analysis suggested that these SmosOBPs belong to the Classic OBP subfamily. Ligand-binding analysis showed that all three SmosOBPs preferentially bound alcohol, ester and ketone compounds, and SmosOBP11 and 16 also selectively bound terpenoid compounds. In particular, the three SmosOBPs had high binding affinities (K_i < 20 μmol/L) to 3-hexanol and cis-3-hexenylacetate that elicited strong electroantennogram (EAG) response from female antennae. In addition, SmosOBP11 displayed significantly higher binding (K_i < 8 μmol/L) than SmosOBP16 and 21 to 1-octen-3-ol, D-panthenol, α-pinene and heptyl acetate which elicited significant EAG response, suggesting that SmosOBP11 plays a major role in recognition and transportation of these volatiles. These findings have provided important insight into the molecular mechanism by which S. mosellana specifically recognizes plant volatiles for host selection, and have facilitated identification of effective volatile attractants that are potentially useful for pest monitoring and trapping.

The Drosophila odorant-binding protein 28a is involved in the detection of the floral odour ß-ionone

Odorant-binding proteins (OBPs) are small soluble proteins that are thought to transport hydrophobic odorants across the aqueous sensillar lymph to olfactory receptors. A recent study revealed that OBP28a, one of the most abundant Drosophila OBPs, is not required for odorant transport, but acts in buffering rapid odour variation in the odorant environment. To further unravel and decipher its functional role, we expressed recombinant OBP28a and characterized its binding specificity. Using a fluorescent binding assay, we found that OBP28a binds a restricted number of floral-like chemicals, including ß-ionone, with an affinity in the micromolar range. We solved the X-ray crystal structure of OBP28a, which showed extensive conformation changes upon ligand binding. Mutant flies genetically deleted for the OBP28a gene showed altered responses to ß-ionone at a given concentration range, supporting its essential role in the detection of specific compounds present in the natural environment of the fly.

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).

Mating-based regulation and ligand binding of an odorant-binding protein support the inverse sexual communication of the greater wax moth, Galleria mellonella (Lepidoptera: Pyralidae)

In moths, sex pheromones play a key role in mate finding. These chemicals are transported in the antennae by odorant-binding proteins (OBPs). Commonly, males encounter conspecific females; therefore, several OBPs are male-biased. Less is known, however, about how the olfactory system of moths has evolved toward inverse sexual communication, ie where females seek males. Therefore, the objective of this study was to identify the profile of OBPs and their expression patterns in the bee hive pest, Galleria mellonella, a moth that uses inverse sexual communication. Here, OBP-related transcripts were identified by an RNA Sequencing (RNA-Seq) approach and analysed through both Reverse Transcription Polymerase Chain Reaction (RT-PCR) in different tissues and quantitative real-time PCR for two states, virgin and postmating. Our results indicate that G. mellonella has 20 OBPs distributed amongst different tissues. Interestingly, 17 of the 20 OBPs were significantly down-regulated after mating in females, whereas only OBP7 was up-regulated. By contrast, 18 OBP transcripts were up-regulated in males after mating. Additionally, binding assays and structural simulations showed general odorant-binding protein 2 (GOBP2) was able to bind sex pheromone components and analogues. These findings suggest a possible role of OBPs, especially GOBPs, in the inverse sexual communication of G. mellonella, with gene expression regulated as a response to mating.

Functional Analysis of Two Odorant-Binding Proteins, MaltOBP9 and MaltOBP10, in Monochamus alternatus Hope

Odorant-binding proteins (OBPs) are important for the perception of chemical signals by insects. Effective pest management strategies can be developed by understanding the host location mechanism and the physiological functions of OBPs in olfactory detection. In this study, we cloned two OBPs from Monochamus alternatus, where MaltOBP9 was highly expressed in multiple insect tissues and MaltOBP10 was highly expressed in the female antenna according to the results of qRT-PCR. The recombinant proteins were successfully purified in vitro. Immunocytochemistry indicated the high expression of MaltOBP9 and MaltOBP10 in the sensillum lymph of sensilla basiconica, sensilla trichodea, sensilla auricillica, and sensilla chaetica, thereby demonstrating their broad participation in semiochemical detection. Both proteins were localized in the inner cavity of mechanoreceptors and they exhibited broad binding abilities with volatiles from pine bark according to fluorescence competitive binding assays. Due to its broad binding ability and distribution, MaltOBP9 may be involved in various physiological processes as well as olfactory detection. MaltOBP10 appears to play a role in the fundamental olfactory recognition process of female adults according to its broad binding ability. These findings suggest that OBPs may have various physiological functions in insects, thereby providing novel insights into the olfactory receptive mechanism.

Identification and phylogenetics of Spodoptera frugiperda chemosensory proteins based on antennal transcriptome data

Understanding the interaction between the insect olfactory system and the environment is crucial for fully explaining the molecular mechanisms underlying insect behavior, and providing new strategies for integrated pest management. Although there is good evidence that olfactory proteins play a vital role in mediating insect behaviors, the olfactory mechanism of insects remains poorly understood. We identified a total of 71 chemosensory genes; 25 odorant-binding proteins (OBPs), 27 odorant receptors (ORs), 8 ionotropic receptors (IRs), 8 chemosensory proteins (CSPs) and 3 sensory neuron membrane proteins (SNMPs), in the antennae of male and female fall armyworms, Spodoptera frugiperda, an invasive global pest that causes significant economic damage worldwide. We used differential gene expression (DGE) and fragments per kilobase per million fragments (FPKM) values to compare the transcript levels of candidate chemosensory genes, and qRT-PCR to compare the expression levels of the OR gene, in male and female antennae. The expression of candidate OR genes in male and female antennae was consistent with the DGE data, and the expression of the SfruCL4419.Contig1-All and SfruUnigene1070-All genes was sex-biased. These results not only provide new information on the olfactory mechanism of S. frugiperda, and insects in general, but also suggest new gene targets for pest control.

Latrophilin participates in insecticide susceptibility through positively regulating CSP10 and partially compensated by OBPC01 in Tribolium castaneum

Latrophilin (LPH) is an adhesion G protein-coupled receptor (aGPCR) that participates in multiple essential physiological processes. Our previous studies have shown that lph is not only indispensable for the development and reproduction of red flour beetles (Tribolium castaneum), but also for their resistance against dichlorvos or carbofuran insecticides. However, the regulatory mechanism of lph-mediated insecticide susceptibility remains unclear. Here, we revealed that knockdown of lph in beetles resulted in opposing changes in two chemoreception genes, chemosensory protein 10 (CSP10) and odorant-binding protein C01 (OBPC01), in which the expression of TcCSP10 was downregulated, whereas the expression of TcOBPC01 was upregulated. TcCSP10 and TcOBPC01 were expressed at the highest levels in early pupal and late larval stages, respectively. High levels of expression of both these genes were observed in the heads (without antennae) of adults. TcCSP10 and TcOBPC01 were significantly induced by dichlorvos or carbofuran between 12 and 72 h (hrs) after exposure, suggesting that they are likely associated with increasing the binding affinity of insecticides, leading to a decrease in sensitivity to the insecticides. Moreover, once these two genes were knocked down, the susceptibility of the beetles to dichlorvos or carbofuran was enhanced. Additionally, RNA interference (RNAi) targeting of lph followed by exposure to dichlorvos or carbofuran also caused the opposing expression levels of TcCSP10 and TcOBPC01 compared to the expression levels of wild-type larvae treated with insecticides alone. All these results indicate that lph is involved in insecticide susceptibility through positively regulating TcCSP10; and the susceptibility could also further partially compensated for through the negative regulation of TcOBPC01 when lph was knockdown in the red flour beetle. Our studies shed new light on the molecular regulatory mechanisms of lph related to insecticide susceptibility.

Structural investigation of selective binding dynamics for the pheromone-binding protein 1 of the grapevine moth, Lobesia botrana

The European grapevine moth, Lobesia botrana (Denis & Schiffermüller), is a serious pest in vineyards in North and South America. Mating disruption techniques have been used to control and monitor L. botrana on the basis of its sexual communication. This needs a well-tuned olfactory system, in which it is believed that pheromone-binding proteins (PBPs) are key players that transport pheromones in the antennae of moths. In this study, the selectivity of a PBP, named as LbotPBP1, was tested by fluorescence binding assays against 11 sex pheromone components and 6 host plant volatiles. In addition, its binding mechanism was predicted on the basis of structural analyses by molecular docking and complex and steered molecular dynamics (SMD). Our results indicate that LbotPBP1 binds selectively to sex pheromone components over certain host plant volatiles, according to both in vitro and in silico tests. Thus, chain length (14 carbon atoms) and functional groups (i.e., alcohol and ester) appear to be key features for stable binding. Likewise, residues such as Phe12, Phe36, and Phe118 could participate in unspecific binding processes, whilst Ser9, Ser56, and Trp114 could participate in the specific recognition and stabilization of sex pheromones instead of host plant volatiles. Moreover, our SMD approach supported 11-dodecenyl acetate as the best ligand for LbotPBP1. Overall, the dynamics simulations, contact frequency analysis and SMD shed light on the binding mechanism of LbotPBP1 and could overcome the imprecision of molecular docking, supporting the in vitro binding assays. Finally, the role of LbotPBP1 in the chemical ecology of L. botrana is discussed.

Antennal transcriptome analysis and expression profiles of odorant binding proteins in Clostera restitura

Clostera restitura Walker (Lepidoptera: Notodontidae) is one of the most destructive defoliators of poplars in China. We constructed an antennal transcriptome using Illumina Hiseq 2500™ sequencing and characterized the expression profiles of odorant binding proteins for better understanding of the olfactory receptive system and the role of putative olfactory proteins in C. restitura. A total of 165 transcripts were identified, including 43 transcripts encoding putative odorant-binding proteins (OBPs), 13 chemosensory proteins (CSPs), 78 odorant receptors (ORs), 15 ionotropic receptors (IRs), 13 gustatory receptors (GRs), and 3 sensory neuron membrane proteins (SNMPs). Furthermore, we systematically analyzed expression patterns of eight OBPs from different tissues of both C. restitura sexes by using reverse transcription PCR and quantitative real time PCR (RT-qPCR). The expression level of CresGOBP2 in female antennae was approximately two times higher than in males, and two pheromone binding proteins PBPs (CresPBP1 and -PBP3) and three OBPs (CresOBP9, -10, and -16) were more highly enriched in male antennae than in female antennae. CresOBP10 showed a remarkably high expression in legs compared to other studied insects. Our results suggested that these proteins might play a key role in foraging, seeking mates, and host recognition in C. restitura. Our findings provided a foundation for future studies on the molecular mechanisms controlling the olfactory system in C. restitura and potential novel targets for pest control strategies.

Evidence of peripheral olfactory impairment in the domestic silkworms: insight from the comparative transcriptome and population genetics

BACKGROUND

The insect olfactory system is a highly specific and sensitive chemical detector, which plays important roles in feeding, mating and finding an appropriate oviposition site. The ecological niche of Bombyx mori has changed greatly since domestication from B. mandarina, and its olfactory response to environmental odorants clearly decreased. However, the mechanisms that result in the olfactory impairment are largely unknown.

RESULTS

The antennal transcriptomes were compared between the domestic and wild silkworms. Comparison of the same sex between the domestic and wild silkworms revealed 1410 and 1173 differentially expressed genes (DEGs) in males and females, respectively. To understand the olfactory impairment, we mainly focused on the olfactory-related genes. In total, 30 olfactory genes and 19 odorant-degrading enzymes (ODEs) showed differential expression in the two comparisons, in which 19 and 14 were down-regulated in the domestic silkworm, respectively. Based on population genomic data, the down-regulated odorant receptors (ORs) showed a higher ratio of unique non-synonymous polymorphisms to synonymous polymorphisms (N/S ratio) in the domestic populations than that in the wild silkworms. Furthermore, one deleterious mutation was found in OR30 of the domestic population, which was located in transmembrane helix 6 (TM6).

CONCLUSIONS

Our results suggested that down-regulation of the olfactory-related genes and relaxed selection might be the major reasons for olfactory impairment of the domestic silkworm reared completely indoor environment. Reversely, wild silkworm may increase expression and remove deleterious polymorphisms of olfactory-related genes to retain sensitive olfaction.

In vivo functional characterisation of pheromone binding protein-1 in the silkmoth, Bombyx mori

Male moths detect sex pheromones emitted by conspecific females with high sensitivity and specificity by the olfactory sensilla on their antennae. Pheromone binding proteins (PBPs) are highly enriched in the sensillum lymph of pheromone sensitive olfactory sensilla and are supposed to contribute to the sensitivity and selectivity of pheromone detection in moths. However, the functional role of PBPs in moth sex pheromone detection in vivo remains obscure. In the silkmoth, Bombyx mori, female moths emit bombykol as a single attractive sex pheromone component along with a small amount of bombykal that negatively modulates the behavioural responses to bombykol. A pair of olfactory receptor neurons, specifically tuned to bombykol or bombykal, co-localise in the trichodeum sensilla, the sensillum lymph of which contains a single PBP, namely, BmPBP1. We analysed the roles of BmPBP1 using BmPBP1-knockout silkmoth lines generated by transcription activator-like effector nuclease-mediated gene targeting. Electroantennogram analysis revealed that the peak response amplitudes of BmPBP1-knockout male antennae to bombykol and bombykal were significantly reduced by a similar percentage when compared with those of the wild-type males. Our results indicate that BmPBP1 plays a crucial role in enhancing the sensitivity, but not the selectivity, of sex pheromone detection in silkmoths.

Pheromone Binding Protein EhipPBP1 Is Highly Enriched in the Male Antennae of the Seabuckthorn Carpenterworm and Is Binding to Sex Pheromone Components

The seabuckthorn carpenterworm moth Eogystia hippophaecolus is a major threat to seabuckthorn plantations, causing considerable ecological and economic losses in China. Transcriptomic analysis of E. hippophaecolus previously identified 137 olfactory proteins, including three pheromone-binding proteins (PBPs). We investigated the function of E. hippophaecolus PBP1 by studying its mRNA and protein expression profiles and its binding ability with different compounds. The highest levels of expression were in the antennae, particularly in males, with much lower levels of expression in the legs and external genitals. Recombinant PBP1 showed strong binding to sex-pheromone components, suggesting that antennal EhipPBP1 is involved in binding sex-pheromone components during pheromone communication.

Binding Specificity of Two PBPs in the Yellow Peach Moth Conogethes punctiferalis (Guenée)

Pheromone binding proteins (PBPs) play an important role in olfaction of insects by transporting sex pheromones across the sensillum lymph to odorant receptors. To obtain a better understanding of the molecular basis between PBPs and semiochemicals, we have cloned, expressed, and purified two PBPs (CpunPBP2 and CpunPBP5) from the antennae of Conogethes punctiferalis. Fluorescence competitive binding assays were used to investigate binding affinities of CpunPBP2 and CpunPBP5 to sex pheromone and volatiles. Results indicate both CpunPBP2 and CpunPBP5 bind sex pheromones E10-16:Ald, Z10-16:Ald and hexadecanal with higher affinities. In addition, CpunPBP2 and CpunPBP5 also could bind some odorants, such as 1-tetradecanol, trans-caryopyllene, farnesene, and β-farnesene. Homology modeling to predict 3D structure and molecular docking to predict key binding sites were used, to better understand interactions of CpunPBP2 and CpunPBP5 with sex pheromones E10-16:Ald and Z10-16:Ald. According to the results, Phe9, Phe33, Ser53, and Phe115 were key binding sites predicted for CpunPBP2, as were Ser9, Phe12, Val115, and Arg120 for CpunPBP5. Binding affinities of four mutants of CpunPBP2 and four mutants of CpunPBP5 with the two sex pheromones were investigated by fluorescence competitive binding assays. Results indicate that single nucleotides mutation may affect interactions between PBPs and sex pheromones. Expression levels of CpunPBP2 and CpunPBP5 in different tissues were evaluated using qPCR. Results show that CpunPBP2 and CpunPBP5 were largely amplified in the antennae, with low expression levels in other tissues. CpunPBP2 was expressed mainly in male antennae, whereas CpunPBP5 was expressed mainly in female antennae. These results provide new insights into understanding the recognition between PBPs and ligands.

Structural Transformation Detection Contributes to Screening of Behaviorally Active Compounds: Dynamic Binding Process Analysis of DhelOBP21 from Dastarcus helophoroides

In light of reverse chemical ecology, the fluorescence competitive binding assays of functional odorant binding proteins (OBPs) is a recent advanced approach for screening behaviorally active compounds of insects. Previous research on Dastareus helophoroides identified a minus-C OBP, DhelOBP21, which preferably binds to several ligands. In this study, only (+)-β-pinene proved attractive to unmated adult beetles. To obtain a more in-depth explanation of the lack of behavioral activity of other ligands we selected compounds with high (camphor) and low (β-caryophyllene) binding affinities. The structural transformation of OBPs was investigated using well-established approaches for studying binding processes, such as fluorescent quenching assays, circular dichroism, and molecular dynamics. The dynamic binding process revealed that the flexibility of DhelOBP21 seems conducive to binding specific ligands, as opposed to broad substrate binding. The compound (+)-β-pinene and DhelOBP21 formed a stable complex through a secondary structural transformation of DhelOBP21, in which its amino-terminus transformed from random coil to an α-helix to cover the binding pocket. On the other hand, camphor could not efficiently induce a stable structural transformation, and its high binding affinities were due to strong hydrogen-bonding, compromising the structure of the protein. The other compound, β-caryophyllene, only collided with DhelOBP21 and could not be positioned in the binding pocket. Studying structural transformation of these proteins through examining the dynamic binding process rather than using approaches that just measure binding affinities such as fluorescence competitive binding assays can provide a more efficient and reliable approach for screening behaviorally active compounds.

The developmental transcriptome of the bamboo snout beetle Cyrtotrachelus buqueti and insights into candidate pheromone-binding proteins

Cyrtotrachelus buqueti is an extremely harmful bamboo borer, and the larvae of this pest attack clumping bamboo shoots. Pheromone-binding proteins (PBPs) play an important role in identifying insect sex pheromones, but the C. buqueti genome is not readily available for PBP analysis. Developmental transcriptomes of eggs, larvae from the first instar to the prepupal stage, pupae, and adults (females and males) from emergence to mating were built by RNA sequencing (RNA-Seq) in the present study to establish a sequence background of C. buqueti to help understand PBPs. Approximately 164.8 million clean reads were obtained and annotated into 108,854 transcripts. These were assembled into 24,338, 21,597, 24,798, 21,886, 24,642, and 83,115 unigenes for eggs, larvae, pupae, females, males, and the combined datasets, respectively. Unigenes were annotated against NCBI non-redundant protein sequences, NCBI non-redundant nucleotide sequences, Gene Ontology (GO), Protein family, Clusters of Orthologous Groups of Proteins/ Clusters of Eukaryotic Orthologous Groups (KOG), Swiss-Prot, and KEGG Orthology databases. A total of 17,213 unigenes were annotated into 55 sub-categories belonging to three main GO categories; 10,672 unigenes were classified into 26 functional categories by KOG classification, and 8,063 unigenes were classified into five functional KEGG categories. RSEM software for RNA sequencing showed that 4,816, 3,176, 3,661, 2,898, 4,316, 8,019, 7,273, 5,922, 5,844, and 4,570 genes were differentially expressed between larvae and males, larvae and eggs, larvae and pupae, larvae and females, males and females, males and eggs, males and pupae, females and eggs, females and pupae, and eggs and pupae, respectively. Of these, three were confirmed to be significantly differentially expressed between larvae, females, and males. Furthermore, PBP Cbuq7577_g1 was highly expressed in the antenna of males. A comprehensive sequence resource of a desirable quality was constructed from developmental transcriptomes of C. buqueti eggs, larvae, pupae, and adults. This work enriches the genomic data of C. buqueti, and facilitates our understanding of its metamorphosis, development, and response to environmental change. The identified candidate PBP Cbuq7577_g1 might play a crucial role in identifying sex pheromones, and could be used as a targeted gene to control C. buqueti numbers by disrupting sex pheromone communication.

De novo analysis of the oriental armyworm Mythimna separata antennal transcriptome and expression patterns of odorant-binding proteins

To better understand the olfactory mechanisms in the oriental armyworm Mythimna separate, one of the most serious pests of cereals, an antennal transcriptome was constructed in this study. A total of 130 olfactory related transcripts were identified. These transcripts were predicted to encode 32 odorant-binding proteins (OBPs), 16 chemosensory proteins (CSPs), 71 olfactory receptors (ORs), 8 ionotropic receptors (IRs), 1 gustatory receptor (GR) and 2 sensory neuron membrane proteins (SNMPs). Q-PCR analysis of the temporal expression profiles of seven OBPs in different tissues indicated that, except for MsepOBP19 which was highly expressed in the wings of 0-day-old adult and MsepOBP20 which was low expressed in all tissues, other tested MsepOBPs were significantly more highly expressed in the antenna than in the head (antenna excluded), thorax, abdomen, legs and wings. The expression levels of MsepOBPs were diverse in different life stages (differed on eclosion days). MsepOBP5 exhibited female-biased expression in 0- and 5-day-old adult, while no gender bias in 1- and 3-day-old adult was detected and similar expression profiles were found for MsepOBP7, 20, 24 and 26. In addition, we found that although the expression of MsepOBP22 was female biased in 0- and 5-day-old adult, in the 3-day-old adult it was male-biased. Our findings established a foundation for future studies of the functions of olfactory proteins in M. separata.

A look inside odorant-binding proteins in insect chemoreception

Detection of chemical signals from the environment through olfaction is an indispensable mechanism for maintaining an insect's life, evoking critical behavioral responses. Among several proteins involved in the olfactory perception process, the odorant binding protein (OBP) has been shown to be essential for a normally functioning olfactory system. This paper discusses the role of OBPs in insect chemoreception. Here, structural aspects, mechanisms of action and binding affinity of such proteins are reviewed, as well as their promising application as molecular targets for the development of new strategies for insect population management and other technological purposes.

Functional characterization of a pheromone-binding protein from rice leaffolder Cnaphalocrocis medinalis in detecting pheromones and host plant volatiles

Pheromone-binding proteins (PBPs) are believed to be involved in the recognition of semiochemicals. In the present study, western blot analysis, fluorescence-binding characteristics and immunolocalization of CmedPBP4 from the rice leaffolder, Cnaphalocrocis medinalis, were investigated. Western blot analysis revealed that CmedPBP4 showed obvious antenna-specific expression patterns in female and male antenna, and made a clearly different sex-biased expression. Immunocytochemical labeling revealed that CmedPBP4 showed specific expression in the trichoid sensilla. Competitive fluorescence binding assays indicated that CmedPBP4 could selectively recognize three sex pheromone components (Z13-18:Ac, Z11-16:Al and Z13-18:OH) and eleven rice plant volatiles, including cyclohexanol, nerolidol, cedrol, dodecanal, ionone, (-)-α-cedrene, (Z)-farnesene, β-myrcene, R-(+)-limonene, (-)-limonene, and (+)-3-carene. Meanwhile the CmedPBP4 detection of sex pheromones and host odorants was pH-dependent. Our results, for the first time, provide further evidence that trichoid sensilla might be play an important role in detecting sex pheromones and host plant volatiles in the C. medinalis moth. Our systematic studies provided further detailed evidence for the function of trichoid sensilla in insect semiochemical perception.