NMR characterization of a pH-dependent equilibrium between two folded solution conformations of the pheromone-binding protein from Bombyx mori

Pheromone-Binding Protein 1 Performs a Dual Function for Intra- and Intersexual Signaling in a Moth

Moths use pheromones to ensure intraspecific communication. Nevertheless, few studies are focused on both intra- and intersexual communication based on pheromone recognition. Pheromone-binding proteins (PBPs) are generally believed pivotal for male moths in recognizing female pheromones. Our research revealed that PBP1 of Agriphila aeneociliella (AaenPBP1) serves a dual function in both intra- and intersexual pheromone recognition. Here, a total of 20 odorant-binding protein (OBP) family genes from A. aeneociliella were identified and subjected to transcriptional analysis. Among these, AaenPBP1 was primarily highly expressed in the antennae. Competitive fluorescence binding assays and molecular docking analyses demonstrated that AaenPBP1 exhibits a strong binding affinity for the female sex pheromone (Z)-9-Hexadecenyl acetate and the male pheromone 1-Nonanal. Notably, hydrogen bonds were observed between Ser56 and the ligands. The analysis of pheromone components and PBPs in lepidopteran lineage suggested that their strong and precise interactions, shaped by coevolution, may play a crucial role in facilitating reproductive isolation in moths. Our findings provide valuable insight into the functional significance of PBPs in invertebrates and support the development of behavioral regulation tools as part of an integrated pest management strategy targeting crambid pests.

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.

Odorant-binding protein 19 in Monochamus alternatus involved in the recognition of a volatile strongly emitted from ovipositing host pines

Monochamus alternatus is the primary carrier of pine wood nematodes, which pose a serious threat to Pinus spp. in many countries. Newly emerging M. alternatus adults feed on heathy host pines, while matured adults transfer to stressed host pines for mating and oviposition. Several odorant-binding proteins (OBPs) of M. alternatus have been proved to aid in the complex process of host location. To clarify the corresponding relations between OBPs and pine volatiles, more OBPs need to be studied. In this research, MaltOBP19 showed a specific expression in the antennae and mouthparts of M. alternatus, and it was marked in 4 types of antenna sensilla by immunolocalization. Fluorescence binding assays demonstrated the high binding affinity of MaltOBP19 with camphene and myrcene in vitro. In Y-tube olfactory experiments, M. alternatus adults were attracted by camphene and RNAi of OBP19 via microinjection significantly decreased their attraction index. Myrcene induced phobotaxis, but RNAi had no significant effect on this behavior. Further, we found that ingesting dsOBP19 produced by a bacteria-expressed system with a newly constructed vector could lead to the knockdown of MaltOBP19. These results suggest that MaltOBP19 may play a role in the process of host conversion via the recognition of camphene, which has been identified to be strongly released in stressed host pines. In addition, it is proved that knockdown of OBP can be achieved by oral administration of bacteria-expressed double-stranded RNA in M. alternatus adults, providing a new perspective in the control of M. alternatus.

Backbone and side chain NMR assignments and secondary structure calculation of the pheromone binding protein3 of Ostrinia nubilalis, an agricultural pest

Ostrinia nubilalis, also known as European Corn Borer (ECB), is a serious pest in Europe and North America, as well as in Central Asia and Northern Africa. It damages a variety of agricultural crops such as corn, oats, buckwheat, millet, and soybeans. causing annually at least one billion dollars in loss. The Ostrinia nubilalis pheromone-binding protein3 (OnubPBP3), preferentially expressed in the male moth antenna, has been implicated in the detection of the female-secreted pheromone blend during the mating process. Understanding the structure of and function of OnubPBP3, including the mechanism of pheromone binding and its release at the dendritic olfactory neuron (ORN), is essential if integrated pest management through sensory inhibition is to be achieved. We report here the backbone and side-chain resonance assignments of OnubPBP3 at pH 6.5 using various triple resonance NMR experiments on a 13C, 15N-labeled protein sample. The secondary structure of OnubPBP3 consists of six α-helices and an unstructured C-terminus based on backbone chemical shifts.

Computational insights of the molecular recognition between volatile molecules and odorant binding proteins from the red palm weevil Rhynchophorus ferrugineus

The red palm weevil Rhynchophorus ferrugineus (Coleoptera: Curculionidae) is one of the most harmful pests for palm trees, causing serious economic damage worldwide. The present work aims to model and study the 3D structures of highly expressed odorant binding proteins from R. ferrugineus (RferOBPs) and identify possible binding modes and ligand release mechanism by docking and molecular dynamics. Highly confident 3D structures of a total of 11 odorant binding proteins (OBPs) were obtained with AlphaFold2. All 3D RferOBPs modeled structures displayed six characteristic α-helices, except for RfeOBP7 and RfeOBP10, which had an extra terminal α-helix. Among the eleven modeled RferOBPs, RferOBP4 was highly expressed in the antennae and subsequently selected for further analyses. Molecular docking analyses demonstrated that ferruginol, α-pinene, DEET, and picaridin can favorably bind the RferOBP4 cavity with low affinity energies. Molecular dynamic simulations of RferOBP4 bound to ferruginol at different pH values showed that low pH environments dictate a structural change into an apo-state that modifies the number of tunnels where the ligand can coexist, further triggering ligand release by a pH-dependent mechanism. This is the first report concerning the modelling and study of ligand binding modes and release mechanism of R. ferrugineus OBPs.Communicated by Ramaswamy H. Sarma.

Influence of pH on indole-dependent heterodimeric interactions between Anopheles gambiae odorant-binding proteins OBP1 and OBP4

Insect Odorant Binding Proteins (OBPs) constitute important components of their olfactory apparatus, as they are essential for odor recognition. OBPs undergo conformational changes upon pH change, altering their interactions with odorants. Moreover, they can form heterodimers with novel binding characteristics. Anopheles gambiae OBP1 and OBP4 were found capable of forming heterodimers possibly involved in the specific perception of the attractant indole. In order to understand how these OBPs interact in the presence of indole and to investigate the likelihood of a pH-dependent heterodimerization mechanism, the crystal structures of OBP4 at pH 4.6 and 8.5 were determined. Structural comparison to each other and with the OBP4-indole complex (3Q8I, pH 6.85) revealed a flexible N-terminus and conformational changes in the α4-loop-α5 region at acidic pH. Fluorescence competition assays showed a weak binding of indole to OBP4 that become further impaired at acidic pH. Additional Molecular Dynamic and Differential Scanning Calorimetry studies displayed that the influence of pH on OBP4 stability is significant compared to the modest effect of indole. Furthermore, OBP1-OBP4 heterodimeric models were generated at pH 4.5, 6.5, and 8.5, and compared concerning their interface energy and cross-correlated motions in the absence and presence of indole. The results indicate that the increase in pH may induce the stabilization of OBP4 by increasing its helicity, thereby enabling indole binding at neutral pH that further stabilizes the protein and possibly promotes the creation of a binding site for OBP1. A decrease in interface stability and loss of correlated motions upon transition to acidic pH may provoke the heterodimeric dissociation allowing indole release. Finally, we propose a potential OBP1-OBP4 heterodimer formation/disruption mechanism induced by pH change and indole binding.

Ostrinia furnacalis PBP2 solution NMR structure: Insight into ligand binding and release mechanisms

Ostrinia furnacalis is an invasive lepidopteran agricultural pest that relies on olfaction for mating and reproduction. Male moths have an extremely sensitive olfactory system that can detect the sex pheromones emitted by females over a great distance. Pheromone-binding proteins present in the male moth antenna play a key role in the pheromone uptake, transport, and release at the dendritic membrane of the olfactory neuron. Here, we report the first high-resolution NMR structure of a pheromone-binding protein from an Ostrinia species at pH 6.5. The core of the Ostrinia furnacalis PBP2 (OfurPBP2) consists of six helices, α1a (2-14), α1b (16-22), α2 (27-37), α3 (46-60), α4 (70-80), α5 (84-100), and α6 (107-124) surrounding a large hydrophobic pocket. The structure is stabilized by three disulfide bridges, 19-54, 50-108, and 97-117. In contrast to the unstructured C-terminus of other lepidopteran PBPs, the C-terminus of OfurPBP2 folds into an α-helix (α7) at pH 6.5. The protein has nanomolar affinity towards both pheromone isomers. Molecular docking of both pheromones, E-12 and Z-12-tetradecenyl acetate, to OfurPBP2 revealed that the residues Met5, Lys6, Met8, Thr9, Phe12, Phe36, Trp37, Phe76, Ser115, Phe118, Lys119, Ile122, His123, and Ala128 interact with both isomers, while Thr9 formed a hydrogen bond with the acetate head group. NMR structure and thermal unfolding studies with CD suggest that ligand release at pH 4.5 is likely due to the partial unfolding of the protein.

Silencing of an odorant binding protein (SaveOBP10) involved in the behavioural shift of the wheat aphid Sitobion avenae (Fabricius)

Insects are highly reliant on their active olfactory system in which odorant binding proteins play a role to selectivity and sensitivity during odour perception and processing. This study sets out to determine whether and to which extent the antennal loaded SaveOBP10 in English grain aphid Sitobion avenae, contributes in olfactory processing during host selection. To understand this possible relationship, we purified the SaveOBP10 recombinant protein and performed fluorescence ligand binding tests, molecular docking, RNA interference (RNAi) and behavioural trials. The results showed that SaveOBP10 had strong binding affinities (Ki ≤5 μM) with most of wheat plant volatiles at pH 5.0 as compared to pH 7.4. In Y-tube olfactometer bioassays, the S. avenae was attracted behaviourally towards pentadecane, butylated hydroxytoluene, tetradecane and β-caryophyllene however repelled by naphthalene. After RNAi of SaveOBP10, the aphid showed nonattraction towards β-caryophyllene and nonsignificant behavioural response to pentadecane, butylated hydroxytoluene and tetradecane. Furthermore, the three-dimensional structure modelling and molecular docking of SaveOBP10 were performed to the volatiles with high binding abilities. Together these findings indicate that SaveOBP10 can bind more strongly to the volatiles that involved in S. avenae behaviour regulation and possibly will contribute effectively in S. avenae integrated pest management.

Insights Into Chemosensory Proteins From Non-Model Insects: Advances and Perspectives in the Context of Pest Management

Nowadays, insect chemosensation represents a key aspect of integrated pest management in the Anthropocene epoch. Olfaction-related proteins have been the focus of studies due to their function in vital processes, such ashost finding and reproduction behavior. Hence, most research has been based on the study of model insects, namely Drosophila melanogaster, Bombyx mori or Tribolium castaneum. Over the passage of time and the advance of new molecular techniques, insects considered non-models have been studied, contributing greatly to the knowledge of insect olfactory systems and enhanced pest control methods. In this review, a reference point for non-model insects is proposed and the concept of model and non-model insects is discussed. Likewise, it summarizes and discusses the progress and contribution in the olfaction field of both model and non-model insects considered pests in agriculture.

Host-Guest Inclusion Complexes of Natural Products and Nanosystems: Applications in the Development of Repellents

Repellents are compounds that prevent direct contact between the hosts and the arthropods that are vectors of diseases. Several studies have described the repellent activities of natural compounds obtained from essential oils. In addition, these chemical constituents have been pointed out as alternatives to conventional synthetic repellents due to their interesting residual protection and low toxicity to the environment. However, these compounds have been reported with short shelf life, in part, due to their volatile nature. Nanoencapsulation provides protection, stability, conservation, and controlled release for several compounds. Here, we review the most commonly used polymeric/lipid nanosystems applied in the encapsulation of small organic molecules obtained from essential oils that possess repellent activity, and we also explore the theoretical aspects related to the intermolecular interactions, thermal stability, and controlled release of the nanoencapsulated bioactive compounds.

Enantiomeric Discrimination in Insects: The Role of OBPs and ORs

Olfaction is a complex recognition process that is critical for chemical communication in insects. Though some insect species are capable of discrimination between compounds that are structurally similar, little is understood about how this high level of discrimination arises. Some insects rely on discriminating between enantiomers of a compound, demonstrating an ability for highly selective recognition. The role of two major peripheral olfactory proteins in insect olfaction, i.e., odorant-binding proteins (OBPs) and odorant receptors (ORs) has been extensively studied. OBPs and ORs have variable discrimination capabilities, with some found to display highly specialized binding capability, whilst others exhibit promiscuous binding activity. A deeper understanding of how odorant-protein interactions induce a response in an insect relies on further analysis such as structural studies. In this review, we explore the potential role of OBPs and ORs in highly specific recognition, specifically enantiomeric discrimination. We summarize the state of research into OBP and OR function and focus on reported examples in the literature of clear enantiomeric discrimination by these proteins.

Structural and Functional Characterization of European Corn Borer, Ostrinia nubilalis, Pheromone Binding Protein 3

Ostrinia nubilalis, a lepidopteran moth, also known as the European corn borer, has a major impact on the production of economically important crops in the United States and Europe. The female moth invites the male moth for mating through the release of pheromones, a volatile chemical signal. Pheromone binding proteins (PBPs) present in the male moth antennae are believed to pick up the pheromones, transport them across the aqueous sensillum lymph, and deliver them to the olfactory receptor neurons. Here we report for the first time the cloning, expression, refolding, purification, and structural characterization of Ostrinia nubilalis PBP3 (OnubPBP3). The recombinant protein showed nanomolar affinity to each isomer of the Ostrinia pheromones, E- and Z-11-tetradecenyl acetate. In a pH titration study by nuclear magnetic resonance, the protein exhibited an acid-induced unfolding at pH below 5.5. The molecular dynamics simulation study demonstrated ligand-induced conformational changes in the protein with both E- and Z-isomers of the Ostrinia pheromone. The simulation studies showed that while protein flexibility decreases upon binding to E-pheromone, it increases when bound to Z-pheromone. This finding suggests that the OnubPBP3 complex with E-pheromone is more stable than with Z-pheromone.

Two carboxylesterase genes in Plutella xylostella associated with sex pheromones and plant volatiles degradation

BACKGROUND

Carboxyl/cholinesterases (CCEs) are thought to play a pivotal role in the degradation of sex pheromones and plant-derived odorants in insects, but their exact biochemistry and physiological functions remain unclear.

RESULTS

In this study, two paralogous antennae-enriched CCEs from Plutella xylostella (PxylCCE16a and 16c) were identified and functionally characterized. High-purity protein preparations of active recombinant PxylCCE16a and 16c have been obtained from Sf9 insect cells by Ni²⁺ affinity purification. Our results revealed that the purified recombinant PxylCCE016c is able to degrade two sex pheromone components Z9-14:Ac and Z11-16:Ac at 27.64 ± 0.79% and 24.40 ± 3.07%, respectively, while PxylCCE016a presented relatively lower activity. Additionally, a similar difference in activity was measured in plant-derived odorants. Furthermore, both CCEs displayed obvious preferences for the two sex pheromone components, especially on Z11-16:Ac (K_m values are in the range 7.82-45.06 μmol L^-1 ) which much lower than plant odorants (K_m values are in the range 1290-4030 μmol L^-1 ). Furthermore, the activity of the two newly identified CCEs is pH-dependent. The activity at pH 6.5 is obviously higher than that at pH 5.0. Interestingly, only PxylCCE016c can be inhibited by a common esterase inhibitor triphenyl phosphate (TPP) with LC₅₀ of 1570 ± 520 μmol L^-1 .

CONCLUSION

PxylCCE16c plays a more essential role in odorant degradation than PxylCCE16a. Moreover, the current study provides novel potential pesticide targets for the notorious moth Plutella xylostella. © 2021 Society of Chemical Industry.

Discovery of behaviorally active semiochemicals in Aenasius bambawalei using a reverse chemical ecology approach

BACKGROUND

The invasive mealybug, Phenacoccus solenopsis, has caused serious damage to cotton crops throughout the world. Aenasius bambawalei is a dominant endoparasitoid of P. solenopsis. Exploration of behaviorally active semiochemicals may promote the efficacy of parasitoids used in biological control. Reverse chemical ecology, based on the physiological function of odorant-binding proteins (OBPs), provides an effective approach to screen behaviorally active compounds to target insect pests. Determination of the binding mechanisms and specificity towards different odorants in A. bambawalei may facilitate the development of more-efficient biological control strategies.

RESULTS

We characterized the expression profile and analyzed the binding affinity of OBP28 in A. bambawalei. AbamOBP28 showed high expression in the wings and antennae of both male and female A. bambawalei. A fluorescence competitive binding assay indicated that AbamOBP28 displayed strong binding affinity to most candidate ligands. Circular dichroism spectra demonstrated that 1-octen-3-one, myrcene, dodecane, 2,4,4-trimethyl-2-pentene, nonanal, and limonene elicited conformational changes in AbamOBP28. Electrophysiological and behavioral bioassays revealed that diethyl sebacate, 2,4,4-trimethyl-2-pentene, and 1-octen-3-one evoked significant electroantennography responses and functioned as attractants in A. bambawalei at specific concentrations. Furthermore, three-dimensional structure modeling and molecular docking showed that hydrogen bonds were formed by Glu1 and Ser75 of AbamOBP28 with diethyl sebacate, respectively.

CONCLUSION

These results demonstrate that AbamOBP28 is involved in the chemoreception of A. bambawalei. The identified protein provides a potential target for efficient enemy utilization and pest control, and the overall results may help develop protocols for more effective screening of behaviorally active semiochemicals. © 2021 Society of Chemical Industry.

Responses of the Pheromone-Binding Protein of the Silk Moth Bombyx mori on a Graphene Biosensor Match Binding Constants in Solution

An electronic biosensor for odors was assembled by immobilizing the silk moth Bombyx mori pheromone binding protein (BmorPBP1) on a reduced graphene oxide surface of a field-effect transistor. At physiological pH, the sensor detects the B. mori pheromones, bombykol and bombykal, with good affinity and specificity. Among the other odorants tested, only eugenol elicited a strong signal, while terpenoids and other odorants (linalool, geraniol, isoamyl acetate, and 2-isobutyl-3-methoxypyrazine) produced only very weak responses. Parallel binding assays were performed with the same protein and the same ligands, using the common fluorescence approach adopted for similar proteins. The results are in good agreement with the sensor’s responses: bombykol and bombykal, together with eugenol, proved to be strong ligands, while the other compounds showed only poor affinity. When tested at pH 4, the protein failed to bind bombykol both in solution and when immobilized on the sensor. This result further indicates that the BmorPBP1 retains its full activity when immobilized on a surface, including the conformational change observed in acidic conditions. The good agreement between fluorescence assays and sensor responses suggests that ligand-binding assays in solution can be used to screen mutants of a binding protein when selecting the best form to be immobilized on a biosensor.

A candidate aldehyde oxidase in the antennae of the diamondback moth, Plutella xylostella (L.), is potentially involved in the degradation of pheromones, plant-derived volatiles and the detoxification of xenobiotics

Insect antennae play a fundamental role in perceiving and recognizing a broad spectrum of conventional semiochemicals and host plant-derived odors. As such, genes that are tightly associated with the antennae are thought to have olfactory-related roles related to signal transduction mechanisms. Several mechanisms suggest that enzymatic inactivation could contribute to the signal termination process, such as odorant-degrading enzymes (ODEs). To date, a few ODEs have been identified and characterized in detail in insect herbivores, but little is known about aldehyde oxidases (AOXs); moreover, direct in vivo experimental evidence is needed. AOXs are a major family of metabolic enzymes that oxidize a variety of aromatic aldehydes, and they may also play a significant role in detoxification and degradation of environmental chemical cues. Here, we report on the identification and characterization of a novel cDNA encoding the putative odorant-degrading enzyme, PxylAOX3, from the antennae of the diamondback moth, (DBM), Plutella xylostella (L.) (Lepidoptera: Plutellidae). The purified recombinant protein showed a wide-range of substrate zymography oxidizing both sex pheromone compounds as well as plant-derived aldehydes with distinct activities. Our data suggest PxylAOX3 might be involved in the degradation of many structurally diverse aldehyde odorants. Furthermore, PxylAOX3 could participate in olfactory neuron protection by inactivation of redundant odorants and xenobiotic detoxification, making it a potential target for pesticide development as well.

Expression, Affinity, and Functional Characterization of the Specific Binding of Two Putative Pheromone-Binding Proteins in the Omnivorous German Cockroach Blattella germanica

The German cockroach Blattella germanica (L.) is an important pest in medical, veterinary, and public health. Studies on the olfaction mechanism of hemimetabolous insects have rarely been reported, especially in cockroaches. Pheromone-binding proteins (PBPs) play a vital role in insect sex pheromone recognition, which solubilize and carry the hydrophobic pheromonal compounds through the antennal lymph to receptors. In this study, two potential PBPs (BgerOBP26 and BgerOBP40) were identified on the basis of their biased expression in male antennae using tissue transcriptome data and verified by the quantitative real-time polymerase chain reaction approach. We then expressed and purified the two identified odorant-binding proteins (OBPs) using the Escherichia coli expression system and affinity purification. In vitro binding studies showed that the two OBPs display stronger binding affinities to the female volatile sex pheromone blattellaquinone than to its analogues and contact sex pheromone components. Finally, three-dimensional modeling of the two OBPs and dock conformation with sex pheromone molecules showed BgerOBP26 has a larger odorant cavity and more conservative active amino acid residues than BgerOBP40. These results illuminated the binding characteristics of potential PBPs of B. germanica, which could lay the groundwork for improved understanding of many aspects of the chemical ecology of B. germanica. Moreover, this information complements the understanding of the olfactory molecular mechanism in cockroaches and provides potential gene targets for B. germanica control.

An Odorant Binding Protein (SaveOBP9) Involved in Chemoreception of the Wheat Aphid Sitobion avenae

Odorant binding proteins play a key role in the olfactory system and are involved in the odor perception and discrimination of insects. To investigate the potential physiological functions of SaveOBP9 in Sitobion avenae, fluorescence ligand binding experiments, molecular docking, RNA interference, and behavioral tests were performed. Fluorescence binding assay results showed that SaveOBP9 had broad and high (Ki < 10 μM) binding abilities with most of the wheat volatiles, but was more obvious at pH 7.4 than pH 5.0. The binding sites of SaveOBP9 to the volatiles were predicted well by three-dimensional docking structure modeling and molecular docking. Moreover, S. avenae showed a strong behavioral response with the four compounds of wheat. The reduction in mRNA transcript levels after the RNA interference significantly reduced the expression level of SaveOBP9 and induced the non-significant response of S. avenae to the tetradecane, octanal, decanal, and hexadecane. This study provides evidence that SaveOBP9 might be involved in the chemoreception of wheat volatile organic compounds and can successfully contribute in the integrated management programs of S. avenae.

Crystal structure of Epiphyas postvittana pheromone binding protein 3

The insect olfactory system operates as a well-choreographed ensemble of molecules which functions to selectively translate volatile chemical messages present in the environment into neuronal impulses that guide insect behaviour. Of these molecules, binding proteins are believed to transport hydrophobic odorant molecules across the aqueous lymph present in antennal sensilla to receptors present in olfactory sensory neurons. Though the exact mechanism through which these proteins operate is still under investigation, these carriers clearly play a critical role in determining what an insect can smell. Binding proteins that transport important sex pheromones are colloquially named pheromone binding proteins (PBPs). Here, we have produced a functional recombinant PBP from the horticultural pest, Epiphyas postvittana (EposPBP3), and experimentally solved its apo-structure through X-ray crystallography to a resolution of 2.60 Å. Structural comparisons with related lepidopteran PBPs further allowed us to propose models for the binding of pheromone components to EposPBP3. The data presented here represent the first structure of an olfactory-related protein from the tortricid family of moths, whose members cause billions of dollars in losses to agricultural producers each year. Knowledge of the structure of these important proteins will allow for subsequent studies in which novel, olfactory molecule-specific insecticides can be developed.

Ligand- and pH-Induced Structural Transition of Gypsy Moth Lymantria dispar Pheromone-Binding Protein 1 (LdisPBP1)

Pheromone-binding proteins (PBPs) are small, water-soluble proteins found in the lymph of pheromone-sensing hairs. PBPs are essential in modulating pheromone partitioning in the lymph and at pheromone receptors of olfactory sensory neurons. The function of a PBP is associated with its ability to structurally convert between two conformations. Although mechanistic details remain unclear, it has been proposed that the structural transition between these forms is affected by two factors: pH and the presence or absence of ligand. To better understand the PBP conformational transition, the structure of the gypsy moth (Lymantria dispar) LdisPBP1 was elucidated at pH 4.5 and 35 °C using nuclear magnetic resonance spectroscopy. In addition, the effects of sample pH and binding of the species' pheromone, (+)-disparlure, (7R,8S)-epoxy-2-methyloctadecane, and its enantiomer were monitored via ¹⁵N HSQC spectroscopy. LdisPBP1 in acidic conditions has seven helices, with its C-terminal residues forming the seventh helix within the pheromone-binding pocket and its N-terminal residues disordered. Under conditions where this conformation is made favorable, free LdisPBP1 would have limited ligand binding capacity due to the seventh helix occupying the internal binding pocket. Our findings suggest that even in the presence of 4-fold ligand at acidic pH, LdisPBP1 is only ∼60% in its pheromone-bound form. Furthermore, evidence of a different LdisPBP1 form is seen at higher pH, with the transition pH between 5.6 and 6.0. This suggests that LdisPBP1 at neutral pH exists as a mixture of at least two conformations. These findings have implications concerning the PBP ligand binding and release mechanism.

Site-directed mutagenesis of odorant-binding proteins

Modifying the affinity of odorant-binding proteins (OBPs) to small ligands by replacement of specific residues in the binding pocket may lead to several technological applications. Thanks to their compact and stable structures, OBPs are currently regarded as the best candidates to be used in biosensing elements for odorants and volatiles detection. The wide and rich information on the structure of these proteins both in their apo-forms and in complexes with specific ligands provides guidelines to design reliable mutants to monitor specific targets. The same engineered proteins may also find applications in the slow release of pheromones and other chemicals in the environment, as well as in the fine purification of drugs, including the resolution of racemates. Apart from such useful applications, site-directed mutagenesis represents an interesting approach to dissect the specific interactions between small chemicals and amino acid residues in the binding pocket. These studies can lead to design of better ligands, such as pheromone analogues with desired physico-chemical characteristics. In this chapter we examine the different uses of mutagenesis applied to OBPs and report a couple of protocols that have been successful in our hands.

Ligand-binding assays with OBPs and CSPs

Assessing the ligand-binding properties of OBPs and CSPs is essential for understanding their physiological function. It also provides basic information when these proteins are used as biosensing elements for instrumental measurement of odors. Although different approaches have been applied in the past to evaluate the affinity of receptors and soluble binding proteins to their ligands, using a fluorescent reporter represents the method of choice for OBPs and CSPs. It offers the advantages of working at the equilibrium, being simple, fast and inexpensive, without requiring the use of radioactive tracers. However, as an indirect method, the fluorescence competitive binding approach presents drawbacks and sometimes requires an elaborate analysis to explain unexpected results. Here, after a brief survey of the different approaches to evaluate affinity constants, we focus on the fluorescence binding assay as applied to OBPs and CSPs, discussing situations that may require closer inspection of the results.

1H, 13C, and 15N resonance assignment and secondary structure of the pheromone-binding protein2 from the agricultural pest Ostrinia furnacalis (OfurPBP2)

Ostrinia furnacalis, a lepidopteran moth, is an invasive pest found in Asia, Australia, Africa, and parts of the United States. The O. furnacalis pheromone-binding protein2 (OfurPBP2), present in the male moth antenna, plays a role in the detection of female-secreted pheromone in a process that leads to mating. To understand the structural mechanism of pheromone binding and release in this pest, we have initiated characterization of OfurPBP2 by solution NMR. Here, we report the backbone resonance assignments and the secondary structural elements of OfurPBP2 at pH 6.5 using uniformly ¹³C, ¹⁵N-labeled protein with various triple resonance NMR experiments. The assignments are 97% completed for backbone and 88% completed for side-chain resonances. The secondary structure of OfurPBP2, based on backbone chemical shifts, consists of eight α-helices, including a well-structured C-terminal helix.

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.

Aedes aegypti Odorant Binding Protein 22 selectively binds fatty acids through a conformational change in its C-terminal tail

Aedes aegypti is the primary vector for transmission of Dengue, Zika and chikungunya viruses. Previously it was shown that Dengue virus infection of the mosquito led to an in increased expression of the odorant binding protein 22 (AeOBP22) within the mosquito salivary gland and that siRNA mediated knockdown of AeOBP22 led to reduced mosquito feeding behaviors. Insect OBPs are implicated in the perception, storage and transport of chemosensory signaling molecules including air-borne odorants and pheromones. AeOBP22 is unusual as it is additionally expressed in multiple tissues, including the antenna, the male reproductive glands and is transferred to females during reproduction, indicating multiple roles in the mosquito life cycle. However, it is unclear what role it plays in these tissues and what ligands it interacts with. Here we present solution and X-ray crystallographic studies that indicate a potential role of AeOBP22 binding to fatty acids, and that the specificity for longer chain fatty acids is regulated by a conformational change in the C-terminal tail that leads to creation of an enlarged binding cavity that enhances binding affinity. This study sheds light onto the native ligands for AeOBP22 and provides insight into its potential functions in different tissues.

Pheromone Perception: Mechanism of the Reversible Coil-Helix Transition in Antheraea polyphemus Pheromone-Binding Protein 1

Pheromone-binding protein (PBP) in male moth antennae transports pheromone to the olfactory receptor neuron by undergoing a pH-dependent conformational switch, from PBP^B at higher pH to PBP^A at lower pH, associated with ligand binding and release, respectively. The characteristic feature of the dramatic protein switch is the pH-dependent reversible coil-helix transition of the C-terminus. In the PBP^B conformation at pH >6.0, the C-terminus is exposed to the solvent as a coil while the ligand occupies the hydrophobic pocket. However, in the PBP^A conformation at acidic pH, the C-terminus switches to a helix and releases the ligand by outcompeting it for the hydrophobic pocket. In Antheraea polyphemus PBP1 (ApolPBP1), the C-terminus (P¹²⁹-V¹⁴²) is composed predominantly of hydrophobic residues except for three strategically located acidic residues: Asp¹³², Glu¹³⁷, and Glu¹⁴¹. Here, we report for the first time on the consequences of the mutation of one or more acidic residues in the pH-driven reversible coil-helix transition of the ApolPBP1 C-terminus through biophysical characterization. Mutation of any single acidic residue in the C-terminus to its neutral counterpart destabilizes the helix formation at lower pH; these mutants exist as a mixture of both conformations. However, mutation of the two terminal acidic residues together knocks out the protein switch and adversely affects both ligand binding and release functions. Thus, these mutant proteins remain in the open (PBP^B) conformation at all pH levels.

Molecular Screening of Behaviorally Active Compounds with CmedOBP14 from the Rice Leaf Folder Cnaphalocrocis medinalis

Odorant binding proteins (OBPs) play a key role in chemoreception in insects. In an earlier study, we identified CmedOBP14 from the rice leaf folder, Cnaphalocrocis medinalis, with potential physiological functions in olfaction. Here, we performed a competitive binding assay under different pH conditions as well as knockdown via RNA interference to determine the specific role of CmedOBP14 in C. medinalis. CmedOBP14 displayed broad binding affinities to many host-related compounds, with higher affinities at pH 7.4 compared with pH 5.0. After treatment with CmedOBP14-dsRNA, the transcript level of OBP14 was significantly decreased at 72 h compared with controls, and the electroantennogram response evoked by nerolidol, L-limonene and beta-ionone was reduced. Furthermore, behavioral assays revealed consistent patterns among these compounds, especially for nerolidol, with adults could no longer able to differentiate 0.1% nerolidol from controls. RNAi experiments suggest that at least in part, CmedOBP14 mediates the ability to smell nerolidol and beta-ionone.

The diverse small proteins called odorant-binding proteins

The term ‘odorant-binding proteins (Obps)’ is used to refer to a large family of insect proteins that are exceptional in their number, abundance and diversity. The name derives from the expression of many family members in the olfactory system of insects and their ability to bind odorants in vitro. However, an increasing body of evidence reveals a much broader role for this family of proteins. Recent results also provoke interesting questions about their mechanisms of action, both within and outside the olfactory system. Here we describe the identification of the first Obps and some cardinal properties of these proteins. We then consider their function, discussing both the prevailing orthodoxy and the increasing grounds for heterodox views. We then examine these proteins from a broader perspective and consider some intriguing questions in need of answers.

Complete NMR chemical shift assignments of odorant binding protein 22 from the yellow fever mosquito, Aedes aegypti, bound to arachidonic acid

Aedes aegypti mosquitoes are the vector for transmission of Dengue, Zika and chikungunya viruses. These mosquitos feed exclusively on human hosts for a blood meal. Previous studies have established that Dengue virus infection of the mosquito results in increased expression of the odorant binding proteins 22 and 10 within the mosquito salivary gland and silencing of these genes dramatically reduces blood-feeding behaviors. Odorant binding proteins are implicated in modulating the chemosensory perception of external stimuli that regulate behaviors such as host location, feeding and reproduction. However, the role that AeOBP22 plays in the salivary gland is unclear. Here, as a first step to a more complete understanding of the function of AeOBP22, we present the complete backbone and side chain chemical shift assignments of the protein in the complex it forms with arachidonic acid. These assignments reveal that the protein consists of seven α-helices, and that the arachidonic acid is bound tightly to the protein. Comparison with the chemical shift assignments of the apo-form of the protein reveals that binding of the fatty acid is accompanied by a large conformational change in the C-terminal helix, which appears disordered in the absence of lipid. This NMR data provides the basis for determining the structure of AeOBP22 and understanding the nature of the conformational changes that occur upon ligand binding. This information will provide a path to discover novel compounds that can interfere with AeOBP22 function and impact blood feeding by this mosquito.

Functional characteristics of chemosensory proteins in the sawyer beetle Monochamus alternatus Hope

The Japanese pine sawyer, Monochamus alternatus Hope (Coleoptera: Cerambycidae), is a major pest of pines and it is also the key vector of the exotic pinewood nematode in China. In the present study, we cloned, expressed, and purified a chemosensory protein (CSP) in M. alternatus. We surveyed its expression in various developmental stages of male and female adult tissues and determined its binding affinities for different pine volatiles using a competitive binding fluorescence assay. A CSP known as CSP5 in M. alternatus was obtained from an antennal cDNA library and expressed in Escherichia coli. Quantitative reverse transcription polymerase chain reaction results indicated that the CSP5 gene was mainly expressed in male and female antennae. Competitive binding assays were performed to test the binding affinity of recombinant CSP5 to 13 odour molecules of pine volatiles. The results showed that CSP5 showed very strong binding abilities to myrcene, (+)-β-pinene, and (-)-isolongifolene, whereas the volatiles 2-methoxy-4-vinylphenol, p-cymene, and (+)-limonene oxide have relatively weak binding affinity at pH 5.0. Three volatiles myrcene, (+)-β-pinene, and (-)-isolongifolene may play crucial roles in CSP5 binding with ligands but this needs further study for confirmation. The sensitivity of insect to host plant volatiles can effectively be used to control and monitor the population through mass trapping as part of integrated pest management programs.

Structure and Function Studies of Asian Corn Borer Ostrinia furnacalis Pheromone Binding Protein2

Lepidopteran male moths have an extraordinarily sensitive olfactory system that is capable of detecting and responding to minute amounts of female-secreted pheromones over great distances. Pheromone-binding proteins (PBPs) in male antennae ferry the hydrophobic ligand across the aqueous lymph to the olfactory receptor neuron triggering the response. PBPs bind ligands at physiological pH of the lymph and release them at acidic pH near the receptor while undergoing a conformational change. In Anthereae polyphemus PBP1, ligand binding to the hydrophobic pocket and its release is regulated by two biological gates: His70 and His95 at one end of the pocket and C-terminus tail at the other end. Interestingly, in Asian corn borer Ostrinia furnacalis PBP2 (OfurPBP2), critical residues for ligand binding and release are substituted in both biological gates. The impact of these substitutions on the ligand binding and release mechanism in OfurPBP2 is not known. We report here overexpression of soluble OfurPBP2 and structural characterization at high and low pH by circular dichroism (CD) and NMR. Ligand binding and ab initio model development were carried out with fluorescence and small-angle X-ray scattering (SAXS) respectively. OfurPBP2 in solution at pH 6.5 is homogeneous, well-folded and has a compact globular shape.

Odorant-Binding Proteins as Sensing Elements for Odour Monitoring

Odour perception has been the object of fast growing research interest in the last three decades. Parallel to the study of the corresponding biological systems, attempts are being made to model the olfactory system with electronic devices. Such projects range from the fabrication of individual sensors, tuned to specific chemicals of interest, to the design of multipurpose smell detectors using arrays of sensors assembled in a sort of artificial nose. Recently, proteins have attracted increasing interest as sensing elements. In particular, soluble olfaction proteins, including odorant-binding proteins (OBPs) of vertebrates and insects, chemosensory proteins (CSPs) and Niemann-Pick type C2 (NPC2) proteins possess interesting characteristics for their use in sensing devices for odours. In fact, thanks to their compact structure, their soluble nature and small size, they are extremely stable to high temperature, refractory to proteolysis and resistant to organic solvents. Moreover, thanks to the availability of many structures solved both as apo-proteins and in complexes with some ligands, it is feasible to design mutants by replacing residues in the binding sites with the aim of synthesising proteins with better selectivity and improved physical properties, as demonstrated in a number of cases.

Deciphering the Odorant Binding, Activation, and Discrimination Mechanism of Dhelobp21 from Dastarus Helophoroides

Odorant-binding proteins (OBPs) play a pivotal role in transporting odorants through the sensillar lymph of insect chemosensory sensilla and increasing the sensitivity of the olfactory system. To address the ligand binding, activation, and release mechanisms of OBPs, we performed a set of conventional molecular dynamics simulations for binding of the odorant-binding protein DhelOBP21 from Dastarcus helophoroides with 18 ligands (1-NPN and 17 volatiles), as well as four constant-pH molecular dynamics simulations. We found that the open pocket DhelOBP21 at pH 5.0 could bind volatiles and form a closed pocket complex via transformation of its N-terminus into regular Helix at pH 7.0 and vice versa. Moreover, the discrimination of volatiles (selectivity and promiscuity) was determined by the characteristics of both the volatiles and the ‘essential’ and ‘selective’ amino acid residues in OBP binding pockets, rather than the binding affinity of the volatiles. This study put forward a new hypothesis that during the binding of volatiles there are two transitions for the DhelOBP21 amino-terminus: pH- and odorant binding-dependent random-coil-to-helix. Another important finding is providing a framework for the exploration of the complete coil-to-helix transition process and theoretically analyzing its underlying causes at molecular level.

From radioactive ligands to biosensors: binding methods with olfactory proteins

In this paper, we critically review the binding protocols currently reported in the literature to measure the affinity of odorants and pheromones to soluble olfactory proteins, such as odorant-binding proteins (OBPs), chemosensory proteins (CSPs) and Niemann-Pick class C2 (NPC2) proteins. The first part contains a brief introduction on the principles of binding and a comparison of the techniques adopted or proposed so far, discussing advantages and problems of each technique, as well as their suitable application to soluble olfactory proteins. In the second part, we focus on the fluorescent binding assay, currently the most widely used approach. We analyse advantages and drawbacks, trying to identify the causes of anomalous behaviours that have been occasionally observed, and suggest how to interpret the experimental data when such events occur. In the last part, we describe the state of the art of biosensors for odorants, using soluble olfactory proteins immobilised on biochips, and discuss the possibility of using such approach as an alternative way to measure binding events and dissociation constants.

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.

Distinct Subfamilies of Odorant Binding Proteins in Locust (Orthoptera, Acrididae): Molecular Evolution, Structural Variation, and Sensilla-Specific Expression

Odorant binding proteins (OBPs) play an important role in insect olfaction, facilitating transportation of odorant molecules in the sensillum lymph. While most of the researches are concentrated on Lepidopteran and Dipteran species, our knowledge about Orthopteran species is still very limited. In this study, we have investigated OBPs of the desert locust Schistocerca gregaria, a representative Orthopteran species. We have identified 14 transcripts from a S. gregaria antennal transcriptome encoding SgreOBPs, and recapitulated the phylogenetic relationship of SgreOBPs together with OBPs from three other locust species. Two conserved subfamilies of classic OBPs have been identified, named I-A and II-A, exhibiting both common and subfamily-specific amino acid motifs. Distinct evolutionary features were observed for subfamily I-A and II-A OBPs. Surface topology and interior cavity were elucidated for OBP members from the two subfamilies. Antennal topographic expression revealed distinct sensilla- and cellular- specific expression patterns for SgreOBPs from subfamily I-A and II-A. These findings give first insight into the repertoire of locust OBPs with respect to their molecular and evolutionary features as well as their expression in the antenna, which may serve as an initial step to unravel specific roles of distinct OBP subfamilies in locust olfaction.

C-terminus Methionene Specifically Involved in Binding Corn Odorants to Odorant Binding Protein4 in Macrocentrus cingulum

The soluble carrier proteins, OBPs carry odor components through sensilium lymph to specific receptors within the antennal sensilla to trigger behavioral responses. Herein, McinOBP4 was characterized from the Macrocentrus cingulum, which is the specialist parasitic insect of Ostrinia furnacalis for better understanding of olfactory recognition mechanism of this wasp. The classical odorant binding protein McinOBP4 showed good binding affinity to corn green leaf volatiles. RT-qPCR results showed that the McinOBP4 was primarily expressed in male and female wasp antennae, with transcripts levels differing by sex. Fluorescence assays indicate that, McinOBP4 binds corn green leaf volatiles including terpenoides and aliphatic alcohols as well as aldehydes with good affinity. We have also conducted series of binding assay with first mutant (M1), which lacked the last 8 residues and a second mutant (M2), with Met119 replaced by Leucine (Leu119). In the acidic conditions, affinity N-phenylnaphthylamine (1-NPN) to McinOBP4 and M1 were substantially decreased, but increase in basic condition with no significant differences. The lack of C-terminus showed reduced affinity to terpenoides and aliphatic alcohols as well as aldehydes compounds of corn odorants. The mutant M2 with Met119 showed significant reduction in binding affinity to tested odorants, it indicating that Met119 forming hydrophobic chain with the odorants functional group to binding. This finding provides detailed insight of chemosensory function of McinOBP4 in M. cingulum and help to develop low release agents that attract of this wasp to improve ecologically-friendly pest management strategy.

Multiple Binding Poses in the Hydrophobic Cavity of Bee Odorant Binding Protein AmelOBP14

In the first step of olfaction, odorants are bound and solubilized by small globular odorant binding proteins (OBPs) which shuttle them to the membrane of a sensory neuron. Low ligand affinity and selectivity at this step enable the recognition of a wide range of chemicals. Honey bee Apis mellifera’s OBP14 (AmelOBP14) binds different plant odorants in a largely hydrophobic cavity. In long molecular dynamics simulations in the presence and absence of ligand eugenol, we observe a highly dynamic C-terminal region which forms one side of the ligand-binding cavity, and the ligand drifts away from its crystallized orientation. Hamiltonian replica exchange simulations, allowing exchanges of conformations sampled by the real ligand with those sampled by a noninteracting dummy molecule and several intermediates, suggest an alternative, quite different ligand pose which is adopted immediately and which is stable in long simulations. Thermodynamic integration yields binding free energies which are in reasonable agreement with experimental data.

Three amino acid residues of an odorant-binding protein are involved in binding odours in Loxostege sticticalis L

Odorant-binding proteins (OBPs) play an important role in insect olfactory processes and are thought to be responsible for the transport of pheromones and other semiochemicals across the sensillum lymph to the olfactory receptors within the antennal sensilla. As an important general odorant binding protein in the process of olfactory recognition, LstiGOBP1 of Loxostege sticticalis L. has been shown to have good affinity to various plant volatiles. However, the binding specificity of LstiGOBP1 should be further explored in order to better understand the olfactory recognition mechanism of L. sticticalis. In this study, real-time PCR experiments indicated that LstiGOBP1 was expressed primarily in adult antennae. Homology modelling and molecular docking were then conducted on the interactions between LstiGOBP1 and 1-heptanol to understand the interactions between LstiGOBP1 and their ligands. Hydrogen bonds formed by amino acid residues might be crucial for the ligand-binding specificity on molecular docking, a hypothesis that was tested by site-directed mutagenesis. As predicted binding sites for LstiGOBP1, Thr15, Trp43 and Val14 were replaced by alanine to determine the changes in binding affinity. Finally, fluorescence assays revealed that the mutants Thr15 and Trp43 had significantly decreased binding affinity to most odours; in mutants that had two-site mutations, the binding to the six odours that were tested was completely abolished. This result indicates that Thr15 and Trp43 were involved in binding these compounds, possibly by forming multiple hydrogen bonds with the functional groups of the ligands. These results provide new insights into the detailed chemistry of odours' interactions with proteins.

An antenna-biased carboxylesterase is specifically active to plant volatiles in Spodoptera exigua

Odorant-degrading enzymes (ODEs) in sensillar lymph are proposed to play important roles in the maintenance of the sensitivity of the olfactory sensilla, by timely degrading the odorants that have already fulfilled the activation of the odorant receptor (OR). Here we reported the cloning and characterization of an ODE gene (SexiCXE10) from the polyphagous insect pest Spodoptera exigua. SexiCXE10 is a carboxylesterase (CXE) gene, encoding a protein with 538 amino acid residues, and bearing typical characteristics of Carboxyl/cholinesterase (CCE, EC 3.1.1.1.) gene family. Tissue-temporal expression pattern by qPCR revealed that the SexiCXE10 mRNA was highly antenna biased, and maintained at high level throughout the adult stage. Further fluorescence in situ hybridization demonstrated that SexiCXE10 mRNA signal was detected under sensilla basiconica and short and long sensilla trichodea. Finally, enzymatic study using purified recombinant enzyme showed that SexiCXE10 had high activity specifically for ester plant volatiles with 7-10 carbon atoms, while no activity was found with S. exigua sex pheromone components and plant volatiles with more carbon atoms. In addition, SexiCXE10 displayed lower activity at acidic pH (pH 5.0), while higher activity was found at neutral and alkaline conditions (pH 6.5-9.0). Our results suggest that SexiCXE10 may play an important role in the degradation of the host plant volatiles, and thus contributes to the high sensitivity of the olfactory system in S. exigua. Meanwhile, the CXE would be a potential target for developing behavioral antagonists and pesticides against S. exigua.

Two subclasses of odorant-binding proteins in Spodoptera exigua display structural conservation and functional divergence

Although many studies on lepidopteran pheromone-binding proteins (PBPs)/ general odorant-binding proteins (GOBPs) have been reported, the functional differentiation within and between the two odorant-binding protein (OBP) subclasses is still elusive. Here we conducted a comparative study on three SexiPBPs and two SexiGOBPs in Spodoptera exigua. Results showed that all five SexiPBP/GOBP genes have the same intron numbers and conserved exon/intron splice sites. Reverse transcription PCR results showed that these five SexiPBP/GOBPs were primarily expressed in antennae of both sexes and some were also detected in other tissues. Further, quantitative real-time PCR showed that five SexiPBP/GOBPs had different sex-biased expression patterns, with PBP1 being highly male-biased (5.96-fold difference) and PBP3 slightly female-biased (2.43-fold difference), while PBP2 and two GOBPs were approximately sex-equivalent (the absolute value<1.90-fold difference). Binding assays showed that all three SexiPBPs could bind all six sex pheromone components, but SexiPBP1 had much higher affinities [dissociation constant (Ki ) <1.10 μM] than did the other two SexiPBPs (Ki  >1.20 μM). Very intriguingly, SexiGOBP2 displayed even stronger binding to five sex pheromone components (Ki  <0.40 μM) than SexiPBP1. In contrast, SexiGOBP1 only exhibited weak binding to three alcohol-pheromone components. Similar results were obtained for tested pheromone analogues. In addition, each of SexiPBP/GOBPs selectively bound some plant odorants with considerable affinities (Ki  <10.0 μM). Taken together, of the three SexiPBPs, SexiPBP1 may play the most important role in female sex pheromone reception, and additionally all three SexiPBPs can detect some plant odorants, while SexiGOBP2 may be involved in the detection of female sex pheromones in addition to plant odorants. The results strongly suggest functional differentiation within and between the two OBP sub-classes.

Ligands binding and molecular simulation: the potential investigation of a biosensor based on an insect odorant binding protein

Based on mimicking biological olfaction, biosensors have been applied for the detection of various ligands in complex environment, which could represent one of the most promising research fields. In this study, the basic characters of one insect odorant binding protein (OBP) as a biosensor were explored. To explore the molecular recognition process, the tertiary structure of the protein was modeled and the protein-ligand interactions with 1,536,550 chemicals were investigated by the molecular docking. The availability of large amount of recombinant SlitOBP1 overcame the difficulty to obtain biological sensing material. After obtained the purified recombinant protein, the result of fluorescence binding assays proved the candidate protein has good affinities with the majority of the tested chemicals. With the aid of simulation docking, the key conserved amino acids within the binding site were identified and then mutated to alanine. After mutation, the protein-ligand binding characteristics were recorded, and the competitive binding assays were carried out to provide experimental verification. The detailed information on its structure and affinities investigated in this study could allow the design of specific mutants with desired characteristics, which provides a solid base for tailoring OBP for biosensor and provides a role model for screening the other elements in olfactory system for different applications.

An antennae-enriched carboxylesterase from Spodoptera exigua displays degradation activity in both plant volatiles and female sex pheromones

Carboxyl/cholinesterase (CCE) is a large gene family of diverse functions, but in insects its function with respect to catabolism of sex pheromone components and plant volatiles is not well understood. In the present study, we cloned and functionally characterized one putative odorant-degrading enzyme (ODE) of the CCE family, SexiCXE14, from Spodoptera exigua. The tissue-temporal expression pattern revealed that the mRNA level of SexiCXE14 is antennae-enriched, sex equivalent and peaks at 3 days after moth eclosion. Functional study using the recombinant enzyme determined that SexiCXE14 has high degrading activity (Vmax) to host plant volatiles, suggesting its role in degradation of these volatiles. In addition, SexiCXE14 may also play a role in the degradation of sex pheromone components, as the Vmax and affinity parameter (Km) values with the sex pheromones are similar to those of reported pheromone degrading enzymes (PDEs). Further analysis of the relationship between substrate structure and enzymatic activity demonstrated that carbon chain length is a major influential factor, while the number of double bonds also affects the enzymatic activity. In addition, SexiCXE14 displays lower activity at acidic pH levels (pH 5.0) than in neutral conditions (pH 6.5). By characterizing this new ODE the present study provides insights in understanding of the high sensitivity of the moth olfactory system.

Structure and biotechnological applications of odorant-binding proteins

Odorant-binding proteins (OBPs) are small soluble polypeptides found in sensory organs of vertebrates and insects as well as in secretory glands and are dedicated to detection and release of chemical stimuli. OBPs of vertebrates belong to the family of lipocalin proteins, while those of insects are folded into α-helical domains. Both types of architectures are extremely stable to temperature, organic solvents and proteolytic digestion. These characteristics make OBPs suitable elements for fabricating biosensors to be used in the environment, as well as for other biotechnological applications. The affinity of OBPs for small volatile organic compounds is in the micromolar range, and they have broad specificity to a range of ligands. For biotechnological applications, OBPs can be expressed in bacterial systems at low cost and are easily purified. The large amount of information available on their structures and affinities to different molecules should allow the design of specific mutants with desired characteristics and represent a solid base for tailoring OBPs for different applications.

Pheromone discrimination by a pH-tuned polymorphism of the Bombyx mori pheromone-binding protein

The Bombyx mori pheromone-binding protein (BmorPBP) is known to adopt two different conformations. These are BmorPBP(A), where a regular helix formed by the C-terminal dodecapeptide segment, α7, occupies the ligand-binding cavity, and BmorPBP(B), where the binding site is free to accept ligands. NMR spectra of delipidated BmorPBP solutions at the physiological pH of the bulk sensillum lymph near pH 6.5 show only BmorPBP(A), and in mixtures, the two species are in slow exchange on the chemical shift frequency scale. This equilibrium has been monitored at variable pH and ligand concentrations, demonstrating that it is an intrinsic property of BmorPBP that is strongly affected by pH variation and ligand binding. This polymorphism tunes BmorPBP for optimal selective pheromone transport: Competition between α7 and lipophilic ligands for its binding cavity enables selective uptake of bombykol at the pore endings in the sensillum wall, whereas compounds with lower binding affinity can only be bound in the bulk sensillum lymph. After transport across the bulk sensillum lymph into the lower pH area near the dendritic membrane surface, bombykol is ejected near the receptor, whereas compounds with lower binding affinity are ejected before reaching the olfactory receptor, rendering them susceptible to degradation by enzymes present in the sensillum lymph.