Protein structure encodes the ligand binding specificity in pheromone binding proteins

Lectin histochemistry of the olfactory mucosa of Korean native cattle, Bos taurus coreanae

BACKGROUND

The olfactory mucosa (OM) is crucial for odorant perception in the main olfactory system. The terminal carbohydrates of glycoconjugates influence chemoreception in the olfactory epithelium (OE).

OBJECTIVES

The histological characteristics and glycoconjugate composition of the OM of Korean native cattle (Hanwoo, Bos taurus coreae) were examined to characterize their morphology and possible functions during postnatal development.

METHODS

The OM of neonate and adult Korean native cattle was evaluated using histological, immunohistochemical, and lectin histochemical methods.

RESULTS

Histologically, the OM in both neonates and adults consists of the olfactory epithelium and the lamina propria. Additionally, using periodic acid Schiff and Alcian blue (pH 2.5), the mucus specificity of the Bowman's gland duct and acini in the lamina propria was determined. Immunohistochemistry demonstrated that mature and immature olfactory sensory neurons of OEs express the olfactory marker protein and growth associated protein-43, respectively. Lectin histochemistry indicated that numerous glycoconjugates, including as N-acetylglucosamine, mannose, galactose, N-acetylgalactosamine, complex type N-glycan, and fucose groups, were expressed at varied levels in the different cell types in the OMs of neonates and adults at varying levels. According to our observations, the cattle possessed a well-developed olfactory system, and the expression patterns of glycoconjugates in neonatal and adult OMs varied considerably.

CONCLUSIONS

This is the first study to describe the morphological assessment of the OM of Korean native cattle with a focus on lectin histochemistry. The findings suggest that glycoconjugates may play a role in olfactory chemoreception, and that their labeling properties may be closely related to OM development and maturity.

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.

Odor-Specific Deactivation Defects in a Drosophila Odorant-Binding Protein Mutant

Insect odorant-binding proteins (OBPs) are a large, diverse group of low-molecular weight proteins secreted into the fluid bathing olfactory and gustatory neuron dendrites. The best-characterized OBP, LUSH (OBP76a) enhances pheromone sensitivity enabling detection of physiological levels of the male-specific pheromone, 11-cis vaccenyl acetate. The role of the other OBPs encoded in the Drosophila genome is largely unknown. Here, using clustered regularly interspaced short palindromic repeats/Cas9, we generated and characterized the loss-of-function phenotype for two genes encoding homologous OBPs, OS-E (OBP83b) and OS-F (OBP83a). Instead of activation defects, these extracellular proteins are required for normal deactivation of odorant responses to a subset of odorants. Remarkably, odorants detected by the same odorant receptor are differentially affected by the loss of the OBPs, revealing an odorant-specific role in deactivation kinetics. In stark contrast to lush mutants, the OS-E/F mutants have normal activation kinetics to the affected odorants, even at low stimulus concentrations, suggesting that these OBPs are not competing for these ligands with the odorant receptors. We also show that OS-E and OS-F are functionally redundant as either is sufficient to revert the mutant phenotype in transgenic rescue experiments. These findings expand our understanding of the roles of OBPs to include the deactivation of odorant responses.

Pheromone-binding proteins based phylogenetics and phylogeography of Maruca spp. from Asia, Africa, Oceania, and South America

Variations in the functional response of legume pod borer (Maruca vitrata) populations to sex pheromone blends were observed in Asia and Africa. Hence, this study was carried out to understand the differences in pheromone-binding proteins (PBPs) among Maruca populations in Asia, Africa, Oceania, and South America. A de novo transcriptome assembly was adopted to sequence the entire transcribed mRNAs in M. vitrata from Taiwan. The raw-sequence data were assembled using homologous genes from related organisms in GenBank to detect M. vitrata PBPs (MvitPBPs). Sections of the cDNA of MvitPBP of different length were used to design primers to amplify the full-length cDNA of PBPs. All three PBP sequences comprised three exons interspersed by two introns. In total, 92 MvitPBP1 haplotypes, 77 MvitPBP2 haplotypes, and 64 MvitPBP3 haplotypes were identified in 105, 98, and 68 Maruca individuals, respectively. High pairwise F ST values (0.41-0.73) and phylogenetic analyses distinguished the putative Maruca species in South America from those occurring in rest of the world, and possibly two putative subspecies in Asia and Africa. The haplotype networks and Automatic Barcode Gap Discovery analyses also confirmed these results. The negative Tajima's D and Fu's F S values showed the recent demographic expansion of Maruca populations. Thus, this study confirmed the presence of different Maruca species and/or subspecies in different continents based on the diversity within PBP genes. Additional sampling and studies are suggested for Oceania and South America. The genetic differences among Maruca populations should be carefully considered while using sex pheromone lures and bio-control agents.

Molecular characterization and functional analysis of pheromone binding proteins and general odorant binding proteins from Carposina sasakii Matsumura (Lepidoptera: Carposinidae)

BACKGROUND

The peach fruit moth, Carposina sasakii Matsumura (Lepidoptera: Carposinidae), is one of the most destructive pests of pome and stone fruits, while few studies of their molecular biology and physiology have been conducted. Research into CsasPBPs (Carposina sasakii pheromone binding proteins) and CsasGOBPs (Carposina sasakii general odorant binding proteins) may provide insights in to the mechanisms of olfaction in Carposina sasakii.

RESULTS

In our study, results of real time quantitative polymerase chain reaction (qPCR) assays demonstrated that CsasPBP1-3 and CsasGOBP1-2 transcripts were abundantly expressed in the antennae of both sexes, suggesting they play a vital role in olfaction. In addition, to examine specific functional differences between pheromone binding proteins (PBPs) and general odorant binding proteins (GOBPs), fluorescence competitive binding assays were used to measured the binding affinities for the two sex pheromones and 18 apple plant volatiles. As a result, both PBPs and GOBPs showed stronger binding affinities to Z-7-eicosene-11-one than Z-7-nonadecene-11-one in two sex pheromones, whereas only PBP3 exhibited specific affinity towards both these two sex pheromone components, and PBP1 showed a high binding affinity to the sex pheromone components, and to other plant volatiles. In addition, GOBP1-2 displayed high binding affinity to general components of plant volatiles.

CONCLUSION

Our study suggested CsasPBPs and CsasGOBPs play distinct physiological roles in the perception of sex pheromones and host plant volatiles. © 2018 Society of Chemical Industry.

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.

Molecular and Functional Characterization of pheromone binding protein 1 from the Oriental Fruit Moth, Grapholita molesta (Busck)

Pheromone binding protein (PBP) is thought primarily to bind and transport the sex pheromone in moths. The accumulated studies suggest that three PBPs were identified in moth species. In Grapholita molesta, the functions of GmolPBP2 and GmolPBP3 have been previously studied. However, the function of GmolPBP1 is still unclear. Furthermore, the Cydia pomonella sex pheromone Codlemone can act as a sex pheromone synergist of G. molesta. In C. pomonella, CpomPBP1 specifically bind the Codlemone. CpomPBP1 displays high identity with GmolPBP1 (70%), indicating that the two PBPs may share a similar 3D structure thus can bind the similar or same ligands. In this study, we explored the molecular and functional characterization of GmolPBP1. GmolPBP1, bearing the typical characteristics of Lepidopteran odorant binding proteins, was closest phylogenetically to CpomPBP1. Binding studies demonstrated that GmolPBP1 exhibited strong binding affinities with (Z)-8-dodecenyl alcohol, 1-dodecanol and Codlemone. Molecular docking showed that GmolPBP1 has different ligand recognition mechanism for the three ligands. Our results suggest that GmolPBP1 functions as recognizer of (Z)-8-dodecenyl alcohol and 1-dodecanol of the female sex pheromone blend, and may be the potential transporter of Codlemone, which contributes to the synergism of the pheromone response of G. molesta by Codlemone.

Correlation of pheromone-binding protein–ligand equilibrium dissociation constants with electroantennogram response patterns

Pheromone-binding proteins (PBPs) are water-soluble proteins found at high concentration in the lymph fluid of pheromone-sensing hairs on insect antennae. PBPs could function as pheromone transporters, ferrying the hydrophobic odorants to their cognate odorant receptors. However, it is also possible for these proteins to bind the odorants near the dendritic membrane of pheromone-sensing neurons and, therefore, function as scavengers. The two functions are not mutually exclusive. In this paper, the transporter and (or) scavenger roles of PBPs in pheromone perception were investigated using the pheromone of the gypsy moth (7R, 8S)-epoxy-2-methyloctadecane and analogues with heteroatom (O or S) substitutions in the hydrocarbon chain. PBP–ligand equilibrium dissociation constants (Kd) were correlated with electroantennogram (EAG) response patterns of male gypsy moth antennae to the pheromone, its enantiomer, and their respective analogues. EAG measures the potential drop across the antenna due to odorant receptor activation and subsequent ion channel opening. Three quantifiable properties of the EAG responses were used: lag times from stimulus to response onset, depolarization rates (rate of receptor activation), and repolarization rates (rate of receptor deactivation). Negative correlations were observed between Kd and lag times and between Kd and repolarization rates. Positive correlations were seen with Kd against depolarization rates. The inverse relationship of Kd constants with lag times and the direct relationship with depolarization rates strongly supports transporter function of PBPs. Interestingly, the inverse correlation of Kd constants with repolarization rates suggests a scavenger effect. These results indicate that PBP affects odorant receptor activity through both odorant transport and scavenger functions. Through differences in ligand binding affinities, PBPs influence pheromone availability for receptor activation.

Differential gene expression in the evolution of sex pheromone communication in New Zealand's endemic leafroller moths of the genera Ctenopseustis and Planotortrix

Background

Sex pheromone communication in moths has attracted the attention of evolutionary biologists due to the vast array of pheromone compounds used, addressing questions of how this diversity arose and how male reception has evolved in step with the female signal. Here we examine the role of changing gene expression in the evolution of mate recognition systems in leafroller moths, particularly focusing on genes involved in the biosynthetic pathways of sex pheromones in female pheromone glands and the peripheral reception repertoire in the antennae of males. From tissue-specific transcriptomes we mined and compared a database of genes expressed in the pheromone glands and antennae of males and females of four closely related species of leafroller moths endemic to New Zealand, Ctenopseutis herana and C. obliquana, and Planotortrix excessana and P. octo. The peculiarity of this group, compared to other Lepidoptera, is the use of (Z)-5-tetradecenyl acetate, (Z)-7-tetradecenyl acetate, and (Z)-8-tetradecenyl acetate as sex pheromone components.

Results

We identify orthologues of candidate genes from the pheromone biosynthesis pathway, degradation and transport, as well as genes of the periphery olfactory repertoire, including large families of binding proteins, receptors and odorant degrading enzymes. The production of distinct pheromone blends in the sibling species is associated with the differential expression of two desaturase genes, deast5 and desat7, in the pheromone glands. In male antennae, three odorant receptors, OR74, OR76a and OR30 are over-expressed, but their expression could not be clearly associated with the detection of species-specific pheromones components. In addition these species contain duplications of all three pheromone binding proteins (PBPs) that are also differentially expressed among species.

Conclusions

While in females differences in the expression of desaturases may be sufficient to explain pheromone blend differences among these New Zealand leafroller species, in males differential expression of several genes, including pheromone binding proteins, may underpin differences in the response by males to changing pheromone components among the species.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4451-1) contains supplementary material, which is available to authorized users.

Functional characterization of PBP1 gene in Helicoverpa armigera (Lepidoptera: Noctuidae) by using the CRISPR/Cas9 system

Pheromone binding proteins (PBPs) are thought to play crucial roles in perception of the sex pheromones particularly in noctuid moths, but this is rarely in vivo evidenced due to lacking an effective technique. Here, we reported an in vivo functional study of PBP1 in the important lepidopteran pest Helicoverpa armigera (HarmPBP1), by using the CRISPR/Cas9 system. Efficient and heritable mutagenesis was achieved by egg injection of mixture of Cas9-mRNA and HarmPBP1-sgRNA. The TA cloning and sequencing revealed various insertion and/or deletion (indel) mutations at the target site. Among those, one mutation resulted in a premature stop codon at the target site, which led to a highly truncated protein with only 10 amino acids. The HarmPBP1 with this mutation would completely loss its function, and thus was used to select the homozygous mutant insects for functional analysis. The electroantennogram recording showed that the mutant male adults displayed severely impaired responses to all three sex pheromone components (Z11-16:Ald, Z9-16:Ald and Z9-14:Ald). Our study provides the first in vivo evidence that HarmPBP1 plays important role in perception of female sex pheromones, and also an effective methodology for using CRISPR/Cas9 system in functional genetic study in H. armigera as well as other insects.

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.

Sex pheromone recognition and characterization of three pheromone-binding proteins in the legume pod borer, Maruca vitrata Fabricius (Lepidoptera: Crambidae)

Pheromone-binding proteins (PBPs) are essential for the filtering, binding and transporting of sex pheromones across sensillum lymph to membrane-associated pheromone receptors of moths. In this study, three novel PBP genes were expressed in Escherichia coli to examine their involvement in the sex pheromone perception of Maruca vitrata. Fluorescence binding experiments indicated that MvitPBP1-3 had strong binding affinities with four sex pheromones. Moreover, molecular docking results demonstrated that six amino acid residues of three MvitPBPs were involved in the binding of the sex pheromones. These results suggested that MvitPBP1-3 might play critical roles in the perception of female sex pheromones. Additionally, the binding capacity of MvitPBP3 with the host-plant floral volatiles was high and was similar to that of MvitGOBP2. Furthermore, sequence alignment and docking analysis showed that both MvitGOBP2 and MvitPBP3 possessed an identical key binding site (arginine, R130/R140) and a similar protein pocket structure around the binding cavity. Therefore, we hypothesized that MvitPBP3 and MvitGOBP2 might have synergistic roles in binding different volatile ligands. In combination, the use of synthetic sex pheromones and floral volatiles from host-plant may be used in the exploration for more efficient monitoring and integrated management strategies for the legume pod borer in the field.

Antennal transcriptome analysis of the Asian longhorned beetle Anoplophora glabripennis

Olfactory proteins form the basis of insect olfactory recognition, which is crucial for host identification, mating, and oviposition. Using transcriptome analysis of Anoplophora glabripennis antenna, we identified 42 odorant-binding proteins (OBPs), 12 chemosensory proteins (CSPs), 14 pheromone-degrading enzymes (PDEs), 1 odorant-degrading enzymes (ODE), 37 odorant receptors (ORs), 11 gustatory receptors (GRs), 2 sensory neuron membrane proteins (SNMPs), and 4 ionotropic receptor (IR). All CSPs and PBPs were expressed in antennae, confirming the authenticity of the transcriptome data. CSP expression profiles showed that AglaCSP3, AglaCSP6, and AglaCSP12 were expressed preferentially in maxillary palps and AglaCSP7 and AglaCSP9 were strongly expressed in antennae. The vast majority of CSPs were highly expressed in multiple chemosensory tissues, suggesting their participation in olfactory recognition in almost all olfactory tissues. Intriguingly, the PBP AglaPBP2 was preferentially expressed in antenna, indicating that it is the main protein involved in efficient and sensitive pheromone recognition. Phylogenetic analysis of olfactory proteins indicated AglaGR1 may detect CO₂. This study establishes a foundation for determining the chemoreception molecular mechanisms of A. glabripennis, which would provide a new perspective for controlling pest populations, especially those of borers.

Identification of Putative Olfactory Genes from the Oriental Fruit Moth Grapholita molesta via an Antennal Transcriptome Analysis

BACKGROUND

The oriental fruit moth, Grapholita molesta, is an extremely important oligophagous pest species of stone and pome fruits throughout the world. As a host-switching species, adult moths, especially females, depend on olfactory cues to a large extent in locating host plants, finding mates, and selecting oviposition sites. The identification of olfactory genes can facilitate investigation on mechanisms for chemical communications.

METHODOLOGY/PRINCIPAL FINDING

We generated transcriptome of female antennae of G.molesta using the next-generation sequencing technique, and assembled transcripts from RNA-seq reads using Trinity, SOAPdenovo-trans and Abyss-trans assemblers. We identified 124 putative olfactory genes. Among the identified olfactory genes, 118 were novel to this species, including 28 transcripts encoding for odorant binding proteins, 17 chemosensory proteins, 48 odorant receptors, four gustatory receptors, 24 ionotropic receptors, two sensory neuron membrane proteins, and one odor degrading enzyme. The identified genes were further confirmed through semi-quantitative reverse transcription PCR for transcripts coding for 26 OBPs and 17 CSPs. OBP transcripts showed an obvious antenna bias, whereas CSP transcripts were detected in different tissues.

CONCLUSION

Antennal transcriptome data derived from the oriental fruit moth constituted an abundant molecular resource for the identification of genes potentially involved in the olfaction process of the species. This study provides a foundation for future research on the molecules involved in olfactory recognition of this insect pest, and in particular, the feasibility of using semiochemicals to control this pest.

The Lepidoptera Odorant Binding Protein gene family: Gene gain and loss within the GOBP/PBP complex of moths and butterflies

Butterflies and moths differ significantly in their daily activities: butterflies are diurnal while moths are largely nocturnal or crepuscular. This life history difference is presumably reflected in their sensory biology, and especially the balance between the use of chemical versus visual signals. Odorant Binding Proteins (OBP) are a class of insect proteins, at least some of which are thought to orchestrate the transfer of odor molecules within an olfactory sensillum (olfactory organ), between the air and odor receptor proteins (ORs) on the olfactory neurons. A Lepidoptera specific subclass of OBPs are the GOBPs and PBPs; these were the first OBPs studied and have well documented associations with olfactory sensilla. We have used the available genomes of two moths, Manduca sexta and Bombyx mori, and two butterflies, Danaus plexippus and Heliconius melpomene, to characterize the GOBP/PBP genes, attempting to identify gene orthologs and document specific gene gain and loss. First, we identified the full repertoire of OBPs in the M. sexta genome, and compared these with the full repertoire of OBPs from the other three lepidopteran genomes, the OBPs of Drosophila melanogaster and select OBPs from other Lepidoptera. We also evaluated the tissue specific expression of the M. sexta OBPs using an available RNAseq databases. In the four lepidopteran species, GOBP2 and all PBPs reside in single gene clusters; in two species GOBP1 is documented to be nearby, about 100 kb from the cluster; all GOBP/PBP genes share a common gene structure indicating a common origin. As such, the GOBP/PBP genes form a gene complex. Our findings suggest that (1) the lepidopteran GOBP/PBP complex is a monophyletic lineage with origins deep within Lepidoptera phylogeny, (2) within this lineage PBP gene evolution is much more dynamic than GOBP gene evolution, and (3) butterflies may have lost a PBP gene that plays an important role in moth pheromone detection, correlating with a shift from olfactory (moth) to visual (butterfly) communication, at least regarding long distance mate recognition. These findings will be clarified by additional lepidopteran genomic data, but the observation that moths and butterflies share most of the PBP/GOBP genes suggests that they also share common chemosensory-based behavioral pathways.

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.

Molecular characterization and phylogenetic analysis of three odorant binding protein gene transcripts in Dendrolimus species (Lepidoptera: Lasiocampidae)

Pine caterpillar moths, Dendrolimus spp. (Lepidoptera: Lasiocampidae), are serious economic pest of pines. Previously, phylogenetic analyses of Dendrolimus using different methods yielded inconsistent results. The chemosensory systems of insects may play fundamental roles in promoting speciation. Odorant-binding proteins (OBPs) participate in the first step of odor detection. Studying the evolution of OBPs in closely related species may help us to identify their role in speciation. We identified three OBPs - one pheromone-binding protein and two general odorant-binding proteins - from male antennae of four Dendrolimus species, D. superans (Butler), D. punctatus (Walker), D. kikuchii Matsumura, and D. houi Lajonquiere, the olfactory recognition systems of which had not been previously investigated. We analyzed their molecular characteristics and compared their sequences to those of OBPs in D. tabulaeformis Tsai et Liu. Ka/Ks ratio analyses among the five Dendrolimus species indicate that PBP1 genes experienced more evolutionary pressure than the GOBPs. Phylogenetic relationships of PBP1 and GOBP1 both indicated that D. houi was the basal species, then branched D. kikuchii, while D. tabulaeformis, D. punctatus, and D. superans evolved more recently. These relationships are consistent with the changes in sex pheromone components of these five species. Dendrolimus tabulaeformis and D. punctatus are closely related sister species. However, the distances among GOBP2 sequences in the five Dendrolimus were very short, and the relationships of D. houi and D. kikuchii could not be resolved. Integrating our results with those of previous studies, we hypothesized that D. kikuchii, D. punctatus and D. superans evolved from the basal ancestor because of sex pheromone mutations and environmental pressure.

Antennal transcriptome analysis and comparison of olfactory genes in two sympatric defoliators, Dendrolimus houi and Dendrolimus kikuchii (Lepidoptera: Lasiocampidae)

The Yunnan pine and Simao pine caterpillar moths, Dendrolimus houi Lajonquière and Dendrolimus kikuchii Matsumura (Lepidoptera: Lasiocampidae), are two closely related and sympatric pests of coniferous forests in southwestern China, and olfactory communication systems of these two insects have received considerable attention because of their economic importance. However, there is little information on the molecular aspect of odor detection about these insects. Furthermore, although lepidopteran species have been widely used in studies of insect olfaction, few work made comparison between sister moths on the olfactory recognition mechanisms. In this study, next-generation sequencing of the antennal transcriptome of these two moths were performed to identify the major olfactory genes. After comparing the antennal transcriptome of these two moths, we found that they exhibit highly similar transcripts-associated GO terms. Chemosensory gene families were further analyzed in both species. We identified 23 putative odorant binding proteins (OBP), 17 chemosensory proteins (CSP), two sensory neuron membrane proteins (SNMP), 33 odorant receptors (OR), and 10 ionotropic receptors (IR) in D. houi; and 27 putative OBPs, 17 CSPs, two SNMPs, 33 ORs, and nine IRs in D. kikuchii. All these transcripts were full-length or almost full-length. The predicted protein sequences were compared with orthologs in other species of Lepidoptera and model insects, including Bombyx mori, Manduca sexta, Heliothis virescens, Danaus plexippus, Sesamia inferens, Cydia pomonella, and Drosophila melanogaster. The sequence homologies of the orthologous genes in D. houi and D. kikuchii are very high. Furthermore, the olfactory genes were classed according to their expression level, and the highly expressed genes are our target for further function investigation. Interestingly, many highly expressed genes are ortholog gene of D. houi and D. kikuchii. We also found that the Classic OBPs were further separated into three groups according to their motifs, which will help future functional researches. Surprisingly, no pheromone receptor was identified in the two Dendrolimus species, which may indicate a special pheromone identification mechanism in Dendrolimus. Our work allows for further functional studies of pheromones and host volatile recognition genes, and give novel candidate targets for pest management.

Different roles suggested by sex-biased expression and pheromone binding affinity among three pheromone binding proteins in the pink rice borer, Sesamia inferens (Walker) (Lepidoptera: Noctuidae)

Pheromone binding proteins (PBPs) are thought to bind and transport hydrophobic sex pheromone molecules across the aqueous sensillar lymph to specific pheromone receptors on the dendritic membrane of olfactory neurons. A maximum of 3 PBP genes have been consistently identified in noctuid species, and each of them shares high identity with its counterparts in other species within the family. The functionality differences of the 3 proteins are poorly understood. In the present study, 3 PBP cDNAs (SinfPBP1, 2, 3) were identified from the pink rice borer, Sesamia inferens, for the first time. The quantitative real-time PCR indicated that the 3 PBPs displayed similar temporal but very different sex related expression profiles. Expression of SinfPBP1 and SinfPBP2 were highly and moderately male biased, respectively, while SinfPBP3 was slightly female biased, as SinfPBPs were expressed at very different levels (PBP1>PBP2≫PBP3) in male antennae, but at similar levels in female antennae. Furthermore, the 3 SinfPBPs displayed different ligand binding profiles in fluorescence competitive binding assays. SinfPBP1 exhibited high and similar binding affinities to all 3 sex pheromone components (Ki=0.72-1.60 μM), while SinfPBP2 showed selective binding to the alcohol and aldehyde components (Ki=0.78-1.71 μM), and SinfPBP3 showed no obvious binding to the 3 sex pheromone components. The results suggest that SinfPBP1 plays a major role in the reception of female sex pheromones in S. inferens, while SinfPBP3 plays a least role (if any) and SinfPBP2 functions as a recognizer of alcohol and aldehyde components.

Construction and analysis of cDNA libraries from the antennae of Batocera horsfieldi and expression pattern of putative odorant binding proteins

A high-quality cDNA library was constructed from female and male antenna of the longhorned beetle, Batocera horsfieldi (Hope) (Coleoptera: Cerambycidae), a serious pest of Populus (Salicales: Salicaceae). The titer was approximately 2.37 × 106 pfu/mL, and this complies with the test requirement. From the libraries, 692 clones were selected randomly, sequenced, and further analyzed, and the recombinational efficiency reached 93.85%. By alignment and cluster analysis, we identified four odorant binding proteins, two pheromone-binding proteins (have the characteristic six conserved cysteine residues), four Minus-C odorant binding proteins (lost two conserved cysteines), and three chemosensory proteins. In this study, we describe the identification and characterization of four new cDNAs that encode Minus-C odorant binding proteins (Minus-C OBPs) from B. horsfieldi antennal cDNA libraries. Our investigation focused on the expression pattern of the Minus-C OBP genes in various tissues in both sexes at different developmental stages, using reverse transcription PCR (RT-PCR) and realtime PCR (qPCR) strategies. Minus-C OBP1, 2, and 3 were expressed in all tested tissues, with the exception of the head (without antenna, labial palps, and maxillary palps). Minus-C OBP4 was expressed in the antenna, legs, and abdomen, but not in the labial palps, maxillary palps, or head. The qPCR results revealed MinusC OBPs were expressed in the antenna throughout the adult life, and that the transcript levels of these genes depended on the sex, age, and mating status of adults.

Molecular characterization, expression pattern, and ligand-binding property of three odorant binding protein genes from Dendrolimus tabulaeformis

Odorant binding proteins (OBPs) play important roles in insect olfactory processes. The Chinese pine caterpillar moth, Dendrolimus tabulaeformis (Lepidoptera, Lasiocampidae) is a serious economic pest in China, and the pheromones of this species have been identified to monitor their presence. However, the molecular mechanisms by which D. tabulaeformis perceive pheromones and host volatiles remain unknown. In this study, we identified and characterized three new OBPs, including one pheromone binding protein (PBP1) and two general odor binding proteins (GOBPs), from antennal cDNA of D. tabulaeformis. The deduced amino acid sequences of DtabPBP1, DtabGOBP1, and DtabGOBP2 revealed mature proteins of 140, 147, and 140 amino acids, respectively. Each has six cysteine residues in conserved positions relative to other known OBPs. Amino-acid alignments indicated that the two GOBPs are more conserved (DtabGOBP1 is 52.9–67.4 % identical to orthologs from other Lepidoptera, and DtabGOBP2 is 55.2–81.8 % identical) than the PBP (32.5–46.0 %). Real-time PCR indicated tissue- and sex-specific expression patterns of the three genes. DtabPBP1 was mainly expressed in the antennae of males, whereas female antennae had only 1.09 % the expression in male antennae. Both DtabGOBP1 and DtabGOBP2 were more highly expressed in antennae than in other tissues, while DtabGOBP1 was more abundant in male antennae and DtabGOBP2 in female antennae. In addition, the binding specificities of the three proteins were investigated, and all three OBPs exhibited high binding affinities for the pheromone component (5Z,7E)-5,7-dodecadien-1-yl propionate (Z5,E7-12:OPr). This suggests a role in binding pheromone for GOBPs, as well as PBP1, in D. tabulaeformis.

Identification and tissue distribution of odorant binding protein genes in the beet armyworm, Spodoptera exigua

Odorant binding proteins (OBPs) contribute to the remarkable sensitivity of the insect's olfactory system and play an important role in insect chemical communication. In this study, we identified 11 putative cDNAs encoding OBPs (namely SexiOBP1-11) from the antennal full length cDNA library of the beet armyworm Spodoptera exigua (Lepidoptera: Noctuidae) and examined their expression profiles in different adult body tissues (antennae, heads, thoraxes, abdomens, legs and wings) by real-time quantitative PCR (qPCR). All SexiOBPs had the characteristic typical features of the OBP family, with the exception of SexiOBP11, which lacked the predicted signal peptide sequence at the N-terminus. qPCR revealed that all of these genes were highly transcribed in the antennae. SexiOBP1-4 and SexiOBP10 were dominantly restricted to antennae. Within antennae, SexiOBP2-4 and SexiOBP10 exhibited female-biased expression patterns, while the expression of SexiOBP7 was male-biased, indicating that they might be involved in interacting with sex pheromones. In general, these OBPs were mainly expressed in chemosensory-specific tissues, although some displayed non-chemosensory or ubiquitous tissue expression. The data is helpful for further determining the potential physiological functions of S. exigua OBPs, and paves the way towards a better understanding of the chemosensory perception of this pest, which may help to uncover new targets for behavioral interference used as a control strategy.

Kinetics of olfactory responses might largely depend on the odorant-receptor interaction and the odorant deactivation postulated for flux detectors

Experimental data together with modeling of pheromone perireceptor and receptor events in moths (Bombyx mori, Antheraea polyphemus) suggest that the kinetics of olfactory receptor potentials largely depend on the association of the odorant with the neuronal receptor molecules and the deactivation of the odorant accumulated around the receptor neuron. The first process could be responsible for the reaction times (mean about 400 ms) of the nerve impulses at threshold. The second process has been postulated for flux detectors such as olfactory sensilla of moths. The odorant deactivation could involve a modification of the pheromone-binding protein (PBP) that "locks" the pheromone inside the inner binding cavity of the protein. The model combines seemingly contradictory functions of the PBP such as pheromone transport, protection of the pheromone from enzymatic degradation, pheromone deactivation, and pheromone-receptor interaction. Model calculations reveal a density of at least 6,000 receptor molecules per µm(2) of neuronal membrane. The volatile decanoyl-thio-1,1,1-trifluoropropanone specifically blocks pheromone receptor neurons, probably when bound to the PBP and by competitive binding to the receptor molecules. The shallow dose-response curve of the receptor potential and altered response properties observed with pheromone derivatives or after adaptation may indicate shortened opening of ion channels.

Functional differentiation of pheromone-binding proteins in the common cutworm Spodoptera litura

Pheromone-binding proteins (PBPs), a sub-family of odorant-binding proteins, are thought primarily to bind and transport the sex pheromones in moths. Considering multiple components of sex pheromone and multiple PBP genes exist in a single species, PBPs may contribute to the discrimination of different sex pheromone components. However, so far this discrimination is still unclear. Our previous ligand-binding assays showed that Spodoptera litura PBP1 (SlitPBP1) did not exhibit an obvious binding specificity among different sex pheromone components. In this study, binding specificity of the other two PBPs in S. litura (SlitPBP2 and SlitPBP3) was further investigated. As a result, SlitPBP2 was capable of binding all four sex pheromone components with similar affinities; whereas SlitPBP3 showed very weak binding affinities to them except Z9,E12-14:Ac. Similar results were also obtained from studied pheromone analogs, to which SlitPBP2 showed much stronger affinities than SlitPBP3. However, both SlitPBP2 and SlitPBP3 exhibited overall weaker affinities to sex pheromones and their analogs than SlitPBP1. In addition, quantitative real time PCR showed that three SlitPBP genes exhibited a very different sex-biased expression in adult antenna with male-biased for SlitPBP1 and SlitPBP2 while female-biased for SlitPBP3. Finally, ligand-binding assays indicated that the two SlitPBPs showed a similar pH-dependent conformational change as reported SlitPBP1, but these three SlitPBPs showed different behavior across a pH range or something similar. Taken together, our data suggest that in S. litura PBP1 and PBP2 may play critical roles in the perception of female sex pheromones, but do not show an obvious discriminative ability among different sex pheromone components; whereas PBP3 may have other functions.

Quantitative analysis of pheromone-binding protein specificity

Many pheromones have very low water solubility, posing experimental difficulties for quantitative binding measurements. A new method is presented for determining thermodynamically valid dissociation constants for ligands binding to pheromone-binding proteins, using β-cyclodextrin as a solubilizer and transfer agent. The method is applied to LUSH, a Drosophila odorant-binding protein that binds the pheromone 11-cis vaccenyl acetate (cVA). Refolding of LUSH expressed in Escherichia coli was assessed by measuring N-phenyl-1-naphthylamine (NPN) binding and Förster resonance energy transfer between LUSH tryptophan 123 (W123) and NPN. Binding of cVA was measured from quenching of W123 fluorescence as a function of cVA concentration. The equilibrium constant for transfer of cVA between β-cyclodextrin and LUSH was determined from a linked equilibria model. This constant, multiplied by the β-cyclodextrin-cVA dissociation constant, gives the LUSH-cVA dissociation constant: ∼100 nM. It was also found that other ligands quench W123 fluorescence. The LUSH-ligand dissociation constants were determined to be ∼200 nM for the silk moth pheromone bombykol and ∼90 nM for methyl oleate. The results indicate that the ligand-binding cavity of LUSH can accommodate a variety ligands with strong binding interactions. Implications of this for the Laughlin, Ha, Jones and Smith model of pheromone reception are discussed.

Expression patterns and binding properties of three pheromone binding proteins in the diamondback moth, Plutella xyllotella

Pheromone binding proteins (PBPs) play a key role in transporting hydrophobic sex pheromone components emitted by con-specific female across aqueous sensillar lymph to the surface of olfactory receptor neurons. A number of PBPs have been cloned, however, details of their function are still largely unknown. Here three pheromone binding protein genes in the diamondback moth, Plutella xyllotella were cloned. The three PxylPBP genes are not only expressed in chemosensory tissues but also expressed in female reproductive organs and male legs. To better understand the functions of PxylPBPs in the initial steps of pheromone recognition, three PxylPBPs were expressed in Escherichia coli and the ligand-binding specificities of purified recombinant PBPs were investigated. Fluorescence binding assays indicate that three PxylPBPs not only robustly bound all four sex pheromone components but also significantly bound pheromone analogs with at least one double bond, while weakly bound tested plant volatiles. Although pheromone analogs bound PBPs, they could not elicit the moth's electrophysiological response. These experiments provide evidence that PxylPBPs have limited selectivity of pheromone components and analogs and some downstream components such as odor receptors might be involved in selectivity and specificity of pheromone perception in P. xyllotella.

Molecular characterization, expression patterns, and ligand-binding properties of two odorant-binding protein genes from Orthaga achatina (Butler) (Lepidoptera: Pyralidae)

It is postulated that insect pheromone-binding proteins (PBPs) are involved in sex pheromone reception, while the general odorant-binding proteins (GOBPs) are involved in reception of the general odorants including plant volatiles. However, this functional specificity is not completely conclusive. In the present study, full-length sequences of two new OBP genes were molecularly identified as OachPBP1 and OachGOBP2 from Orthaga achatina, an important pest of the camphor tree Cinnamomum camphora. Quantification of transcript levels by qRT-PCR showed that the two genes highly expressed in antennae, with OachPBP1 male-biased and OachGOBP2 similar between sexes. These expression patterns are consistent with the generally proposed functions of PBPs and GOBPs. With the recombinant proteins obtained by a bacterial expression system, the binding specificity of these proteins was further investigated and compared using the competitive binding assay. OachPBP1 exhibited high binding affinities with all three putative sex pheromones and 10 pheromone analogs, supporting its role in pheromone reception. On the other hand, in addition to binding with some plant volatiles, OachGOBP2 surprisingly displayed similar or even higher binding affinities with the sex pheromones than OachPBP1. Therefore, we propose that OachGOBP2 might play roles in reception of sex pheromone. Additionally, plant volatiles farnesol and farnesene showed high binding with both OachGOBP2 and OachPBP1, suggesting that these volatile chemicals have regulatory functions in the behavior of O. achatina.

Characterization of three pheromone-binding proteins (PBPs) of Helicoverpa armigera (Hübner) and their binding properties

Three pheromone-binding proteins of Helicoverpa armigera were cloned and expressed in Escherichia coli. In order to characterize their physiological properties, ligand-binding experiments were performed using five biologically relevant substances including sex pheromones and interspecific signals. The results showed that one of the pheromone-binding proteins, HarmPBP1, binds strongly to each of the two principal pheromone components of H. armigera, (Z)-11-tetradecenal and (Z)-9-hexadecenal, but not to the interspecific signal (Z)-9-tetracecenal. The two remaining pheromone-binding proteins, HarmPBP2 and HarmPBP3, showed only weak affinities with the ligands tested. The 3-D structure of HarmPBP1 was predicted and the docking experiments indicate that the key binding site of (Z)-9-hexadecenal to HarmPBP1 includes Thr112, Lys111, and Phe119 whereas that of (Z)-11-tetradecenal includes Ser9, Trp37, Phe36, and Phe119.

Constant pH molecular dynamics (CpHMD) and mutation studies: insights into AaegOBP1 pH-induced ligand releasing mechanism

AaegOBP1, isolated from the male and female antenna of yellow fever mosquitoes, may serve as crucial molecular targets for the development of mosquitoes' attractants and for the control of mosquito populations. Nowadays crystal structures of AaegOBP1 in the neutral environment have been obtained, whereas little research is focused on the conformational change of AaegOBP1 in the acid solution. In our study, the conformational change and the ligand bound poses in different solution pH were investigated using constant pH molecular dynamics (CpHMD) as well as mutation studies. Results demonstrate that the protein changes dramatically in low pH solution and that the pH-sensing triad (Arg23-Tyr54-Ile125) plays an indispensable role in the structural stability and ligand binding. In addition, we have proved that the residue Arg23 is the most important one of the pH-sensing triad. This work could provide more penetrating understanding of the pH-induced ligand-releasing mechanism.

Anopheles gambiae odorant binding protein crystal complex with the synthetic repellent DEET: implications for structure-based design of novel mosquito repellents

Insect odorant binding proteins (OBPs) are the first components of the olfactory system to encounter and bind attractant and repellent odors emanating from various sources for presentation to olfactory receptors, which trigger relevant signal transduction cascades culminating in specific physiological and behavioral responses. For disease vectors, particularly hematophagous mosquitoes, repellents represent important defenses against parasitic diseases because they effect a reduction in the rate of contact between the vectors and humans. OBPs are targets for structure-based rational approaches for the discovery of new repellent or other olfaction inhibitory compounds with desirable features. Thus, a study was conducted to characterize the high resolution crystal structure of an OBP of Anopheles gambiae, the African malaria mosquito vector, in complex with N,N-diethyl-m-toluamide (DEET), one of the most effective repellents that has been in worldwide use for six decades. We found that DEET binds at the edge of a long hydrophobic tunnel by exploiting numerous non-polar interactions and one hydrogen bond, which is perceived to be critical for DEET's recognition. Based on the experimentally determined affinity of AgamOBP1 for DEET (K (d) of 31.3 μΜ) and our structural data, we modeled the interactions for this protein with 29 promising leads reported in the literature to have significant repellent activities, and carried out fluorescence binding studies with four highly ranked ligands. Our experimental results confirmed the modeling predictions indicating that structure-based modeling could facilitate the design of novel repellents with enhanced binding affinity and selectivity.

Binding properties of pheromone-binding protein 1 from the common cutworm Spodoptera litura

Pheromone-binding proteins (PBPs) were formerly thought to act as passive pheromone carriers. However, recent studies, particularly in Drosophila melanogaster, suggest that PBPs are involved in the recognition of semiochemicals, thus making ligand-binding studies more meaningful. Previously, we cloned three PBPs from Spodoptera litura (Slit), and showed that SlitPBP1 is much more abundant than the other two, particularly in male antennae. To investigate the ligand specificity of SlitPBP1, we expressed the protein in a bacterial system and performed binding experiments with the three components of the specific sex pheromones (Z9-14:Ac, Z9,E11-14:Ac and Z9,E12-14:Ac), as well as with 26 volatile ligands. The results indicated that SlitPBP1 bound all three sex pheromone components with dissociation constants between 0.6 and 1.1 μM. The same protein also bound with comparable affinities several pheromone analogs, but not plant volatiles. The presence of a double bond was the most important element for a strong binding, while its position and configuration also affected the affinity. Finally, the binding of pheromone components is strongly affected by pH, showing a critical pH value corresponding to isoelectric point of the protein. This suggests that a pH-dependent conformational mechanism might exist in SlitPBP1 for pheromone binding and release.

Asian corn borer pheromone binding protein 3, a candidate for evolving specificity to the 12-tetradecenyl acetate sex pheromone

Most moth species in the genus Ostrinia use varying ratios of (Z)-11- and (E)-11-tetradecenyl acetate as their main sex pheromone components. The Asian corn borer is unique within the genus having evolved to use pheromone components with a shift in the location of the double bond, (Z)-12- and (E)-12-tetradecenyl acetate. We identified cDNAs representing five pheromone binding proteins (PBPs) and two sensory neuron membrane protein genes from an antennal transcriptome. The coding regions of the orthologous genes were cloned from the Asian corn borer and the (E) and (Z) sex pheromone races of the European corn borer. Their nucleotide sequences and transcript expression levels were analyzed to identify candidate genes from the Asian corn borer that may have evolved specificity to the 12-tetradecenyl acetate ligand. PBP2 and PBP3 transcripts were expressed at high male-biased levels. PBP3 had the most nonsynonymous nucleotide substitutions resulting in ten amino acid changes. Based on the predicted three-dimensional structure of PBP3, six of these ten amino acid changes occur in domains that may interact with the pheromone ligand. Future studies will determine whether PBP3 has evolved specificity to the Asian corn borer sex pheromone.

Effects of aromatic compounds on antennal responses and on the pheromone-binding proteins of the gypsy moth (Lymantria dispar)

Female gypsy moths emit a pheromone, (+)-disparlure, which the males follow until they locate the emitter. The male moths' antennae are covered with innervated sensory hairs, specialized in detection of the pheromone. The neurons in these sensory hairs are bathed by a solution rich in pheromone-binding protein (PBP). PBPs are soluble proteins that bind the pheromone and other odorants reversibly with variable thermodynamic and kinetic selectivity and are essential for olfactory responses. Here, we have studied the interaction between 2 gypsy moth PBPs with aromatic compounds that modulate the responses of male moth antennae to (+)-disparlure. The aromatic compounds do not elicit responses by themselves, but when administered together with pheromone, they inhibit, enhance, or prolong the electrophysiological response to the pheromone. Three interactions between the compounds and PBPs were studied: 1) the equilibrium binding of the compounds by themselves to the PBPs, 2) the equilibrium binding of the compounds in the presence of pheromone or a fluorescent reporter ligand, and 3) the effect of the compounds on the conformation of the pheromone-PBP complex. A subset of compounds causes a prolongation of the electroantennogram response, and from this study, we conclude that these compounds follow a structure-activity pattern and stabilize a particular conformer of the PBPs that appears to activate the olfactory response.

Characterisations of odorant-binding proteins in the tsetse fly Glossina morsitans morsitans

Odorant-binding proteins (OBPs) play an important role in insect olfaction by mediating interactions between odorants and odorant receptors. We report for the first time 20 OBP genes in the tsetse fly Glossina morsitans morsitans. qRT-PCR revealed that 8 of these genes were highly transcribed in the antennae. The transcription of these genes in the antennae was significantly lower in males than in females and there was a clear correlation between OBP gene transcription and feeding status. Starvation over 72 h post-blood meal (PBM) did not significantly affect the transcription. However, the transcription in the antennae of 10-week-old flies was much higher than in 3-day-old flies at 48 h PBM and decreased sharply after 72 h starvation, suggesting that the OBP gene expression is affected by the insect's nutritional status. Sequence comparisons with OBPs of other Dipterans identified several homologs to sex pheromone-binding proteins and OBPs of Drosophila melanogaster.

Odorant-binding proteins in insects

Our understanding of the molecular and biochemical mechanisms that mediate chemoreception in insects has been greatly improved after the discovery of olfactory and taste receptor proteins. However, after 50 years of the discovery of first insect sex pheromone from the silkmoth Bombyx mori, it is still unclear how hydrophobic compounds reach the dendrites of sensory neurons in vivo across aqueous space and interact with the sensory receptors. The presence of soluble polypeptides in high concentration in the lymph of chemosensilla still poses unanswered questions. More than two decades after their discovery and despite the wealth of structural and biochemical information available, the physiological function of odorant-binding proteins (OBPs) is not well understood. Here, I review the structural properties of different subclasses of insect OBPs and their binding to pheromones and other small ligands. Finally, I discuss current ideas and models on the role of such proteins in insect chemoreception.

A receptor and binding protein interplay in the detection of a distinct pheromone component in the silkmoth Antheraea polyphemus

Male moths respond to conspecific female-released pheromones with remarkable sensitivity and specificity, due to highly specialized chemosensory neurons in their antennae. In Antheraea silkmoths, three types of sensory neurons have been described, each responsive to one of three pheromone components. Since also three different pheromone binding proteins (PBPs) have been identified, the antenna of Antheraea seems to provide a unique model system for detailed analyzes of the interplay between the various elements underlying pheromone reception. Efforts to identify pheromone receptors of Antheraea polyphemus have led to the identification of a candidate pheromone receptor (ApolOR1). This receptor was found predominantly expressed in male antennae, specifically in neurons located beneath pheromone-sensitive sensilla trichodea. The ApolOR1-expressing cells were found to be surrounded by supporting cells co-expressing all three ApolPBPs. The response spectrum of ApolOR1 was assessed by means of calcium imaging using HEK293-cells stably expressing the receptor. It was found that at nanomolar concentrations ApolOR1-cells responded to all three pheromones when the compounds were solubilized by DMSO and also when DMSO was substituted by one of the three PBPs. However, at picomolar concentrations, cells responded only in the presence of the subtype ApolPBP2 and the pheromone (E,Z)-6,11-hexadecadienal. These results are indicative of a specific interplay of a distinct pheromone component with an appropriate binding protein and its related receptor subtype, which may be considered as basis for the remarkable sensitivity and specificity of the pheromone detection system.

Molecular characterization and expression pattern of two general odorant binding proteins from the diamondback moth, Plutella xylostella

In the Lepidoptera, odorant signals are thought to be mediated by general odorant binding proteins (GOBPs) in the sensillar lymph surrounding the olfactory receptors. We describe the identification and characterization of two new cDNAs encoding GOBPs from the antennae of the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), a species for which no GOBPs have been identified to date. We focused our investigation on this olfactory protein family by using reverse transcription-polymerase chain reaction strategies. The deduced amino acid sequences of PxylGOBP1 and PxylGOBP2 revealed open reading frames of 168 and 163 amino acids, respectively, with six cysteine residues in conserved positions relative to other known GOBPs. The alignment of the mature PxylGOBPs with other Lepidoptera GOBPs showed high sequence identity (70-80%) with other full-length sequences from GenBank. Sequence identity between PxylGOBP1and PxylGOBP2 was only 50%, suggesting that the two proteins belong to different classes of lepidopteran GOBPs. The expression patterns of the two PxylGOBP genes, with respect to tissue distribution and sex, were further investigated by reverse transcription-polymerase chain reaction (RT-PCR) and real-time PCR. Although the two GOBP genes were expressed only in the antennae of both sexes, reflecting the antennal specificity of GOBPs, the transcription levels of these genes depended on the sex, the age, the mating status, and the genes.

Olfactory perireceptor and receptor events in moths: a kinetic model revised

Modelling reveals that within about 3 ms after entering the sensillum lymph, 17% of total pheromone is enzymatically degraded while 83% is bound to the pheromone-binding protein (PBP) and thereby largely protected from enzymatic degradation. The latter proceeds within minutes, 20,000-fold more slowly than with the free pheromone. In vivo the complex pheromone-PBP interacts with the receptor molecule. At weak stimulation the half-life of the active complex is 0.8 s due to the postulated pheromone deactivation. Most likely this process is enzymatically catalysed; it changes the PBP into a scavenger form, possibly by interference with the C-terminus. The indirectly determined PBP concentration (3.8 mM) is close to direct measurements. The calculated density of receptor molecules within the plasma membrane of the receptor neuron reaches up to 6,000 units per mum(2). This is compared with the estimated densities of the sensory-neuron membrane protein and of ion channels. The EC(50) of the model pheromone-PBP complex interacting with the receptor molecules is 6.8 muM, as compared with the EC(50) = 1.5 muM of bombykol recently determined using heterologous expression. A possible mechanism widening the range of stimulus intensities covered by the dose-response curve of the receptor-potential is proposed.

Ligand binding turns moth pheromone-binding protein into a pH sensor: effect on the Antheraea polyphemus PBP1 conformation

In moths, pheromone-binding proteins (PBPs) are responsible for the transport of the hydrophobic pheromones to the membrane-bound receptors across the aqueous sensillar lymph. We report here that recombinant Antheraea polyphemus PBP1 (ApolPBP1) picks up hydrophobic molecule(s) endogenous to the Escherichia coli expression host that keeps the protein in the "open" (bound) conformation at high pH but switches to the "closed" (free) conformation at low pH. This finding has bearing on the solution structures of undelipidated lepidopteran moth PBPs determined thus far. Picking up a hydrophobic molecule from the host expression system could be a common feature for lipid-binding proteins. Thus, delipidation is critical for bacterially expressed lipid-binding proteins. We have shown for the first time that the delipidated ApolPBP1 exists primarily in the closed form at all pH levels. Thus, current views on the pH-induced conformational switch of PBPs hold true only for the ligand-bound open conformation of the protein. Binding of various ligands to delipidated ApolPBP1 studied by solution NMR revealed that the protein in the closed conformation switches to the open conformation only at or above pH 6.0 with a protein to ligand stoichiometry of approximately 1:1. Mutation of His(70) and His(95) to alanine drives the equilibrium toward the open conformation even at low pH for the ligand-bound protein by eliminating the histidine-dependent pH-induced conformational switch. Thus, the delipidated double mutant can bind ligand even at low pH in contrast to the wild type protein as revealed by fluorescence competitive displacement assay using 1-aminoanthracene and solution NMR.

Binding specificity of locust odorant binding protein and its key binding site for initial recognition of alcohols

Odorant binding proteins (OBPs) are required for olfaction perception, and thus may be possible targets for controlling the population of pests by interfering with their chemical communication. A single OBP LmigOBP1 has been identified in the antennae of Locusta migratoria, though four isoforms have been detected. Here, we have investigated the ligand-binding specificity of LmigOBP1 using 67 volatile odor compounds. Fluorescence assays indicate that LmigOBP1 does not bind fecal volatiles or green leaf odors, but shows high affinity for some linear aliphatic compounds, with pentadecanol and 2-pentadecanone being the strongest binding ligands. A 3-dimensional (3D) model of LmigOBP1 was built by homology modeling. Docking simulations based on this model suggested that Asn74 of LmigOBP1 is a key binding site, and this was validated by site-directed mutagenesis and fluorescence assays. We suggest that, as a general rule, a hydrophilic amino acid at the entrance of the binding cavity participates in initial recognition of ligands, and contributes to ligand-binding specificity of OBPs.

Ligand-interaction kinetics of the pheromone- binding protein from the gypsy moth, L. dispar: insights into the mechanism of binding and release

The pheromone-binding proteins (PBPs), which exist at a high concentration in the sensillum lymph surrounding olfactory neurons, are proposed to be important in pheromone detection and discrimination in insects. Here, we present a systematic study of PBP-ligand interaction kinetics. We find that PBP2, from the gypsy moth, Lymantria dispar, associates and dissociates slowly with its biofunctional ligands, (+)- and (-)-disparlure. Tryptophan anisotropy measurements detect PBP multimers in solution as well as an increase in the multimeric state of the protein upon long exposure to ligand. We propose a kinetic model that includes monomer/multimer equilibria and a two-step binding process: (1) external binding of the pheromone assisted by the C terminus of PBP2, and (2) slow embedding of the pheromone into the internal pocket. This experimentally derived model sheds light on the potential biological function and mechanism of PBPs as ligand scavengers.

Cloning, expression and functional analysis of a general odorant-binding protein 2 gene of the rice striped stem borer, Chilo suppressalis (Walker) (Lepidoptera: Pyralidae)

A full-length cDNA encoding a general odorant binding protein 2 (GOBP2) was cloned from the antennae of the rice striped stem borer, Chilo suppressalis (Lepidoptera: Pyralidae), by the combination of reverse transcription PCR (RT-PCR) and rapid amplification of cDNA ends PCR (RACE-PCR). The cDNA contains a 489 bp open reading frame, which encodes a 162 amino acid protein, termed as Ch. suppressalis GOBP2 (CsupGOBP2). CsupGOBP2 is similar in the number of amino acids and protein sequence to GOBP2s in other species of Lepidoptera. RT-PCR results showed that CsupGOBP2 mRNA was highly expressed in the adult antennae of both females and males, as was CsupGOBP2 protein as revealed by Western blot analysis. CsupGOBP2 expressed in Escherichia coli was purified by affinity chromatography, refolding and gel filtration from the inclusion body. Fluorescence emission spectra and competitive binding assays by using N-phenyl-1-naphthylamine as first binding ligand and odorants as potential competitors revealed that the CsupGOBP2 protein has significant affinity to cis-11-hexadecenal (Z11-16:Ald), the main component of Ch. suppressalis pheromone and to laurinaldehyd and benzaldehyde, two general plant volatile aldehydes.

The major antennal chemosensory protein of red imported fire ant workers

Some chemosensory proteins (CSPs) are expressed in insect sensory appendages and are thought to be involved in chemical signalling by ants. We identified 14 unique CSP sequences in expressed sequence tag (EST) libraries of the red imported fire ant, Solenopsis invicta. One member of this group (Si-CSP1) is highly expressed in worker antennae, suggesting an olfactory function. A shotgun proteomic analysis of antennal proteins confirmed the high level of Si-CSP1 expression, and also showed expression of another CSP and two odorant-binding proteins (OBPs). We cloned and expressed the coding sequence for Si-CSP1. We used cyclodextrins as solubilizers to investigate ligand binding. Fire ant cuticular lipids strongly inhibited Si-CSP1 binding to the fluorescent dye N-phenyl-naphthylamine, suggesting cuticular substances are ligands for Si-CSP1. Analysis of the cuticular lipids showed that the endogenous ligands of Si-CSP1 are not cuticular hydrocarbons.

Queen bee pheromone binding protein pH-induced domain swapping favors pheromone release

In honeybee (Apis mellifera) societies, the queen controls the development and the caste status of the members of the hive. Queen bees secrete pheromonal blends comprising 10 or more major and minor components, mainly hydrophobic. The major component, 9-keto-2(E)-decenoic acid (9-ODA), acts on the workers and male bees (drones), eliciting social or sexual responses. 9-ODA is captured in the antennal lymph and transported to the pheromone receptor(s) in the sensory neuron membranes by pheromone binding proteins (PBPs). A key issue is to understand how the pheromone, once tightly bound to its PBP, is released to activate the receptor. We report here on the structure at physiological pH of the main antennal PBP, ASP1, identified in workers and male honeybees, in its apo or complexed form, particularly with the main component of the queen mandibular pheromonal mixture (9-ODA). Contrary to the ASP1 structure at low pH, the ASP1 structure at pH 7.0 is a domain-swapped dimer with one or two ligands per monomer. This dimerization is disrupted by a unique residue mutation since Asp35 Asn and Asp35 Ala mutants remain monomeric at pH 7.0, as does native ASP1 at pH 4.0. Asp35 is conserved in only approximately 30% of medium-chain PBPs and is replaced by other residues, such as Asn, Ala and Ser, among others, thus excluding that they may perform domain swapping. Therefore, these different medium-chain PBPs, as well as PBPs from moths, very likely exhibit different mechanisms of ligand release or receptor recognition.

Multifunctionality and mechanism of ligand binding in a mosquito antiinflammatory protein

The mosquito D7 salivary proteins are encoded by a multigene family related to the arthropod odorant-binding protein (OBP) superfamily. Forms having either one or two OBP domains are found in mosquito saliva. Four single-domain and one two-domain D7 proteins from Anopheles gambiae and Aedes aegypti (AeD7), respectively, were shown to bind biogenic amines with high affinity and with a stoichiometry of one ligand per protein molecule. Sequence comparisons indicated that only the C-terminal domain of AeD7 is homologous to the single-domain proteins from A. gambiae, suggesting that the N-terminal domain may bind a different class of ligands. Here, we describe the 3D structure of AeD7 and examine the ligand-binding characteristics of the N- and C-terminal domains. Isothermal titration calorimetry and ligand complex crystal structures show that the N-terminal domain binds cysteinyl leukotrienes (cysLTs) with high affinities (50-60 nM) whereas the C-terminal domain binds biogenic amines. The lipid chain of the cysLT binds in a hydrophobic pocket of the N-terminal domain, whereas binding of norepinephrine leads to an ordering of the C-terminal portion of the C-terminal domain into an alpha-helix that, along with rotations of Arg-176 and Glu-268 side chains, acts to bury the bound ligand.