Proteins that smell: pheromone recognition and signal transduction

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.

Identification of Chemosensory Genes, Including Candidate Pheromone Receptors, in Phauda flammans (Walker) (Lepidoptera: Phaudidae) Through Transcriptomic Analyses

Olfactory and gustatory systems play an irreplaceable role in all cycles of growth of insects, such as host location, mating, and oviposition. Many chemosensory genes in many nocturnal moths have been identified via omics technology, but knowledge of these genes in diurnal moths is lacking. In our recent studies, we reported two sex pheromone compounds and three host plant volatiles that play a vital role in attracting the diurnal moth, Phauda flammans. The antennal full-length transcriptome sequence of P. flammans was obtained using the Pacbio sequencing to further explore the process of sex pheromone and host plant volatile recognition in P. flammans. Transcriptome analysis identified 166 candidate olfactory and gustatory genes, including 58 odorant-binding proteins (OBPs), 19 chemosensory proteins (CSPs), 59 olfactory receptors (ORs), 16 ionotropic receptors (IRs), 14 gustatory receptors (GRs), and 2 sensory neuron membrane proteins (SNMPs). Subsequently, a phylogenetic tree was established using P. flammans and other lepidopteran species to investigate orthologs. Among the 17 candidate pheromone receptor (PR) genes, the expression levels of PflaOR21, PflaOR25, PflaOR35, PflaOR40, PflaOR41, PflaOR42, PflaOR44, PflaOR49, PflaOR51, PflaOR61, and PflaOR63 in the antennae were significantly higher than those in other non-antennae tissues. Among these PR genes, PflaOR21, PflaOR27, PflaOR29, PflaOR35, PflaOR37, PflaOR40, PflaOR42, PflaOR44, PflaOR60, and PflaOR62 showed male-biased expression, whereas PflaOR49, PflaOR61, and PflaOR63 revealed female-biased expression. The functions of related OR genes were also discussed. This research filled the gap of the chemosensory genes of P. flammans and provided basic data for future functional molecular mechanisms studies on P. flammans olfaction.

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

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

Transcriptome Characterization and Expression Analysis of Chemosensory Genes in Chilo sacchariphagus (Lepidoptera Crambidae), a Key Pest of Sugarcane

Insect chemoreception involves many families of genes, including odourant/pheromone binding proteins (OBP/PBPs), chemosensory proteins (CSPs), odourant receptors (ORs), ionotropic receptors (IRs), and sensory neuron membrane proteins (SNMPs), which play irreplaceable roles in mediating insect behaviors such as host location, foraging, mating, oviposition, and avoidance of danger. However, little is known about the molecular mechanism of olfactory reception in Chilo sacchariphagus, which is a major pest of sugarcane. A set of 72 candidate chemosensory genes, including 31 OBPs/PBPs, 15 CSPs, 11 ORs, 13 IRs, and two SNMPs, were identified in four transcriptomes from different tissues and genders of C. sacchariphagus. Phylogenetic analysis was conducted on gene families and paralogs from other model insect species. Quantitative real-time PCR (qRT-PCR) showed that most of these chemosensory genes exhibited antennae-biased expression, but some had high expression in bodies. Most of the identified chemosensory genes were likely involved in chemoreception. This study provides a molecular foundation for the function of chemosensory proteins, and an opportunity for understanding how C. sacchariphagus behaviors are mediated via chemical cues. This research might facilitate the discovery of novel strategies for pest management in agricultural ecosystems.

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.

Two Sympatric Spodoptera Species Could Mutually Recognize Sex Pheromone Components for Behavioral Isolation

Spodoptera exigua and S. litura are two sympatric species in China and many other countries. Both moths employ a multiple component sex pheromone blend, including a common component Z9,E12-14:OAc, and two specific components Z9-14:OH and Z11-16:OAc for S. exigua, and one specific component Z9,E11-14:OAc for S. litura. For the two species, it has been well documented that males are able to recognize and behaviorally attracted by their species-specific sex pheromone, which functions as a means of reproductive isolation, but whether males could mutually recognize pheromone components of its sympatric species is unknown. In the present study, the electroantennogram (EAG) and field evaluation were conducted to address this topic. The EAG recordings revealed that males of each species could significantly respond to specific components of its sympatric species, although the response values were lower than that to its own major component. In field tests, the specific components Z9-14:OH and Z11-16:OAc of S. exigua strongly inhibited the male catches of S. litura to its conspecific sex pheromone, while specific component Z9,E11-14:OAc of S. litura significantly reduced the male catches of S. exigua to its sex pheromone. Furthermore, the combined lure of the two species completely inhibited male catches of S. litura, and significantly decreased the male catches of S. exigua, compared to the species-specific lure alone. The results demonstrated that males of the two sibling species could perceive the specific components of its counterpart, suggesting that mutual recognition of pheromone components may function to strengthen the behavioral isolation between the two species. Our study has added new knowledge to the reproductive isolation via sex pheromone communication system in sympatric moth species, and provided a base for designing of mating disruption tactics targeting multispecies by using insect sex pheromones.

Physicochemical Basis and Comparison of Two Type II Sex Pheromone Components Binding with Pheromone-Binding Protein 2 from Tea Geometrid, Ectropis obliqua

Lepidopteran geometrid moth can produce complex Type II sex pheromone components to attract males and trigger mating behavior. Although several sex pheromone components have been identified, it remains unclear whether their physicochemical roles in sex pheromone sensing are the same. Therefore, we utilized tea geometrid ( Ectropis obliqua) as an example model to investigate and compare the physicochemical basis of two key Type II sex pheromone components, cis-6,7-epoxy-(3Z,9Z)-3,9-octadecadiene ( Z3 Z9-6,7-epo-18:Hy) and ( Z, Z, Z)-3,6,9-octadecatriene (Z3Z6Z9-18:Hy), interacting with pheromone-binding protein 2 ( EoblPBP2) from E. obliqua. Multispectral, thermodynamic, docking, and site-directed mutagenesis indicated that the major sex pheromone component Z3Z9-6,7-epo-18:Hy is more susceptible to pH-tuned than the minor component Z3Z6Z9-18:Hy, whereas Z3Z6Z9-18:Hy seems to be more susceptible to temperature and amino acid mutations than Z3Z9-6,7-epo-18:Hy. Our study suggests that different components of Type II sex pheromone play different binding characters under specific conditions in the physicochemical behavior. This deeply supplements the theoretical knowledge of Type II pheromones involved in the recognition and discrimination in the Lepidopteran sex pheromones family.

Identification and comparative expression analysis of odorant binding protein genes in the tobacco cutworm Spodoptera litura

Insect odorant binding proteins (OBPs) are thought to involve in insects' olfaction perception. In the present study, we identified 38 OBP genes from the antennal transcriptomes of Spodoptera litura. Tissue expression profiles analysis revealed that 17 of the 38 SlitOBP transcripts were uniquely or primarily expressed in the antennae of both sexes, suggesting their putative role in chemoreception. The RPKM value analysis revealed that seven OBPs (SlitPBP1-3, SlitGOBP1-2, SlitOBP3 and SlitOBP5) are highly abundant in male and female antennae. Most S. litura antennal unigenes had high homology with Lepidoptera insects, especially genes of the genus Spodoptera. Phylogenetic analysis of the Lepidoptera OBPs demonstrated that the OBP genes from the genus Spodoptera (S. litura, Spodoptera littoralis and Spodoptera exigua) had a relatively close evolutionary relationship. Some regular patterns and key conserved motifs of OBPs in genus Spodoptera are identified by MEME, and their putative roles in detecting odorants are discussed here. The motif-patterns between Lepidoptera OBPs and CSPs are also compared. The SlitOBPs identified here provide a starting point to facilitate functional studies of insect OBPs at the molecular level both in vivo and in vitro.

Antennal transcriptome analysis and comparison of chemosensory gene families in two closely related noctuidae moths, Helicoverpa armigera and H. assulta

To better understand the olfactory mechanisms in the two lepidopteran pest model species, the Helicoverpa armigera and H. assulta, we conducted transcriptome analysis of the adult antennae using Illumina sequencing technology and compared the chemosensory genes between these two related species. Combined with the chemosensory genes we had identified previously in H. armigera by 454 sequencing, we identified 133 putative chemosensory unigenes in H. armigera including 60 odorant receptors (ORs), 19 ionotropic receptors (IRs), 34 odorant binding proteins (OBPs), 18 chemosensory proteins (CSPs), and 2 sensory neuron membrane proteins (SNMPs). Consistent with these results, 131 putative chemosensory genes including 64 ORs, 19 IRs, 29 OBPs, 17 CSPs, and 2 SNMPs were identified through male and female antennal transcriptome analysis in H. assulta. Reverse Transcription-PCR (RT-PCR) was conducted in H. assulta to examine the accuracy of the assembly and annotation of the transcriptome and the expression profile of these unigenes in different tissues. Most of the ORs, IRs and OBPs were enriched in adult antennae, while almost all the CSPs were expressed in antennae as well as legs. We compared the differences of the chemosensory genes between these two species in detail. Our work will surely provide valuable information for further functional studies of pheromones and host volatile recognition genes in these two related species.

Pheromone reception in moths: from molecules to behaviors

Male moths detect and find their mates using species-specific sex pheromones emitted by conspecific females. Olfaction plays a vital role in this behavior. Since the first discovery of an insect sex pheromone from the silkmoth Bombyx mori, great efforts have been spent on understanding the sensing of the pheromones in vivo. Much progress has been made in elucidating the molecular mechanisms that mediate chemoreception in insects in the past few decades. In this review, we focus on pheromone reception and detection in moths, from the molecular to the behavioral level. We trace the information pathway from the capture of pheromone by male antennae, binding and transportation to olfactory receptor neurons, receptor activation, signal transduction, molecule inactivation, through brain processing and behavioral response. We highlight the impact of recent studies and also provide our insights into pheromone processing.

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.

Sex pheromone recognition and immunolocalization of three pheromone binding proteins in the black cutworm moth Agrotis ipsilon

Insect pheromone binding proteins (PBPs) are believed to solubilize and transport hydrophobic sex pheromones across sensillum lymph to membrane-associated pheromone receptors. To address the molecular mechanisms of PBPs in insect pheromone perception, we undertook a systemic study on the PBPs of the black cutworm Agrotis ipsilon at transcript as well as protein level from tissue distribution and cellular localization to pheromone binding affinity. We cloned three full-length PBP genes AipsPBP1-3 from A. ipsilon antennae, and demonstrated that AipsPBP1-3 transcripts were highly expressed in male antennae. The electron microscopic examinations revealed at least six types of olfactory sensilla on male and female antenna: trichodea, chaetica, basiconica, coeloconica, squamiformia and Böhm bristles. The immunocytochemistry results demonstrated that AipsPBP1-3 proteins were strongly expressed in the sensillum lymph of the trichoid sensilla of male moth. The binding assays showed that AipsPBP1 had high binding affinities with the major sex pheromone components Z7-12:Ac and Z9-14:Ac among five related chemicals and was clustered together with the long trichoid sensilla-expressing LdisPBPs of Lymantria dispar. AipsPBP2 showed high binding affinities also with Z11-16:Ac. AipsPBP3 displayed a high affinity only with Z11-16:Ac. Our studies provide further detail evidences for the involvement of moth PBPs in pheromone discrimination and selective recognition of specific components of the female sex pheromone blends.

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.

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.

Partition coefficients of disparlure at hydrophobic/aqueous interfaces: A comparative experimental and theoretical study

The gypsy moth, Lymantria dispar, uses pheromones, such as (+)-disparlure, for communication. These pheromones are detected by the sensitive antennae of the male moth and travel through their aqueous environment to the sensory neuron, possibly with assistance from pheromone binding proteins (PBPs). The hydrophobicity of disparlure, which is of primary importance for its interactions with PBPs, can be measured by its partition coefficients. These coefficients for heptane/water and chloroform/water interfaces were obtained in this work experimentally and using molecular dynamics (MD) simulations. The agreement between the experimental and calculated values validates the choice of the force field parameters, which can thus be used in future MD simulations of the PBP–pheromone interactions.

Molecular characterization and expression pattern of two pheromone-binding proteins from Spodoptera litura (Fabricius)

Pheromone perception is thought to be mediated by pheromone-binding proteins (PBPs) in the lymph surrounding the olfactory receptors. We cloned and characterized two PBP genes (SlitPBP1 and SlitPBP2) from the common cutworm, Spodoptera litura (F.; Lepidoptera: Noctuidae), which encode PBPs belonging to two different PBP groups. Western blot analysis of the crude antennal extracts with SexigPBP1 antibody revealed a single immunoreactive band (much stronger in male than in female) of approximately 16 kDa, in agreement with the calculated values for SlitPBPs. From genomic DNA, two introns and a similar exon/intron structural pattern were identified in each PBP genes, but the introns differed in length within and between PBP genes. The expression patterns of two SlitPBP genes, with respect to tissue distribution and sex, were further investigated by reverse transcriptase-polymerase chain reaction (RT-PCR) and real-time PCR. Although the two PBP genes were expressed only in the antennae of both sexes, reflecting the antennal specificity of PBPs, the transcription levels of PBP genes differed between the sexes and the genes. The transcription levels of SlitPBP1 and SlitPBP2 in females were only 2.1% and 7.0%, respectively, relative to those in males, and the levels of PBP2 compared with PBP1 were 31.4% and 95.3% in males and females, respectively. These differential expression levels might suggest different roles played by the two SlitPBPs in the perception of sex pheromone both in males and females.

Analysis of odorant-binding proteins in antennae of a geometrid species, Ascotis selenaria cretacea, which produces lepidopteran Type II sex pheromone components

Information on the olfactory system in antennae of Geometridae moths is very limited, and odorant-binding proteins (OBPs) working as transporters of lipophilic odors have not been identified. In the first investigation on this family of insects, we examined antennal OBPs of the Japanese giant looper, Ascotis selenaria cretacea. RT-PCR experiments using several pairs of degenerate primers designed from known cDNA sequences encoding lepidopteran OBPs successfully amplified partial sequences of two pheromone-binding proteins (PBPs), named AscrPBP1 and AscrPBP2 in reference to their corresponding nucleotide sequence homologies with other PBPs. Using 5'- and 3'-rapid amplification of cDNA end strategies, a cDNA clone for AscrPBP1 encoding a protein of 141 amino acids was isolated. Western blotting with the antiserum against recombinant AscrPBP1 overexpressed in Escherichia coli showed that the AscrPBP1 gene was more strongly expressed in male antennae than in female antennae. Furthermore, natural AscrPBP1was isolated by immunoprecipitation with the antiserum, and its binding ability was evaluated by using synthetic sex pheromonal compounds with a C(19) chain. The result indicated that AscrPBP1 bound not only the pheromone components, 3,6,9-nonadecatriene and its 3,4-epoxy derivative, but also unnatural 6,7- and 9,10-epoxy derivatives. While no general odorant-binding proteins (GOBPs) were amplified in the RT-PCR experiments, two antisera prepared from GOBP1 and GOBP2 of Bombyx mori suggested the occurrence of at least two GOBPs in the A. s. cretacea antennae.

Structure and physical properties of biomembranes and model membranes

Biomembranes belong to the most important structures of the cell and the cell organels. They play not only structural role of the barrier separating the external and internal part of the membrane but contain also various functional molecules, like receptors, ionic channels, carriers and enzymes. The cell membrane also preserves non-equillibrium state in a cell which is crucial for maintaining its excitability and other signaling functions. The growing interest to the biomembranes is also due to their unique physical properties. From physical point of view the biomembranes, that are composed of lipid bilayer into which are incorporated integral proteins and on their surface are anchored peripheral proteins and polysaccharides, represent liquid scrystal of smectic type. The biomembranes are characterized by anisotropy of structural and physical properties. The complex structure of biomembranes makes the study of their physical properties rather difficult. Therefore several model systems that mimic the structure of biomembranes were developed. Among them the lipid monolayers at an air-water interphase, bilayer lipid membranes (BLM), supported bilayer lipid membranes (sBLM) and liposomes are most known. This work is focused on the introduction into the "physical word" of the biomembranes and their models. After introduction to the membrane structure and the history of its establishment, the physical properties of the biomembranes and their models areare stepwise presented. The most focus is on the properties of lipid monolayers, BLM, sBLM and liposomes that were most detailed studied. This contribution has tutorial character that may be usefull for undergraduate and graduate students in the area of biophysics, biochemistry, molecular biology and bioengineering, however it contains also original work of the author and his co-worker and PhD students, that may be usefull also for specialists working in the field of biomembranes and model membranes.

Disulfide connectivity and reduction in pheromone-binding proteins of the gypsy moth, Lymantria dispar

Males of the gypsy moth, Lymantria dispar, are attracted by a pheromone released by females. Pheromones are detected by olfactory neurons housed in specialized sensory hairs located on the antennae of the male moth. Once pheromone molecules enter the sensilla lymph, a highly abundant pheromone-binding protein (PBP) transports the molecule to the sensory neuron. The PBPs are members of the insect odorant-binding protein family, with six conserved cysteine residues. In this study, the disulfide bond connectivities of the pheromone-binding proteins PBP1 and PBP2 from the gypsy moth were found to be cysteines 19-54, 50-109, and 97-118 for PBP1, and cysteines 19-54, 50-110, and 97-119 for PBP2, as determined by cyanylation reactions and cyanogen bromide chemical cleavage. We have discovered that the second disulfide linkage is the most easily reduced of the three, and this same linkage is missing among four cysteine-containing insect odorant-binding proteins (OBPs). We are the first to identify the unique steric and electronic properties of this second disulfide linkage.

Cellular calcium mobilization in response to phosphoinositide delivery

Intracellular calcium [Ca(2+)](i) is mobilized in many cell types in response to activation of phosphoinositide (PIP(n)) signaling pathways involving PtdIns(4,5)P(2) or PtdIns(3,4,5)P(3). To further explore the relationship between increases in intracellular PIP(n) concentrations and mobilization of [Ca(2+)](i), each of the seven phosphorylated phosphoinositides (PIP(n)s) were delivered into cells and the metabolism and physiological effects of the exogenously administered PIP(n)s were determined. The efficient cellular delivery of fluorophore-tagged and native PIP(n)s was accomplished using histone protein, neomycin, and dendrimeric polyamines. PtdIns(4,5)P(2) fluorophore-tagged analogs with short- and long-acyl chains were substrates for cellular enzymes in vitro and for phospholipases in stimulated fibroblasts. PtdIns(4)P, PtdIns(3,4)P(2) and PtdIns(4,5)P(2), each induced calcium mobilization rapidly after exogenous addition to fibroblasts. PtdIns(3,4,5)P(3) induced a significant, but smaller increase in intracellular calcium. These observations suggest that PIP(n)s other than PtdIns(4,5)P(2) or PtdIns(3,4,5)P(3) may have direct roles in signaling involving [Ca(2+)](i).

The Influence of Fluorinated Molecules (Semiochemicals and Enzyme Substrate Analogues) on the Insect Communication System

Can the introduction of fluorine atoms affect the bioactivity of natural semiochemicals? Can fluorine contribute in the creation of specific enzyme inhibitors to interrupt or disrupt the insect communication system? The first step for the bioactivity of a molecule is interaction with the biological sensor. Hydrogen and fluorine are almost bioisosteric and the receptor site of the enzyme can still recognize and accept the fluoro analogue of its natural substrate. However, the peculiar electronegativity of the fluorine atom can affect the binding, absorption, and transport of the molecule. The differences in the molecule's electronic properties can lead to differences in the chemical interactions between the receptor and the fluorinated substrate. Fluorine introduction can modify the metabolic stability and pathway of the semiochemicals in many different ways. Fluorinated analogues can show synergism, inhibition, or hyperagonism effects on insect behaviors, that is, the activity of the nonfluorinated parent compounds can be mimicked, lost, or increased. In any case, the fluorinated molecules can interact with the bioreceptors in a new and disrupting way. The semiochemicals are olfactory substances: fluorine can affect their volatility or smell. Production of semiochemicals from exogenous substances, perception at antennal receptors, and processing of biological responses are the main steps of communication among insects. In the production step, the fluorinated molecules can interact with enzymes that catalyze the biosynthesis of the natural pheromones. In the perception step, fluorinated semiochemicals can interact with the olfactory receptor cells; this often leads to totally unpredictable behaviors. Fluorinated molecules have been developed as probes to elucidate the complex chemorecognition processes of insects. Many of these molecules have been tested to find highly effective behavior-modifying chemicals. New analogues have been synthesized to investigate the metabolic pathway of a pheromone molecule and many of them are promising disrupting agents. Despite such titanic research efforts, the results have often been random, rational trends in the induced behaviors have sometimes been impossible to find, and practical applications of the fluorinated semiochemicals are still uncertain.

Pheromone binding proteins of Epiphyas postvittana (Lepidoptera: Tortricidae) are encoded at a single locus

The light brown apple moth, Epiphyas postvittana (Tortricidae: Lepidoptera) uses a blend of (E)-11-tetradecenyl acetate and (E,E)-9,11-tetradecadienyl acetate as its sex pheromone. Odorant binding proteins, abundant in the antennae of male and female E. postvittana, were separated by native PAGE to reveal four major proteins with distinct mobilities. Microsequencing of their N-terminal residues showed that two were general odorant binding proteins (GOBPs) while two were pheromone binding proteins (PBPs). Full length cDNAs encoding these proteins were amplified using a combination of PCR and RACE-PCR. Sequence of the GOBPs revealed two genes (EposGOBP1, EposGOBP2), similar to orthologues in other species of Lepidoptera. Eleven cDNAs of the PBP gene were amplified, cloned and sequenced revealing two major phylogenetic clusters of PBP sequences differing by six amino acid substitutions. The position of the six amino acid differences on the protein was predicted by mapping onto the three-dimensional structure of PBP of Bombyx mori. All six substitutions were predicted to fall on the outside of the protein away from the inner pheromone binding pocket. One substitution does fall close to the putative dimerisation region of the protein (Ser63Thr). Expression of three of the cDNAs in a baculovirus expression system revealed that one class encodes an electrophoretically slow form (EposPBP1-12) while the other encodes a fast form (EposPBP1-2, EposPBP1-3). A native Western of these expressed proteins compared with antennal protein extracts demonstrated that PBP is also expressed in female antennae and that PBP may be present as a dimer as well as a monomer in E. postvittana. The fast and slow forms of EposPBP1 are allelic. Westerns on single antennal pair protein extracts and allele-specific PCR from genomic DNA both show a segregating pattern of inheritance in laboratory and wild populations. Radio labelled (E)-11-tetradecenyl acetate binds to both fast and slow PBP forms in gel assays. Taken together, the genetic and biochemical data do not support the hypothesis that these PBPs are specific for each component of the E. postvittana pheromone. However, duplication of this PBP locus in the future might allow such diversification to evolve, as observed in the other species.

NMR structure reveals intramolecular regulation mechanism for pheromone binding and release

Odorants are transmitted by small hydrophobic molecules that cross the aqueous sensillar lymph surrounding the dendrites of the olfactory neurons to stimulate the olfactory receptors. In insects, the transport of pheromones, which are a special class of odorants, is mediated by pheromone-binding proteins (PBPs), which occur at high concentrations in the sensillar lymph. The PBP from the silk moth Bombyx mori (BmPBP) undergoes a pH-dependent conformational transition between the forms BmPBP(A) present at pH 4.5 and BmPBP(B) present at pH 6.5. Here, we describe the NMR structure of BmPBP(A), which consists of a tightly packed arrangement of seven alpha-helices linked by well defined peptide segments and knitted together by three disulfide bridges. A scaffold of four alpha-helices that forms the ligand binding site in the crystal structure of a BmPBP-pheromone complex is preserved in BmPBP(A). The C-terminal dodecapeptide segment, which is in an extended conformation and located on the protein surface in the pheromone complex, forms a regular helix, alpha(7), which is located in the pheromone-binding site in the core of the unliganded BmPBP(A). Because investigations by others indicate that the pH value near the membrane surface is reduced with respect to the bulk sensillar lymph, the pH-dependent conformational transition of BmPBP suggests a novel physiological mechanism of intramolecular regulation of protein function, with the formation of alpha(7) triggering the release of the pheromone from BmPBP to the membrane-standing receptor.

Pheromones cause disease: pheromone/odourant transduction

This paper compares two models of the sense of smell and demonstrates that the new model has advantages over the accepted model with implications for medical research. The accepted transduction model had an odourant or pheromone contacting an aqueous sensory lymph then movement through it to a receptor membrane beneath. If the odourant or pheromone were non-soluble, the odourant/pheromone supposedly would be bound to a soluble protein in the lymph to be carried across. Thus, an odourant/carrier protein complex physically moved through the receptor lymph/mucus to interact with a membrane bound receptor. After the membranous receptor interaction, the molecule would be deactivated and any odourant/pheromone-binding protein recycled. This new electrical chemosensory model being proposed here has the pheromone or other odourant generating an electrical event in the extra-cellular mucus. Before the pheromone arrives, proteins of the 'carrier class' dissolved in the receptor mucus slowly and continuously sequester ions. A sensed pheromonal chemical species sorbs to the mucus and immediately binds to the now ion-holding dissolved protein. The binding of the pheromone to the protein causes a measurable conformational change in the pheromone/odourant-binding protein, desequestering ions. Releasing the bound ions changes the potential differences across a nearby super-sensitive dendritic membrane resulting in dendrite excitation. Pheromones will be implicated in the aetiology of the infectious, psychiatric and autoimmune diseases. This is the third article in a series of twelve to systematically explore this contention (see references 1-9).

Characterization and cloning of a Tenebrio molitor hemolymph protein with sequence similarity to insect odorant-binding proteins

The yellow mealworm beetle, Tenebrio molitor, produces a number of moderately abundant low molecular weight hemolymph proteins ( approximately 12 kDa) which behave in a similar manner during purification and share antigenic epitopes. The cDNA sequence of the major component (THP12) was determined and the deduced protein sequence was found to be similar to those of insect odorant-binding proteins. Southern blot analysis suggests that at least some of the diversity in this family of proteins is encoded at the gene level. Both northern and western blot analysis indicate that THP12 is present in a variety of developmental stages and both sexes. THP12 was originally classified as an antifreeze protein, but the lack of antifreeze activity in the recombinant protein, as well as the clear separation of the antifreeze activity from THP12 following HPLC purification, has ruled out this function. The abundance of THP12, the similarity of THP12 to insect odorant-binding proteins, and the presence of hydrophobic cavities inside the protein (Rothemund et al., A new class of hexahelical insect proteins revealed as putative carriers of small hydrophobic ligands. Structure, 7 (1999) 1325-1332.) suggest that THP12 may function to carry non-water soluble compounds in the hemolymph. THP12 is also similar, particularly in structurally important regions, to other insect proteins from non-sensory tissues, suggesting the existence of a large family of carrier proteins which may perform diverse functions throughout the insect.

Three pheromone-binding proteins in olfactory sensilla of the two silkmoth species Antheraea polyphemus and Antheraea pernyi

Females of the sibling silkmoth species Antheraea polyphemus and A. pernyi use the same three sex pheromone components in different ratios to attract conspecific males. Accordingly, the sensory hairs on the antennae of males contain three receptor cells sensitive to each of the pheromone components. In agreement with the number of pheromones used, three different pheromone-binding proteins (PBPs) could be identified in pheromone-sensitive hairs of both species by combining biochemical and molecular cloning techniques. MALDI-TOF MS of sensillum lymph droplets from pheromone-sensitive sensilla trichodea of male A. polyphemus revealed the presence of three major peaks with m/z of 15702, 15752 and 15780 and two minor peaks of m/z 15963 and 15983. In Western blots with four antisera raised against different silkmoth odorant-binding proteins, immunoreactivity was found only with an anti-(Apol PBP) serum. Free-flow IEF, ion-exchange chromatography and Western blot analyses revealed at least three anti-(Apol PBP) immunoreactive proteins with pI values between 4.4 and 4.7. N-Terminal sequencing of these three proteins revealed two proteins (Apol PBP1a and Apol PBP1b) identical in the first 49 amino acids to the already known PBP (Apol PBP1) [Raming, K. , Krieger, J. & Breer, H. (1989) FEBS Lett. 256, 2215-2218] and a new PBP having only 57% identity with this amino-acid region. Screening of antennal cDNA libraries with an oligonucleotide probe corresponding to the N-terminal end of the new A. polyphemus PBP, led to the discovery of full length clones encoding this protein in A. polyphemus (Apol PBP3) and in A. pernyi (Aper PBP3). By screening the antennal cDNA library of A. polyphemus with a digoxigenin-labelled A. pernyi PBP2 cDNA [Krieger, J., Raming, K. & Breer, H. (1991) Biochim. Biophys. Acta 1088, 277-284] a homologous PBP (Apol PBP2) was cloned. Binding studies with the two main pheromone components of A. polyphemus and A. pernyi, the (E,Z)-6, 11-hexadecadienyl acetate (AC1) and the (E,Z)-6,11-hexadecadienal (ALD), revealed that in A. polyphemus both Apol PBP1a and the new Apol PBP3 bound the 3H-labelled acetate, whereas no binding of the 3H-labelled aldehyde was found. In A. pernyi two PBPs from sensory hair homogenates showed binding affinity for the AC1 (Aper PBP1) and the ALD (Aper PBP2), respectively.

Protein-assisted pericyclic reactions: an alternate hypothesis for the action of quantal receptors

The rules for allowable pericyclic reactions indicate that the photoisomerizations of retinals in rhodopsins can be formally analogous to thermally promoted Diels-Alder condensations of monoenes with retinols. With little change in the seven-transmembrane helical environment these latter reactions could mimic the retinal isomerization while providing highly sensitive chemical reception. In this way archaic progenitors of G-protein-coupled chemical quantal receptors such as those for pheromones might have been evolutionarily plagiarized from the photon quantal receptor, rhodopsin, or vice versa. We investigated whether the known structure of bacteriorhodopsin exhibited any similarity in its active site with those of the two known antibody catalysts of Diels-Alder reactions and that of the photoactive yellow protein. A remarkable three-dimensional motif of aromatic side chains emerged in all four proteins despite the drastic differences in backbone structure. Molecular orbital calculations supported the possibility of transient pericyclic reactions as part of the isomerization-signal transduction mechanisms in both bacteriorhodopsin and the photoactive yellow protein. It appears that reactions in all four of the proteins investigated may be biological analogs of the organic chemists' chiral auxiliary-aided Diels-Alder reactions. Thus the light receptor and the chemical receptor subfamilies of the heptahelical receptor family may have been unified at one time by underlying pericyclic chemistry.

Purification, structural characterization, cloning and immunocytochemical localization of chemoreception proteins from Schistocerca gregaria

Soluble low-molecular-mass protein isoforms were purified from chemosensory organs (antennae, tarsi and labrum) of the desert locust Schistocerca gregaria. Five genes encoding proteins of this group were amplified by PCR from cDNAs of tarsi and sequenced. Their expression products are polypeptide chains of 109 amino acids showing 40-50% sequence identity with putative olfactory proteins from Drosophila melanogaster and Cactoblastis cactorum. Direct structural investigation on isoforms purified from chemosensory organs revealed the presence in the expression products of two of the genes cloned. Two additional protein isoforms were detected and their molecular structure exhaustively characterized. MS analysis of all isoforms demonstrated that the four cysteine residues conserved in the polypeptide chain were involved in disulfide bridges (Cys29-Cys38 and Cys57-Cys60) and indicated the absence of any additional post-translational modifications. Immunocytochemistry experiments, performed with rabbit antiserum raised against the protein isoform mixture, showed selective labelling of the outer lymph in contact sensilla of tarsi, maxillary palps and antennae. Other types of sensilla were not labelled, nor were the cuticle and dendrites of the sensory cells. No binding of radioactively labelled glucose or bicarbonate was detected, in disagreement with the hypothesis that this class of proteins is involved in the CO2-sensing cascade. Our experimental data suggest that the proteins described here could be involved in contact chemoreception in Orthoptera.

Odorant-binding proteins: expression and function

Odorant-binding proteins (OBPs) are a major constituent of the aqueous perireceptor compartment in vertebrates and in insects. Although different in primary structure, they are supposed to serve similar functions in both animal groups: (i) OBPs may act as solubilizers and carriers of the lipophilic odorants in the aqueous mucus or sensillum lymph; (ii) OBPs may act in addition as peripheral filters in odor discrimination by selectively binding certain classes of odorants; (iii) OBPs may present the stimulus molecule in a particular way to the receptor proteins to facilitate signal transduction; (iv) OBPs may clean the perireceptor space from unwanted and toxic compounds; (v) OBPs may rapidly deactivate odorants after stimulation of the receptors. Experimental evidence in favor of this multiple role of OBPs is reviewed.

Attracted or repelled?--a matter of two neurons, one pheromone binding protein, and a chiral center

Two species of scarab beetles, the Osaka beetle (Anomala osakana) and the Japanese beetle (Popillia japonica), utilize the opposite enantiomers of japonilure, (Z)-5-(1-decenyl)oxacyclopentan-2-one, as their sex pheromones. Each species produces only one of the enantiomers that functions as its own sex pheromone and as a very strong behavioral antagonist for the other species. Using an integrated approach we tested whether the discrimination of these two opposite signals is due to selective filtering by pheromone binding proteins or whether it originates in the specificity of ligand-receptor interactions. We found that the antennae of each of these two scarab species contain only a single pheromone binding protein, which associates with both enantiomers to a similar extent. The single neuron recording technique, on the other hand, showed that both species possess olfactory receptor neurons, colocalized in one sensillum, extremely specific to either (R)- or (S)-japonilure. Therefore, pheromone binding proteins (PBPs) alone cannot perform the task of chiral discrimination; enantiomeric specificity must be achieved by the interaction of the pheromone or the appropriate ligand-PBP complex with membrane receptors.

Enantiomers of cis- and trans-3-(4-propyl-cyclopent-2-enyl) propyl acetate. A study on the bioactive conformation and chiral recognition of a moth sex pheromone component

The enantiomers of cis- and trans-3-(4-propyl-cyclopent-2-enyl) propyl acetate, which are conformationally constrained analogues of (Z)-5-decenyl acetate (1), a sex pheromone component of the turnip moth, Agrotis segetum, have been synthesized and tested using the electrophysiological single-sensillum technique. The analogues mimic a cisoid and transoid conformation of 1, respectively. In addition, the enantiomers of each of the cis- and trans-isomers are conformationally constrained analogues of enantiomeric cisoid and transoid conformations of 1. Thus, the compounds prepared and tested are well suited to investigate the nature of the bioactive conformation of the natural pheromone component 1 and the chiral sense of its interaction with the receptor. Electrophysiological single-sensillum recordings show that the activity of the most active cis-isomer, which has a (1S,4R)-configuration, is more than two orders of magnitude higher than that of the most active trans-isomer. Furthermore, the (1S,4R)-isomer is at least 100 times more active than its enantiomer. These results strongly support a previously proposed cisoid bioactive conformation of 1. Furthermore, the (1S,4R)-configuration of most active stereoisomer identifies the chiral sense of the interaction between the natural pheromone component 1 and its receptor.

When is a butterfly like an elephant?

The behaviours of organisms as diverse as elephants and butterflies are affected by pheromones with identical or similar structures. Recent developments in the molecular biology of pheromone detection suggest why.