Single amino acid residue mediates reciprocal specificity in two mosquito odorant receptors.

The southern house mosquito, Culex quinquefasciatus, utilizes two odorant receptors, CquiOR10 and CquiOR2, narrowly tuned to oviposition attractants and well conserved among mosquito species. They detect skatole and indole, respectively, with reciprocal specificity. We swapped the transmembrane (TM) domains of CquiOR10 and CquiOR2 and identified TM2 as a specificity determinant. With additional mutations, we showed that CquiOR10A73L behaved like CquiOR2. Conversely, CquiOR2L74A recapitulated CquiOR10 specificity. Next, we generated structural models of CquiOR10 and CquiOR10A73L using RoseTTAFold and AlphaFold and docked skatole and indole using RosettaLigand. These modeling studies suggested space-filling constraints around A73. Consistent with this hypothesis, CquiOR10 mutants with a bulkier residue (Ile, Val) were insensitive to skatole and indole, whereas CquiOR10A73G retained the specificity to skatole and showed a more robust response than the wildtype receptor CquiOR10. On the other hand, Leu to Gly mutation of the indole receptor CquiOR2 reverted the specificity to skatole. Lastly, CquiOR10A73L, CquiOR2, and CquiOR2L74I were insensitive to 3-ethylindole, whereas CquiOR2L74A and CquiOR2L74G gained activity. Additionally, CquiOR10A73G gave more robust responses to 3-ethylindole than CquiOR10. Thus, we suggest the specificity of these receptors is mediated by a single amino acid substitution, leading to finely tuned volumetric space to accommodate specific oviposition attractants.

Odorant inhibition in mosquito olfaction mediated by inverse agonists.

The insect repellent methyl salicylate elicits excitatory responses upon interaction with CquiOR32, an odorant receptor (OR) from the southern house mosquito, Culex quinquefasciatus. By contrast, eucalyptol binds to CquiOR32 to generate electrophysiological and behavioral inhibitory responses. In an attempt to identify CquiOR32 variants displaying more robust inhibitory responses for more accurate current-voltage analysis, we sequenced 31 CquiOR32 clones. In the Xenopus oocyte recording system, CquiOR32V2/CquiOrco-expressing oocytes yielded eucalyptol-elicited outward (inhibitory) currents relatively larger than methyl salicylate-generated inward (excitatory) currents. Rescuing experiments showed that two of the amino acid substitutions in CquiOR32V2 located in a predicted transmembrane helix of the receptor are determinants of the outward/inward ratios. These findings, along with co-stimulus assays, suggest that odorant and inhibitor may bind to the same binding pocket. Current-voltage relationships obtained with standard perfusion buffer and those devoid of Na+ or Cl- indicated that both excitatory and inhibitory currents are mediated, at least in part, by cation. We then concluded that eucalyptol is an inverse agonist, which shifts the open ⇔ closed equilibrium of the receptor toward the closed conformation, thus reducing the spontaneous activity. By contrast, the binding of methyl salicylate shifts the equilibrium towards the open conformation and, consequently, leads to an increase in cation influx.

Mosquito odorant receptor sensitive to natural spatial repellents and inhibitory compounds.

Previously, we have identified an odorant receptor (OR) from the southern house mosquito Culex quinquefasciatus, CquiOR32, which responded to both odorants (agonists) and inhibitory compounds (antagonists). CquiOR32/CquiOrco-expressing oocytes responded to methyl salicylate and other odorants with inward (regular) currents but gave currents in the reverse direction when challenged with eucalyptol and other inhibitors. To determine whether hitherto unknown ORs show this intrareceptor inhibition, we have now examined two other receptors in the same cluster, CquiOR27 and CquiOR28. We cloned and tested four variants of CquiOR28, but none of the 250 compounds in our panel of odorants, including an Orco ligand candidate (OLC12), elicited inward or upward deflections of the current traces. By contrast, CquiOR27/CquiOrco-expressing oocytes gave robust, dose-dependent inward currents when challenged with γ-octalactone and other odorants. On the other hand, octylamine and other phenolic compounds elicited dose-dependent currents in the reverse direction. When stimulatory and inhibitory compounds were presented in binary mixtures, γ-octalactone-elicited inward currents were attenuated in a dose-dependent manner according to the concentration of octylamine. As part of our chemical ecology approach, we tested the repellency activity of the most potent ligands in the surface landing and feeding assay and a newly reported hand-in cage assay. Protection elicited by γ-octalactone did not differ significantly from that of DEET at the same dose. In the hand-in cage assay, a cream formulation of γ-octalactone showed 97.0 ± 1.3% protection, with 47.6 ± 8.3% and 1.4 ± 0.7% landings per trial in the hands covered with a control and γ-octalactone cream, respectively (N = 8, p = 0.0078, Wilcoxon matched-pairs signed-rank test).

CO2 per se activates carbon dioxide receptors.

Carbon dioxide has been used in traps for more than six decades to monitor mosquito populations and help make informed vector management decisions. CO2 is sensed by gustatory receptors (GRs) housed in neurons in the maxillary palps. CO2-sensitive GRs have been identified from the vinegar fly and mosquitoes, but it remains to be resolved whether these receptors respond to CO2 or bicarbonate. As opposed to the vinegar fly, mosquitoes have three GR subunits, but it is assumed that subunits GR1 and GR3 form functional receptors. In our attempt to identify the chemical species that bind these receptors, we discovered that GR2 and GR3 are essential for receptor function and that GR1 appears to function as a modulator. While Xenopus oocytes coexpressing Culex quinquefasciatus subunits CquiGR1/3 and CquiGR1/2 were not activated, CquiGR2/3 gave robust responses to sodium bicarbonate. Interestingly, CquiGR1/2/3-coexpressing oocytes gave significantly lower responses. That the ternary combination is markedly less sensitive than the GR2/GR3 combination was also observed with orthologs from the yellow fever and the malaria mosquito. By comparing responses of CquiGR2/CquiGR3-coexpressing oocytes to sodium bicarbonate samples (with or without acidification) and measuring the concentration of aqueous CO2, we showed that there is a direct correlation between dissolved CO2 and receptor response. We then concluded that subunits GR2 and GR3 are essential for these carbon dioxide-sensitive receptors and that they are activated by CO2 per se, not bicarbonate.

A popular Indian clove-based mosquito repellent is less effective against Culex quinquefasciatus and Aedes aegypti than DEET.

Insect repellents are widely used as the first line of defense against mosquito bites and transmission of disease-causing agents. However, the cost of daily applications of even the most affordable and the gold standard of insect repellents, DEET, is still high for low-income populations where repellents are needed the most. An Indian clove-based homemade recipe has been presented as a panacea. We analyzed this homemade repellent and confirmed by behavioral measurements and odorant receptor responses that eugenol is the active ingredient in this formulation. Prepared as advertised, this homemade repellent is ineffective, whereas 5x more concentrated extracts from the brand most enriched in eugenol showed moderate repellency activity against Culex quinquefasciatus and Aedes aegypti. DEET showed higher performance when compared to the 5x concentrated formulation and is available in the same market at a lower price than the cost of the ingredients to prepare the homemade formulation.

DEET and other repellents are inhibitors of mosquito odorant receptors for oviposition attractants.

In addition to its primary function as an insect repellent, DEET has many "off-label" properties, including a deterrent effect on the attraction of gravid female mosquitoes. DEET negatively affects oviposition sites. While deorphanizing odorant receptors (ORs) using the Xenopus oocyte recording system, we have previously observed that DEET generated outward (inhibitory) currents on ORs sensitive to oviposition attractants. Here, we systematically investigated these inhibitory currents. We recorded dose-dependent outward currents elicited by DEET and other repellents on ORs from Culex quinquefasciatus, Aedes aegypti, and Anopheles gambiae. Similar responses were observed with other plant-derived and plant-inspired compounds, including methyl jasmonate and methyl dihydrojasmolate. Inward (regular) currents elicited by skatole upon activation of CquiOR21 were modulated when this oviposition attractant was coapplied with a repellent. Compounds that generate outward currents in ORs sensitive to oviposition attractants elicited inward currents in a DEET-sensitive receptor, CquiOR136. The best ligand for this receptor, methyl dihydrojasmolate, showed repellency activity but was not as strong as DEET in our test protocol.

Odorant receptors from Culex quinquefasciatus and Aedes aegypti sensitive to floral compounds.

Mosquitoes rely heavily on the olfactory system to find a host for a bloodmeal, plants for a source of energy and suitable sites for oviposition. Here, we examined a cluster of eight odorant receptors (ORs), which includes one OR, CquiOR1, previously identified to be sensitive to plant-derived compounds. We cloned 5 ORs from Culex quinquefasciatus and two ORs from Aedes aegypti, ie, CquiOR2, CquiOR4, CquiOR5, CquiOR84, CquiOR85, AaegOR14, and AaegOR15 and then deorphanized these receptors using the Xenopus oocyte recording system and a large panel of odorants. 2-Phenylethanol, phenethyl formate, and phenethyl propionate were the best ligands for CquiOR4 somewhat resembling the profile of AaegOR15, which gave the strongest responses to phenethyl propionate, phenethyl formate, and acetophenone. In contrast, the best ligands for CquiOR5 were linalool, PMD, and linalool oxide. CquiOR4 was predominantly expressed in antennae of nonblood fed female mosquitoes, with transcript levels significantly reduced after a blood meal. 2-Phenylethanol showed repellency activity comparable to that of DEET at 1%. RNAi experiments suggest that at least in part 2-phenylethanol-elicited repellency is mediated by CquiOR4 activation.

Odorant Inhibition in Mosquito Olfaction.

How chemical signals are integrated at the peripheral sensory system of insects is still an enigma. Here we show that when coexpressed with Orco in Xenopus oocytes, an odorant receptor from the southern house mosquito, CquiOR32, generated inward (regular) currents when challenged with cyclohexanone and methyl salicylate, whereas eucalyptol and fenchone elicited inhibitory (upward) currents. Responses of CquiOR32-CquiOrco-expressing oocytes to odorants were reduced in a dose-dependent fashion by coapplication of inhibitors. This intrareceptor inhibition was also manifested in vivo in fruit flies expressing the mosquito receptor CquiOR32, as well in neurons on the antennae of the southern house mosquito. Likewise, an orthologue from the yellow fever mosquito, AaegOR71, showed intrareceptor inhibition in the Xenopus oocyte recording system and corresponding inhibition in antennal neurons. Inhibition was also manifested in mosquito behavior. Blood-seeking females were repelled by methyl salicylate, but repellence was significantly reduced when methyl salicylate was coapplied with eucalyptol.

Use of machine learning to identify novel, behaviorally active antagonists of the insect odorant receptor co-receptor (Orco) subunit.

Olfaction is a key component of the multimodal approach used by mosquitoes to target and feed on humans, spreading various diseases. Current repellents have drawbacks, necessitating development of more effective agents. In addition to variable odorant specificity subunits, all insect odorant receptors (ORs) contain a conserved odorant receptor co-receptor (Orco) subunit which is an attractive target for repellent development. Orco directed antagonists allosterically inhibit odorant activation of ORs and we previously showed that an airborne Orco antagonist could inhibit insect olfactory behavior. Here, we identify novel, volatile Orco antagonists. We functionally screened 83 structurally diverse compounds against Orco from Anopheles gambiae. Results were used for training machine learning models to rank probable activity of a library of 1280 odorant molecules. Functional testing of a representative subset of predicted active compounds revealed enrichment for Orco antagonists, many structurally distinct from previously known Orco antagonists. Novel Orco antagonist 2-tert-butyl-6-methylphenol (BMP) inhibited odorant responses in electroantennogram and single sensillum recordings in adult Drosophila melanogaster and inhibited OR-mediated olfactory behavior in D. melanogaster larvae. Structure-activity analysis of BMP analogs identified compounds with improved potency. Our results provide a new approach to the discovery of behaviorally active Orco antagonists for eventual use as insect repellents/confusants.

Reverse chemical ecology-based approach leading to the accidental discovery of repellents for Rhodnius prolixus, a vector of Chagas diseases refractory to DEET.

Rhodnius prolixus is one of the most important vectors of Chagas disease in Central and South America for which repellents and attractants are sorely needed. Repellents like DEET, picaridin, and IR3535 are widely used as the first line of defense against mosquitoes and other vectors, but they are ineffective against R. prolixus. Our initial goal was to identify in R. prolixus genome odorant receptors sensitive to putative sex pheromones. We compared gene expression of 21 ORs in the R. prolixus genome, identified 4 ORs enriched in male (compared with female) antennae. Attempts to de-orphanize these ORs using the Xenopus oocyte recording system showed that none of them responded to putative sex pheromone constituents. One of the them, RproOR80, was sensitive to 4 compounds in our panel of 109 odorants, namely, 2-heptanone, γ-octalactone, acetophenone, and 4-methylcychohexanol. Interestingly, these compounds, particularly 4-methylcyclohexanol, showed strong repellency activity as indicated not only by a significant decrease in residence time close to a host, but also by a remarkable reduction in blood intake. 4-Methylcyclohexanol-elicited repellency activity was abolished in RNAi-treated insects. In summary, our search for pheromone receptors led to the discovery of repellents for R. prolixus.

Methyl dihydrojasmonate and lilial are the constituents with an "off-label" insect repellence in perfumes.

Insect repellents are widely used to fend off nuisance mosquitoes and, more importantly, to reduce or eliminate mosquito bites in areas where viruses and other vector-borne diseases are circulating. Synthesized more than six decades ago, DEET is the most widely used insect repellent. Plant-derived compounds are used in a plethora of commercial formulations and natural recipes to repel mosquitoes. They are also used as fragrances. We analysed Bombshell® to identify the constituent(s) eliciting a previously reported "off- label" repellence activity. The two major fragrance ingredients in Bombshell, i.e., methyl dihydrojasmonate and lilial, demonstrated strong repellence against the southern house mosquito, Culex quinquefasciatus, in laboratory assays. Both compounds activated a previously identified DEET-sensitive odorant receptor, CquiOR136. These compounds were also major constituents of Ivanka Trump eau de parfum. The methyl dihydrojasmonate content was higher in the Ivanka Trump perfume than in Bombshell, the reverse being true for lilial. Both Bombshell and Ivanka Trump eaux de parfums retained activity for as long as 6 hours in laboratory assays. Although wearing these perfumes may repel nuisance mosquitoes, their use as "off-label" repellents against infected mosquitoes is not recommended.

Reverse chemical ecology approach for the identification of an oviposition attractant for Culex quinquefasciatus.

Pheromones and other semiochemicals play a crucial role in today's integrated pest and vector management strategies. These semiochemicals are typically discovered by bioassay-guided approaches. Here, we applied a reverse chemical ecology approach; that is, we used olfactory proteins to lead us to putative semiochemicals. Specifically, we used 7 of the top 10 odorant receptors (ORs) most expressed in the antennae of the southern house mosquito, Culex quinquefasciatus, and which are yet to be deorphanized. We expressed these receptors in the Xenopus oocyte recording system and challenged them with a panel of 230 odorants, including physiologically and behaviorally active compounds. Six of the ORs were silent either because they are not functional or a key odorant was missing. CquiOR36, which showed the highest transcript levels of all OR genes in female antennae, was also silent to all odorants in the tested panel, but yielded robust responses when it was accidentally challenged with an old sample of nonanal in ethanol. After confirming that fresh samples were inactive and through a careful investigation of all possible "contaminants" in the old nonanal samples, we identified the active ligand as acetaldehyde. That acetaldehyde is activating CquiOR36 was further confirmed by electroantennogram recordings from antennae of fruit flies engineered to carry CquiOR36. Antennae of female mosquitoes also responded to acetaldehyde. Cage oviposition and dual-choice assays demonstrated that acetaldehyde is an oviposition attractant in a wide range of concentrations and thus of potential practical applications.

1-Octen-3-ol - the attractant that repels.

Since the discovery in the early 1980s that 1-octen-3-ol, isolated from oxen breath, attracts tsetse fly, there has been growing interest in exploring the use of this semiochemical as a possible generic lure for trapping host-seeking mosquitoes. Intriguingly, traps baited with 1-octen-3-ol captured significantly more females of the malaria mosquito, Anopheles gambiae, and the yellow fever mosquito, Aedes aegypti, than control traps, but failed to attract the southern house mosquito, Culex quinquefasciatus. Additionally, it has been demonstrated that this attractant is detected with enantioselective odorant receptors (ORs) expressed only in maxillary palps. On the basis of indoor behavioral assays it has even been suggested that 1-octen-3-ol might be a repellent to the southern house mosquito. Our approach was two-prong, i.e., to isolate 1-octen-3-ol-sensitive ORs expressed in maxillary palps and antennae of southern house female mosquito, and test the hypothesis that this semiochemical is a repellent. An OR with high transcript levels in maxillary palps, CquiOR118b, showed remarkable selectivity towards ( R)-1-octen-3-ol, whereas an OR expressed in antennae, CquiOR114b, showed higher preference for ( S)-1-octen-3-ol than its antipode. Repellency by a surface landing and feeding assay showed that not only racemic, but enantiopure ( R)- and ( S)-1-octen-3-ol are repellents at 1% dose thus suggesting the occurrence of other ( S)-1-octen-3-ol-sensitive OR(s). Female mosquitoes with ablated maxillary palps were repelled by 1-octen-3-ol, which implies that in addition to OR(s) in the maxillary palps, antennal OR(s) are essential for repellency activity.

Odorant receptor-based discovery of natural repellents of human lice.

The body louse, Pediculus humanus humanus, is an obligate blood-feeding ectoparasite and an important insect vector that mediates the transmission of diseases to humans. The analysis of the body louse genome revealed a drastic reduction of the chemosensory gene repertoires when compared to other insects, suggesting specific olfactory adaptations to host specialization and permanent parasitic lifestyle. Here, we present for the first time functional evidence for the role of odorant receptors (ORs) in this insect, with the objective to gain insight into the chemical ecology of this vector. We identified seven putative full-length ORs, in addition to the odorant receptor co-receptor (Orco), and expressed four of them in the Xenopus laevis oocytes system. When screened with a panel of ecologically-relevant odorants, PhumOR2 responded to a narrow set of compounds. At the behavior level, both head and body lice were repelled by the physiologically-active chemicals. This study presents the first evidence of the OR pathway being functional in lice and identifies PhumOR2 as a sensitive receptor of natural repellents that could be used to develop novel efficient molecules to control these insects.

Mosquito odorant receptor for DEET and methyl jasmonate.

Insect repellents are important prophylactic tools for travelers and populations living in endemic areas of malaria, dengue, encephalitis, and other vector-borne diseases. DEET (N,N-diethyl-3-methylbenzamide) is a 6-decade-old synthetic repellent, which is still considered the gold standard of mosquito repellents. Mosquitoes use their sense of smell to detect DEET, but there are currently two hypotheses regarding its mode of action: activation of ionotropic receptor IR40a vs. odorant receptor(s). Here, we demonstrate that DEET, picaridin, insect repellent 3535, and p-menthan-3,8-diol activate the odorant receptor CquiOR136 of the southern house mosquito, Culex quinquefasciatus. Electrophysiological and behavioral assays showed that CquiIR40a knockdown had no significant effect on DEET detection and repellency. By contrast, reduction of CquiOR136 transcript levels led to a significant decrease in electroantennographic responses to DEET and a complete lack of repellency. Thus, direct activation of an odorant receptor, not an ionotropic receptor, is necessary for DEET reception and repellency in Culex mosquitoes. Interestingly, methyl jasmonate, a repellent derived from the nonvolatile jasmonic acid in the signaling pathway of plant defenses, elicited robust responses in CquiOR136•CquiOrco-expressing Xenopus oocytes, thus suggesting a possible link between natural products with long insect-plant evolutionary history and synthetic repellents.

Differential expression of olfactory genes in the southern house mosquito and insights into unique odorant receptor gene isoforms.

The southern house mosquito, Culex quinquefasciatus, has one of the most acute and eclectic olfactory systems of all mosquito species hitherto studied. Here, we used Illumina sequencing to identify olfactory genes expressed predominantly in antenna, mosquito's main olfactory organ. Less than 50% of the trimmed reads generated by high-quality libraries aligned to a transcript, but approximately 70% of them aligned to the genome. Differential expression analysis, which was validated by quantitative real-time PCR on a subset of genes, showed that approximately half of the 48 odorant-binding protein genes were enriched in antennae, with the other half being predominantly expressed in legs. Similar patterns were observed with chemosensory proteins, "plus-C" odorant-binding proteins, and sensory neuron membrane proteins. Transcripts for as many as 43 ionotropic receptors were enriched in female antennae, thus making the ionotropic receptor family the largest of antennae-rich olfactory genes, second only to odorant receptor (OR) genes. As many as 177 OR genes have been identified, including 36 unique transcripts. The unique OR genes differed from previously annotated ORs in internal sequences, splice variants, and extended N or C terminus. One of the previously unknown transcripts was validated by cloning and functional expression. When challenged with a large panel of physiologically relevant compounds, CquiOR95b responded in a dose-dependent manner to ethyl 2-phenylacteate, which was demonstrated to repel Culex mosquitoes, and secondarily to citronellal, a known insect repellent. This transcriptome study led to identification of key molecular components and a repellent for the southern house mosquito.

RNAi-based demonstration of direct link between specific odorant receptors and mosquito oviposition behavior.

The Southern house mosquito, Culex quinquefasciatus--a vector of West Nile virus--is equipped with 130 odorant receptors (ORs), which enable young females to locate plants and blood-meal sources and older females to find suitable sites for oviposition. In our attempts to de-orphanize ORs expressed in female antennae, we identified CquiOR37 and CquiOR99, which were narrowly tuned to two phenolic compounds, 4-methylphenol and 4-ethylphenol. When tested in the Xenopus oocyte recording system the observed EC50s for 4-methylphenol and 4-ethylphenol were 6.4 and 18.2 µM for CquiOR37 and 14.4 and 0.74 µM for CquiOR99 (goodness of fit, R² = 0.88-0.99), respectively. Indoor behavioral assays demonstrated that gravid female mosquitoes laid significantly more eggs in water trays spiked with these compounds than in control water trays. Field studies with gravid traps corroborated that 4-ethylphenol is active in a wide range of doses from 0.1 to 10 µg/l, as required for practical applications. A dsRNA construct based on the two genes, CquiOR37/99-dsRNA was stable in pupa hemolymph for up to 3 h. Pupae injected with CquiOR37/99-dsRNA, ß-galactosidase-dsRNA or water had more than 40% survival rate at the peak of oviposition (day-9). qPCR analysis showed individual variation, but significant mean reduction in CquiOR37 and CquiOR99 transcript levels in CquiOR37/99-dsRNA-treated mosquitoes. Water-injected females and those treated with the control gene laid significantly more eggs in trays containing 4-ethylphenol than in water trays, whereas CquiOR37/99-dsRNA-treated mosquitoes laid normal number of eggs, but could not discriminate treatment from control. This study linked for the first time specific receptors for 4-ethylphenol with increased oviposition in the important vector Cx. quinquefasciatus.

Silent, generic and plant kairomone sensitive odorant receptors from the Southern house mosquito.

The Southern house mosquito Culex quinquefasciatus has the largest repertoire of odorant receptors (ORs) of all mosquitoes and dipteran species whose genomes have been sequenced to date. Previously, we have identified and de-orphanized two ORs expressed in female antennae, CquiOR2 and CquiOR10, which are sensitive to oviposition attractants. In view of a new nomenclature for the Culex genome (VectorBase) we renamed these ORs as CquiOR21 (formerly CquiOR10) and CquiOR121 (CquiOR2). In addition, we selected ORs from six different phylogenetic groups for deorphanization. We cloned four of them by using cDNA from female antennae as a template. Attempts to clone CquiOR87 and CquiOR110 were unsuccessful either because they are pseudogenes or are not expressed in adult female antennae, the main olfactory tissue. By contrast, CquiOR1, CquiOR44, CquiOR73, and CquiOR161 were highly expressed in female antennae. To de-orphanize these ORs, we employed the Xenopus oocyte recording system. CquiORx-CquiOrco-expressed oocytes were challenged with a panel of 90 compounds, including known oviposition attractants, human and vertebrate host odorants, plant kairomones, and naturally occurring repellents. While CquiOR161 did not respond to any test compound in two different laboratories, CquiOR1 showed the features of a generic OR, with strong responses to 1-octen-3-ol and other ligands. CquiOR44 and CquiOR73 showed preference to plant-derived terpenoids and phenolic compounds, respectively. While fenchone was the best ligand for the former, 3,5-dimethylphenol elicited the strongest responses in the latter. The newly de-orphanized ORs may be involved in reception of plant kairomones and/or natural repellents.

Probing insect odorant receptors with their cognate ligands: insights into structural features.

Odorant receptors (ORs) are essential for insect survival in the environment and thus are ideal molecular targets for the design of insect-inspired modern green chemicals to control populations of agricultural pests and insects of medical importance. Although insect ORs are known for more than a decade, their structural biology is still in its infancy. Here, we unravel the first structural features of ORs from the malaria mosquito, the Southern house mosquito and the silkworm moth. The second extracellular loops (ECL-2s) of their predicted structures are much longer than ECL-1s and ECL-3s. The 27 amino-acid-residue-long of the ECL-2s in mosquito and the 43 amino-acid-residue-long ECL2s in moth ORs are well-conserved. About one-third of the residues are identical, including 3-4 Pro residues. Thorough examination of well-conserved residues in these structures, by point mutation and functional assay with the Xenopus oocyte recording system, strongly suggest that these "loops" include three ß-turns and some degree of folding. In the Southern house mosquito three Pro residues in ECL-2 are essential for full activation of the receptor, which is finely tuned to the oviposition attractant 3-methylindole. Additionally, the "corner residues" of prolines, including Gly, Tyr, and Leu are functionally important thus suggesting that turns are stabilized not only by backbone hydrogen bonds, but also by side-chain interactions. Examination of ECL-2s from a distant taxonomical group suggests these ECL-2 loops might be functionally important in all insect ORs. Two of the four Pro residues in the predicted ECL-2 of the bombykol receptor in the silkworm moth, BmorOR1, are essential for function. Experimental evidence indicates that these loops may not be specificity determinants, but they may form a cover to the yet-to-be-identified membrane embedded binding cavities of insect ORs.

Specificity determinants of the silkworm moth sex pheromone.

The insect olfactory system, particularly the peripheral sensory system for sex pheromone reception in male moths, is highly selective, but specificity determinants at the receptor level are hitherto unknown. Using the Xenopus oocyte recording system, we conducted a thorough structure-activity relationship study with the sex pheromone receptor of the silkworm moth, Bombyx mori, BmorOR1. When co-expressed with the obligatory odorant receptor co-receptor (BmorOrco), BmorOR1 responded in a dose-dependent fashion to both bombykol and its related aldehyde, bombykal, but the threshold of the latter was about one order of magnitude higher. Solubilizing these ligands with a pheromone-binding protein (BmorPBP1) did not enhance selectivity. By contrast, both ligands were trapped by BmorPBP1 leading to dramatically reduced responses. The silkworm moth pheromone receptor was highly selective towards the stereochemistry of the conjugated diene, with robust response to the natural (10E,12Z)-isomer and very little or no response to the other three isomers. Shifting the conjugated diene towards the functional group or elongating the carbon chain rendered these molecules completely inactive. In contrast, an analogue shortened by two omega carbons elicited the same or slightly higher responses than bombykol. Flexibility of the saturated C1-C9 moiety is important for function as addition of a double or triple bond in position 4 led to reduced responses. The ligand is hypothesized to be accommodated by a large hydrophobic cavity within the helical bundle of transmembrane domains.