Key residues involved in the interaction between chlorpyrifos and a chemosensory protein in Rhopalosiphum padi: Implication for tracking chemical residues via insect olfactory proteins

The development of advanced biosensors for tracking chemical residues and detecting environmental pollution is of great significance. Insect chemical sensory proteins, including chemosensory proteins (CSPs), are easy to synthesize and purify and have been used to design proteins for specific biosensor applications. Chlorpyrifos is one of the most commonly used chemicals for controlling insect pests in agriculture. This organophosphate is harmful to aquatic species and has long-term negative consequences for the ecosystem. CSPs can bind and carry a variety of environmental chemicals, including insecticides. However, the mechanism by which CSPs bind to insecticides in aphids has not been clarified. In this study, we discovered that RpCSP1 from Rhopalosiphum padi has a higher affinity for chlorpyrifos, with a Ki value of 4.763 ± 0.491 μM. Multispectral analysis revealed the physicochemical binding mechanism between RpCSP1 and chlorpyrifos. Computational simulation analysis demonstrated that the main factor promoting the development of the RpCSP1-chlorpyrifos complex is polar solvation energy. Four residues (Arg33, Glu94, Gln145, Lys153) were essential in facilitating the interaction between RpCSP1 and chlorpyrifos. Our research has improved knowledge of the relationship between CSPs and organophosphorus pesticides. This knowledge contributes to the advancement of biosensor chips for tracking chemical residues and detecting environmental pollution through the use of CSPs.

Screening of behaviorally active compounds based on the interaction between two chemosensory proteins and mung bean volatiles in Callosobruchus chinensis

Chemosensory proteins (CSPs) are involved in the earliest steps of the olfactory process by binding and transporting odorants and play a crucial role in the insect's search for food and egg-laying sites. In the present study, the tissue expression profiles showed that both CchiCSP3 and CchiCSP5 of Callosobruchus chinensis were highly expressed in the adult antennae. Subsequently, the recombinant CchiCSP3 and CchiCSP5 proteins were analysed using fluorescence competitive binding assays, and both showed binding affinities for the three mung bean volatiles. Molecular docking and site-directed mutagenesis revealed four key amino acid residues in CchiCSP3 (L47, W80, Y81, and L84) and CchiCSP5 (Y28, K46, L49, and I72). Electroantennogram (EAG) and dual-choice biobehavioral assays showed that the antennae of adult C. chinensis were electrophysiologically active in response to stimulation with all three behaviorally active compounds and that octyl 4-methoxycinnamate and β-ionone had a significant luring effect on adult C. chinensis, whereas vanillin had a significant avoidance effect. Our study screened three effective behaviorally active compounds based on the involvement of two CchiCSPs in the recognition of mung bean volatiles, providing an opportunity to develop an alternative control strategy using behavioral disruptors to limit the impact of pests.

Involvement of Holotrichia parallela odorant-binding protein 3 in the localization of oviposition sites

Organic fertilizers-derived volatiles attract Holotrichia parallela during oviposition. However, the mechanisms underlying the perception of oviposition cues in H. parallela remain unclear. Here, H. parallela odorant-binding protein 3 (HparOBP3) was identified as a key OBP. Bioinformatics analysis showed that HparOBP3 clustered together with Holotrichia oblita OBP8. HparOBP3 was mainly expressed in the antennae of both sexes. Recombinant HparOBP3 exhibited distinct binding affinities towards 22 compounds released by organic fertilizers. After 48 h of RNA interference (RNAi), the expression of HparOBP3 in male and female antennae was decreased by 90.77 % and 82.30 %, respectively. In addition, silencing of HparOBP3 significantly reduced the electrophysiological responses and tropism of males to cis-3-hexen-1-ol, 1-hexanol, and (Z)-β-ocimene as well as females to cis-3-hexen-1-ol, 1-hexanol, benzaldehyde, and (Z)-β-ocimene. Molecular docking indicated that hydrophobic residues Leu-83, Leu-87, Phe-108, and Ile-120 of HparOBP3 were important amino acids for interacting with ligands. Mutation of the key residue, Leu-83, significantly diminished the binding ability of HparOBP3. Furthermore, acrylic plastic arena bioassays showed that the attraction and oviposition indexes of organic fertilizers to H. parallela were reduced by 55.78 % and 60.11 %, respectively, after silencing HparOBP3. These results suggest that HparOBP3 is essential in mediating the oviposition behavior of H. parallela.

Ligand-binding properties of chemosensory protein 1 in Callosobruchus chinensis to mung bean volatiles

Callosobruchus chinensis (Coleoptera: Fabaceae) is a worldwide pest that feeds exclusively on legumes, and is the most serious pest affecting mung beans. Usually, the insect olfactory system plays a predominant role in searching for host plants and egg-laying locations. Chemosensory proteins (CSPs), are mainly responsible for transporting specific odour molecules from the environment. In this study, we found that the CSP1 gene of adult C. chinensis displayed antennae-biased expression using quantitative real-time PCR (qRT-PCR) analysis. The binding properties of 23 mung bean volatiles were then determined through several analyses of in vitro recombinant CSP1 protein, including fluorescence competitive binding assay, homology modelling, molecular docking, and site-directed mutagenesis. Fluorescence competitive binding assays showed that CchiCSP1 protein could bind to four mung bean volatiles and was most stable at pH 7.4. After site-directed mutation of three key amino acid bases (L39, V25, and Y35), their binding affinities to each ligand were significantly decreased or lost. This indicated that these three amino acid residues may be involved in the binding of CchiCSP1 to different ligands. We further used Y-tube behavioural bioassays to find that the four mung bean volatiles had a significant attraction or repulsion response in adult C. chinensis. The above findings confirm that the CchiCSP1 protein may be involved in the response of C. chinensis to mung bean volatiles and plays an important role in olfactory-related behaviours. The four active volatiles are expected to develop into new behavioural attractants or repellents in the future.

Functional differentiation of two general odorant-binding proteins to sex pheromones in Spodoptera frugiperda

A precise chemosensory system can help insects complete various important behavioral responses by accurately identifying different external odorants. Therefore, deeply understanding the mechanism of insect recognition of important odorants will help us develop efficient and environmentally-friendly behavioral inhibitors. Spodoptera frugiperda is a polyphagous pest that feeds on >350 different host plants worldwide and also harms maize production in China. However, the molecular mechanism of the first step for males to use odorant-binding proteins (OBPs) to recognize sex pheromones remains unclear. Here, we obtained 50 OBPs from the S. frugiperda genome, and the expression level of SfruGOBP1 in females was significantly higher than that in males, whereas SfruGOBP2 displayed male-biased expression. Fluorescence competitive binding assays showed that only SfruGOBP2 showed binding affinities for the four sex pheromones of female S. frugiperda. Subsequently, we identified some key amino acid residues that can participate in the interaction between SfruGOBP2 and sex pheromones using molecular docking and site-directed mutagenesis methods. These findings will help us explore the interaction mechanism between GOBPs and sex pheromones in moths, and provide important target genes for developing new mating inhibitors of S. frugiperda in the future.

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.

Three Chemosensory Proteins Involved in Chemoreception of Oedaleus asiaticus (Orthopera: Acridoidea)

Chemosensory proteins (CSPs) are thought to play roles in the insect olfactory system by binding and carrying hydrophobic odorants across the aqueous sensillar lymph. The band-winged grasshopper, Oedaleus asiaticus Bei-Bienko, is one of the most important grasshopper pests in northern China, but there is little information about its olfactory system. In order to investigate the olfactory functions of CSPs in this pest, three CSP genes (OasiCSP4, OasiCSP11 and OasiCSP12) were expressed in Escherichia coli, and the binding affinities of the three recombinant CSP proteins were measured for 16 volatiles from the host plant (Stipa krylovii), fecal material and body of live adult O. asiaticus using fluorescence competitive binding assays. To further verify their olfactory functions, RNA interference (RNAi) and electrophysiological recording were conducted. The three recombinant proteins displayed different degrees of binding to various volatiles in ligand-binding assays, with OasiCSP12 having higher binding affinities for more volatiles than OasiCSP4 and OasiCSP11. OasiCSP12 exhibited strong binding affinities (Ki < 20 μΜ) for five host plant volatiles and one volatile from the live body of adult O. asiaticus. The transcript levels of the three OasiCSP genes were significantly lower after silencing the individual genes by RNAi, which in turn reduced the EAG responses in adults of both sexes to most tested compounds. Our study indicates that these three OasiCSPs are involved in the detection of volatile semiochemicals, and may play important roles in finding host plants and in aggregation in O. asiaticus.

The Mouthparts Enriched Odorant Binding Protein 11 of the Alfalfa Plant Bug Adelphocoris lineolatus Displays a Preferential Binding Behavior to Host Plant Secondary Metabolites

Odorant binding proteins (OBPs) are proposed to be directly required for odorant discrimination and represent potential interesting targets for pest control. In the notoriously agricultural pest Adelphocoris lineolatus, our previous functional investigation of highly expressed antennal OBPs clearly supported this viewpoint, whereas the findings of the current study by characterizing of AlinOBP11 rather indicated that OBP in hemipterous plant bugs might fulfill a different and tantalizing physiological role. The phylogenetic analysis uncovered that AlinOBP11 together with several homologous bug OBP proteins are potential orthologs, implying they could exhibit a conserved function. Next, the results of expression profiles solidly showed that AlinOBP11 was predominantly expressed at adult mouthparts, the most important gustatory organ of Hemiptera mirid bug. Finally, a rigorously selective binding profile was observed in the fluorescence competitive binding assay, in which recombinant AlinOBP11 displayed much stronger binding abilities to non-volatile secondary metabolite compounds than the volatile odorants. These results reflect that AlinOBP11, even its orthologous proteins across bug species, could be associated with a distinctively conserved physiological role such as a crucial carrier for non-volatiles host secondary metabolites in gustatory system.