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Zhang C, Tang B, Tan H, Wang X, Dai W. The Orco gene involved in recognition of host plant volatiles and sex pheromone in the chive maggot Bradysia odoriphaga. PESTICIDE BIOCHEMISTRY AND PHYSIOLOGY 2023; 197:105709. [PMID: 38072517 DOI: 10.1016/j.pestbp.2023.105709] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/12/2023] [Revised: 11/06/2023] [Accepted: 11/22/2023] [Indexed: 12/18/2023]
Abstract
The insect olfactory recognition system plays a crucial role in the feeding and reproductive behaviors of insects. The odorant receptor co-receptor (Orco), as an obligatory chaperone, is critical for odorant recognition by way of forming heteromeric complexes with conventional odorant receptors (ORs). To investigate the biological functions of Orco in perceiving host plant volatiles and sex pheromone, the Orco gene was identified from the chive maggot Bradysia odoriphaga transcriptome data. Multiple sequence alignment reveals that BodoOrco exhibits an extremely high sequence identity with Orcos from other dipteran insects. The expression of BodoOrco is significantly higher in adults than in larvae and pupae, and the BodoOrco gene is primarily expressed in the antennae of both sexes. Furthermore, the Y-tube assay indicated that knockdown of BodoOrco leads to significant reductions in B. odoriphaga adults' response to all tested host plant volatiles. The dsOrco-treated unmated male adults show less attraction to unmated females and responded slowly compared with dsGFP control group. These results indicated that BodoOrco is involved in recognition of sex pheromone and host plant volatiles in B. odoriphaga and has the potential to be used as a target for the design of novel active compounds for developing ecofriendly pest control strategies.
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Affiliation(s)
- Chunni Zhang
- State Key Laboratory of Crop Stress Biology for Arid Areas, and Key Laboratory of Plant Protection Resources and Pest Management of the Ministry of Education, College of Plant Protection, Northwest A&F University, Yangling 712100, Shaanxi, China.
| | - Bowen Tang
- State Key Laboratory of Crop Stress Biology for Arid Areas, and Key Laboratory of Plant Protection Resources and Pest Management of the Ministry of Education, College of Plant Protection, Northwest A&F University, Yangling 712100, Shaanxi, China
| | - Haoyu Tan
- State Key Laboratory of Crop Stress Biology for Arid Areas, and Key Laboratory of Plant Protection Resources and Pest Management of the Ministry of Education, College of Plant Protection, Northwest A&F University, Yangling 712100, Shaanxi, China
| | - Xinxiang Wang
- State Key Laboratory of Crop Stress Biology for Arid Areas, and Key Laboratory of Plant Protection Resources and Pest Management of the Ministry of Education, College of Plant Protection, Northwest A&F University, Yangling 712100, Shaanxi, China
| | - Wu Dai
- State Key Laboratory of Crop Stress Biology for Arid Areas, and Key Laboratory of Plant Protection Resources and Pest Management of the Ministry of Education, College of Plant Protection, Northwest A&F University, Yangling 712100, Shaanxi, China
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2
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Structure modelling of odorant receptor from Aedes aegypti and identification of potential repellent molecules. Comput Struct Biotechnol J 2023; 21:2204-2214. [PMID: 37013002 PMCID: PMC10066510 DOI: 10.1016/j.csbj.2023.03.005] [Citation(s) in RCA: 6] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/26/2023] [Revised: 03/05/2023] [Accepted: 03/05/2023] [Indexed: 03/08/2023] Open
Abstract
Odorant receptors (ORs) are important class of proteins involved in olfactory behaviour of insects. These are GPCR-like heptahelical transmembrane proteins with inverted topology compared to GPCR and require a co-receptor (ORco) for their function. OR function can be modulated through small molecules and negative modulation can be beneficial in case of disease vectors like Aedes aegypti. OR4 of A. aegypti is implicated in host recognition through human odour. Aedes aegypti is a vector for viruses that spread diseases like dengue, Zika and Chikungunya. In this study, we have attempted to model the full-length structure of OR4 and the ORco of A. aegypti due to lack of experimental structure. Further, we have screened a library of natural compounds (>0.3 million) along with known repellent molecules against ORco and OR4. Many natural compounds, including those from plants like Ocimum tenuiflorum (Holy Basil) and Piper nigrum (Black pepper), were found to have better binding affinity towards ORco compared to known repellents like DEET providing an alternative to existing repellent molecules. For specific inhibitor of OR4, several natural compounds (including those from plant like Mulberry) were identified. Further, we have utilized multiple docking approaches and conservation analysis to understand the interaction between OR4 and ORco. It was observed that the residues from the seventh transmembrane helix of OR4 and pore forming helix of ORco could play an important role along with known intracellular loop 3 residues in mediating the heteromer formation of OR and ORco.
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Jiang D, Lu X, Zhang L, Tang F. Enhancement of Pathogen Toxicity by Feeding Reticulitermes chinensis Snyder Sonicated Bacteria Expressing Double-Stranded RNA That Interferes with Olfaction. INSECTS 2023; 14:140. [PMID: 36835709 PMCID: PMC9965219 DOI: 10.3390/insects14020140] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 12/28/2022] [Revised: 01/20/2023] [Accepted: 01/26/2023] [Indexed: 06/18/2023]
Abstract
Reticulitermes chinensis Snyder is a serious pest in China, and the odorant receptor co-receptor gene RcOrco plays a crucial role in olfaction. However, the function of RcOrco in the resistance of termites to entomopathogens has not been reported. We constructed dsRcOrco-HT115 engineered bacteria based on the RcOrco sequence from the full-length transcriptome data of R. chinensis. The engineered bacteria expressed dsRNA of RcOrco. Sonication was used to inactivate the dsRNA-HT115 strain and obtain a large amount of dsRcOrco. The dsRcOrco produced using this method overcame the problem that genetically engineered bacteria could not be applied directly and improved its effectiveness against termites. Bioassays using the dsRcOrco generated using this method showed that dsRcOrco significantly increased the toxicity of the bacterial and fungal pathogens to R. chinensis. The present study showed, for the first time, the function of Orco in termite resistance to pathogens, and the results provide a theoretical basis for the development and application of termite RNA biopesticides.
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Affiliation(s)
- Dabao Jiang
- Co-Innovation Center for Sustainable Forestry in Southern China, Nanjing Forestry University, Nanjing 210037, China
- College of Forestry, Nanjing Forestry University, Nanjing 210037, China
| | - Xiaoyu Lu
- Co-Innovation Center for Sustainable Forestry in Southern China, Nanjing Forestry University, Nanjing 210037, China
- College of Forestry, Nanjing Forestry University, Nanjing 210037, China
| | - Ling Zhang
- Co-Innovation Center for Sustainable Forestry in Southern China, Nanjing Forestry University, Nanjing 210037, China
- College of Forestry, Nanjing Forestry University, Nanjing 210037, China
| | - Fang Tang
- Co-Innovation Center for Sustainable Forestry in Southern China, Nanjing Forestry University, Nanjing 210037, China
- College of Forestry, Nanjing Forestry University, Nanjing 210037, China
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Zhang X, Liu P, Qin Q, Li M, Meng R, Zhang T. Characterizing the Role of Orco Gene in Detecting Aggregation Pheromone and Food Resources in Protaetia brevitarsis Leiws (Coleoptera: Scarabaeidae). Front Physiol 2021; 12:649590. [PMID: 33927641 PMCID: PMC8076894 DOI: 10.3389/fphys.2021.649590] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/05/2021] [Accepted: 03/17/2021] [Indexed: 11/20/2022] Open
Abstract
An accurate olfactory system for recognizing semiochemicals and environmental chemical signals plays crucial roles in survival and reproduction of insects. Among all olfaction-related proteins, olfactory receptors (ORs) contribute to the conversion of chemical stimuli to electric signals and thereby are vital in odorant recognition. Olfactory receptor co-receptor (Orco), one of the most conserved ORs, is extremely essential in recognizing odorants through forming a ligand-gated ion channel complex with conventional ligand-binding odorant receptors. We have previously identified aggregation pheromone in Protaetia brevitarsis (Coleoptera: Scarabaeidae), a native agricultural and horticultural pest in East-Asia. However, to our best knowledge, its olfaction recognition mechanisms are still veiled. To illustrate how P. brevitarsis recognize aggregation pheromone and host plants, in the present study we cloned and sequenced the full-length Orco gene from P. brevitarsis antennae (named PbreOrco) and found that PbreOrco is highly conserved and similar to Orcos from other Coleoptera insects. Our real-time quantitative PCR (qRT-PCR) results showed that PbreOrco is mainly expressed in antenna. We also demonstrated that silencing PbreOrco using RNA interference through injecting dsOrco fragment significantly inhibited PbreOrco expression in comparison with injecting control dsGFP and subsequently revealed using electroantennogram and behavioral bioassays that decreasing PbreOrco transcript abundance significantly impaired the responses of P. brevitarsis to intraspecific aggregation pheromone and prolonged the time of P. brevitarsis spending on food seeking. Overall, our results demonstrated that PbreOrco is crucial in mediating odorant perception in P. brevitarsis.
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Affiliation(s)
- Xiaofang Zhang
- Institute of Plant Protection, Hebei Academy of Agriculture and Forestry Sciences, Integrated Pest Management Center of Hebei Province, Baoding, China.,Key Laboratory of IPM on Crops in Northern Region of North China, Ministry of Agriculture, Baoding, China
| | - Panjing Liu
- Institute of Plant Protection, Hebei Academy of Agriculture and Forestry Sciences, Integrated Pest Management Center of Hebei Province, Baoding, China.,Key Laboratory of IPM on Crops in Northern Region of North China, Ministry of Agriculture, Baoding, China
| | - Qiuju Qin
- College of Plant Protection, Hebei Agricultural University, Baoding, China
| | - Min Li
- Institute of Plant Protection, Hebei Academy of Agriculture and Forestry Sciences, Integrated Pest Management Center of Hebei Province, Baoding, China.,Key Laboratory of IPM on Crops in Northern Region of North China, Ministry of Agriculture, Baoding, China
| | - Runjie Meng
- Baoding Vocational and Technical College, Baoding, China
| | - Tao Zhang
- Institute of Plant Protection, Hebei Academy of Agriculture and Forestry Sciences, Integrated Pest Management Center of Hebei Province, Baoding, China.,Key Laboratory of IPM on Crops in Northern Region of North China, Ministry of Agriculture, Baoding, China
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Bobkov YV, Walker Iii WB, Cattaneo AM. Altered functional properties of the codling moth Orco mutagenized in the intracellular loop-3. Sci Rep 2021; 11:3893. [PMID: 33594162 PMCID: PMC7887336 DOI: 10.1038/s41598-021-83024-3] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/09/2020] [Accepted: 01/27/2021] [Indexed: 12/04/2022] Open
Abstract
Amino acid substitutions within the conserved polypeptide sequence of the insect olfactory receptor co-receptor (Orco) have been demonstrated to influence its pharmacological properties. By sequence analysis and phylogenetic investigation, in the Lepidopteran subgroup Ditrysia we identified a fixed substitution in the intracellular loop-3 (ICL-3) of a conserved histidine to glutamine. By means of HEK293 cells as a heterologous system, we functionally expressed Orco from the Ditrysian model Cydia pomonella (CpomOrco) and compared its functional properties with a site-directed mutagenized version where this ICL-3-glutamine was reverted to histidine (CpomOrcoQ417H). The mutagenized CpomOrcoQ417H displayed decreased responsiveness to VUAA1 and reduced response efficacy to an odorant agonist was observed, when co-transfected with the respective OR subunit. Evidence of reduced responsiveness and sensitivity to ligands for the mutagenized Orco suggest the fixed glutamine substitution to be optimized for functionality of the cation channel within Ditrysia. In addition, contrary to the wild type, the mutagenized CpomOrcoQ417H preserved characteristics of VUAA-binding when physiologic conditions turned to acidic. Taken together, our findings provide further evidence of the importance of ICL-3 in forming basic functional properties of insect Orco- and Orco/OR-channels, and suggest involvement of ICL-3 in the potential functional adaptation of Ditrysian Orcos to acidified extra-/intracellular environment.
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Affiliation(s)
- Yuriy V Bobkov
- Whitney Laboratory, Center for Smell and Taste, and McKnight Brain Institute, University of Florida, Gainesville, FL, USA
| | - William B Walker Iii
- Department of Plant Protection Biology, Chemical Ecology Unit, Swedish University of Agricultural Sciences, Alnarp, Sweden
| | - Alberto Maria Cattaneo
- Whitney Laboratory, Center for Smell and Taste, and McKnight Brain Institute, University of Florida, Gainesville, FL, USA. .,Department of Plant Protection Biology, Chemical Ecology Unit, Swedish University of Agricultural Sciences, Alnarp, Sweden.
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Tiwari V, Karpe SD, Sowdhamini R. Topology prediction of insect olfactory receptors. Curr Opin Struct Biol 2019; 55:194-203. [PMID: 31233963 DOI: 10.1016/j.sbi.2019.05.014] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/05/2019] [Revised: 05/10/2019] [Accepted: 05/16/2019] [Indexed: 10/26/2022]
Abstract
Olfactory receptors are important transmembrane proteins that enable organisms to perceive odours and react to them. Structural understanding of insect olfactory receptors is scarce. In this review, we discuss different transmembrane helix prediction methods, consensus methods, topology prediction methods which can enable topology prediction of these proteins. We discuss the current success rates by applying the algorithms on few G-protein coupled receptors of known structure and olfactory receptor sequences and outstanding challenges. Finally, we discuss the impact of topology prediction on biology and modeling of ORs.
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Affiliation(s)
- Vikas Tiwari
- National Centre for Biological Sciences, TIFR, GKVK Campus, Bellary Road, Bangalore 560065, India
| | - Snehal D Karpe
- National Centre for Biological Sciences, TIFR, GKVK Campus, Bellary Road, Bangalore 560065, India
| | - Ramanathan Sowdhamini
- National Centre for Biological Sciences, TIFR, GKVK Campus, Bellary Road, Bangalore 560065, India.
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Du S, Yang Z, Qin Y, Wang S, Duan H, Yang X. Computational investigation of the molecular conformation-dependent binding mode of (E)-β-farnesene analogs with a heterocycle to aphid odorant-binding proteins. J Mol Model 2018; 24:70. [DOI: 10.1007/s00894-018-3612-0] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/10/2017] [Accepted: 01/31/2018] [Indexed: 10/17/2022]
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Harini K, Sowdhamini R. Computational Approaches for Decoding Select Odorant-Olfactory Receptor Interactions Using Mini-Virtual Screening. PLoS One 2015. [PMID: 26221959 PMCID: PMC4519343 DOI: 10.1371/journal.pone.0131077] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022] Open
Abstract
Olfactory receptors (ORs) belong to the class A G-Protein Coupled Receptor superfamily of proteins. Unlike G-Protein Coupled Receptors, ORs exhibit a combinatorial response to odors/ligands. ORs display an affinity towards a range of odor molecules rather than binding to a specific set of ligands and conversely a single odorant molecule may bind to a number of olfactory receptors with varying affinities. The diversity in odor recognition is linked to the highly variable transmembrane domains of these receptors. The purpose of this study is to decode the odor-olfactory receptor interactions using in silico docking studies. In this study, a ligand (odor molecules) dataset of 125 molecules was used to carry out in silico docking using the GLIDE docking tool (SCHRODINGER Inc Pvt LTD). Previous studies, with smaller datasets of ligands, have shown that orthologous olfactory receptors respond to similarly-tuned ligands, but are dramatically different in their efficacy and potency. Ligand docking results were applied on homologous pairs (with varying sequence identity) of ORs from human and mouse genomes and ligand binding residues and the ligand profile differed among such related olfactory receptor sequences. This study revealed that homologous sequences with high sequence identity need not bind to the same/ similar ligand with a given affinity. A ligand profile has been obtained for each of the 20 receptors in this analysis which will be useful for expression and mutation studies on these receptors.
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Affiliation(s)
- K. Harini
- National Centre for Biological Sciences (TIFR), GKVK Campus, Bellary Road, Bangalore, India
| | - Ramanathan Sowdhamini
- National Centre for Biological Sciences (TIFR), GKVK Campus, Bellary Road, Bangalore, India
- * E-mail:
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9
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Tegler LT, Corin K, Hillger J, Wassie B, Yu Y, Zhang S. Cell-free expression, purification, and ligand-binding analysis of Drosophila melanogaster olfactory receptors DmOR67a, DmOR85b and DmORCO. Sci Rep 2015; 5:7867. [PMID: 25597985 PMCID: PMC4297953 DOI: 10.1038/srep07867] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/08/2014] [Accepted: 12/11/2014] [Indexed: 01/25/2023] Open
Abstract
Insects transmit numerous devastating diseases, including malaria, dengue fever, and sleeping sickness. Olfactory cues guide insects to their hosts, and are thus responsible for disease transmission. Understanding the molecular basis of insect olfaction could facilitate the development of interventions. The first step is to heterologously overexpress and purify insect olfactory receptors (ORs). This is challenging, as ORs are membrane proteins. Here, we show that insect ORs and their co-receptor can be expressed in an E. coli cell-free system. After immunoaffinity chromatography, the ORs are ~95% pure, and up to 1 mg/10 ml reaction is obtained. Circular dichroism together with microscale thermophoresis indicate that each receptor is properly folded, and can bind its respective ligand. This is the first time insect ORs have been expressed in an E. coli system. The methods described here could facilitate future structure-function studies, which may aid in developments to alleviate the suffering of millions caused by insect-transmitted diseases.
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Affiliation(s)
- Lotta Tollstoy Tegler
- 1] Center for Bits and Atoms, Massachusetts Institute of Technology, 77 Massachusetts, Cambridge, MA 02139-4307, USA [2] Division of Chemistry, Department of Physics, Chemistry and Biology, Linkoping University, SE-581 83 Linkoping, Sweden
| | - Karolina Corin
- 1] Center for Bits and Atoms, Massachusetts Institute of Technology, 77 Massachusetts, Cambridge, MA 02139-4307, USA [2] Biomedical Engineering Research Group, School of Electrical and Information Engineering University of the Witwatersrand, Johannesburg, South Africa [3] Department of Molecular Medicine and Haematology, University of the Witwatersrand, Johannesburg, South Africa
| | - Julia Hillger
- Center for Bits and Atoms, Massachusetts Institute of Technology, 77 Massachusetts, Cambridge, MA 02139-4307, USA
| | - Brooke Wassie
- Center for Bits and Atoms, Massachusetts Institute of Technology, 77 Massachusetts, Cambridge, MA 02139-4307, USA
| | - Yanmei Yu
- 1] Center for Bits and Atoms, Massachusetts Institute of Technology, 77 Massachusetts, Cambridge, MA 02139-4307, USA [2] College of Electronics and Information Engineering, Sichuan University, Chengdu, China 610065
| | - Shuguang Zhang
- Center for Bits and Atoms, Massachusetts Institute of Technology, 77 Massachusetts, Cambridge, MA 02139-4307, USA
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Nagarathnam B, Karpe SD, Harini K, Sankar K, Iftekhar M, Rajesh D, Giji S, Archunan G, Balakrishnan V, Gromiha MM, Nemoto W, Fukui K, Sowdhamini R. DOR - a Database of Olfactory Receptors - Integrated Repository for Sequence and Secondary Structural Information of Olfactory Receptors in Selected Eukaryotic Genomes. Bioinform Biol Insights 2014; 8:147-58. [PMID: 25002814 PMCID: PMC4069036 DOI: 10.4137/bbi.s14858] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/13/2014] [Revised: 03/20/2014] [Accepted: 03/20/2014] [Indexed: 11/05/2022] Open
Abstract
Olfaction is the response to odors and is mediated by a class of membrane-bound proteins called olfactory receptors (ORs). An understanding of these receptors serves as a good model for basic signal transduction mechanisms and also provides important clues for the strategies adopted by organisms for their ultimate survival using chemosensory perception in search of food or defense against predators. Prior research on cross-genome phylogenetic analyses from our group motivated the addressal of conserved evolutionary trends, clustering, and ortholog prediction of ORs. The database of olfactory receptors (DOR) is a repository that provides sequence and structural information on ORs of selected organisms (such as Saccharomyces cerevisiae, Drosophila melanogaster, Caenorhabditis elegans, Mus musculus, and Homo sapiens). Users can download OR sequences, study predicted membrane topology, and obtain cross-genome sequence alignments and phylogeny, including three-dimensional (3D) structural models of 100 selected ORs and their predicted dimer interfaces. The database can be accessed from http://caps.ncbs.res.in/DOR. Such a database should be helpful in designing experiments on point mutations to probe into the possible dimerization modes of ORs and to even understand the evolutionary changes between different receptors.
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Affiliation(s)
| | - Snehal D Karpe
- National Center for Biological Sciences (TIFR), Bangalore, India
| | - Krishnan Harini
- National Center for Biological Sciences (TIFR), Bangalore, India
| | - Kannan Sankar
- Birla Institute of Technology and Science, Pilani, India. ; Presently in: Bioinformatics and Computational Biology Program, Iowa State University, Ames, IA, USA
| | | | - Durairaj Rajesh
- Department of Animal Sciences, Bharathidasan University, Tiruchirapalli, Tamil Nadu, India
| | - Sadasivam Giji
- National Center for Biological Sciences (TIFR), Bangalore, India
| | - Govidaraju Archunan
- Department of Animal Sciences, Bharathidasan University, Tiruchirapalli, Tamil Nadu, India
| | - Veluchamy Balakrishnan
- Department of Biotechnology, K.S. Rangasamy College of Technology, Tiruchengode, Tamil Nadu, India
| | - M Michael Gromiha
- Department of Biotechnology, Indian Institute of Technology Madras, Chennai, Tamil Nadu, India
| | - Wataru Nemoto
- Current Address: Division of Life Science and Engineering, School of Science and Engineering, Tokyo Denki University, Ishizaka, Hatoyama-cho, Hiki-gun, Saitama, 350-0394, Japan
| | - Kazhuhiko Fukui
- Molecular Profiling Research Center for Drug Discover, National Institute of Advanced Industrial Science and Technology, Aomi, Koto-ku,Tokyo, Japan
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The role of the coreceptor Orco in insect olfactory transduction. J Comp Physiol A Neuroethol Sens Neural Behav Physiol 2013; 199:897-909. [PMID: 23824225 DOI: 10.1007/s00359-013-0837-3] [Citation(s) in RCA: 93] [Impact Index Per Article: 8.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/02/2013] [Revised: 06/19/2013] [Accepted: 06/21/2013] [Indexed: 10/26/2022]
Abstract
Insects sense odorants with specialized odorant receptors (ORs). Each antennal olfactory receptor neuron expresses one OR with an odorant binding site together with a conserved coreceptor called Orco which does not bind odorants. Orco is necessary for localization of ORs to dendritic membranes and, thus, is essential for odorant detection. It forms a spontaneously opening cation channel, activated via phosphorylation by protein kinase C. Thereafter, Orco is also activated via cyclic adenosine monophosphate (cAMP). Orco forms homo-as well as heteromers with ORs with unknown stoichiometry. Contradictory publications suggest different mechanisms of olfactory transduction. On the one hand, evidence accumulates for the employment of more than one G protein-coupled olfactory transduction cascade in different insects. On the other hand, results from other studies suggest that the OR-Orco complex functions as an odorant-gated cation channel mediating ionotropic signal transduction. This review analyzes conflicting hypotheses concerning the role of Orco in insect olfactory transduction. In conclusion, in situ studies in hawkmoths falsify the hypothesis that Orco underlies odorant-induced ionotropic signal transduction in all insect species. Instead, Orco forms a metabotropically gated, slow cation channel which controls odorant response threshold and kinetics of the sensory neuron.
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