title: Towards an understanding of the structural basis for insect olfaction by odorant receptors.
abstract: Insects have co-opted a unique family of seven transmembrane proteins for odour sensing. Odorant receptors are believed to have evolved from gustatory receptors somewhere at the base of the Hexapoda and have expanded substantially to become the dominant class of odour recognition elements within the Insecta. These odorant receptors comprise an obligate co-receptor, Orco, and one of a family of highly divergent odorant "tuning" receptors. The two subunits are thought to come together at some as-yet unknown stoichiometry to form a functional complex that is capable of both ionotropic and metabotropic signalling. While there are still no 3D structures for these proteins, site-directed mutagenesis, resonance energy transfer, and structural modelling efforts, all mainly on Drosophila odorant receptors, are beginning to inform hypotheses of their structures and how such complexes function in odour detection. Some of the loops, especially the second extracellular loop that has been suggested to form a lid over the binding pocket, and the extracellular regions of some transmembrane helices, especially the third and to a less extent the sixth and seventh, have been implicated in ligand recognition in tuning receptors. The possible interaction between Orco and tuning receptor subunits through the final intracellular loop and the adjacent transmembrane helices is thought to be important for transducing ligand binding into receptor activation. Potential phosphorylation sites and a calmodulin binding site in the second intracellular loop of Orco are also thought to be involved in regulating channel gating. A number of new methods have recently been developed to express and purify insect odorant receptor subunits in recombinant expression systems. These approaches are enabling high throughput screening of receptors for agonists and antagonists in cell-based formats, as well as producing protein for the application of biophysical methods to resolve the 3D structure of the subunits and their complexes.
authors.affiliation: The New Zealand Institute for Plant & Food Research Limited, Private Bag 92169, Auckland 1142, New Zealand.
authors.forename: Colm
authors.initials: C
authors.lastname: Carraher
authors.affiliation: Food Nutrition & Health Team, Food & Bio-based Products Group, AgResearch Private Bag 11008, Palmerston North 4442, New Zealand.
authors.forename: Julie
authors.initials: J
authors.lastname: Dalziel
authors.affiliation: The New Zealand Institute for Plant & Food Research Limited, Private Bag 92169, Auckland 1142, New Zealand.
authors.forename: Melissa D
authors.initials: MD
authors.lastname: Jordan
authors.affiliation: School of Biological Sciences, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand.
authors.forename: David L
authors.initials: DL
authors.lastname: Christie
authors.affiliation: The New Zealand Institute for Plant & Food Research Limited, Private Bag 92169, Auckland 1142, New Zealand; School of Biological Sciences, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand.
authors.forename: Richard D
authors.initials: RD
authors.lastname: Newcomb
authors.affiliation: The New Zealand Institute for Plant & Food Research Limited, Private Bag 92169, Auckland 1142, New Zealand. Electronic address: andrew.kralicek@plantandfood.co.nz.
authors.forename: Andrew V
authors.initials: AV
authors.lastname: Kralicek
mesh_terms: D000818:Animals; D004331:Drosophila melanogaster / Q000737:chemistry / Q000235:genetics; D019476:Insect Proteins / Q000737:chemistry* / Q000235:genetics; D007313:Insecta / Q000235:genetics / Q000378:metabolism*; D008024:Ligands; D009812:Odorants; D010802:Phylogeny; D011485:Protein Binding; D021122:Protein Subunits / Q000737:chemistry / Q000235:genetics; D018035:Receptors, Odorant / Q000737:chemistry* / Q000235:genetics; D012903:Smell
chemical_list: D019476:Insect Proteins; D008024:Ligands; D021122:Protein Subunits; D018035:Receptors, Odorant
keywords: Function; Insect olfaction; Odorant receptor; Orco; Structure