Subunit contributions to insect olfactory receptor function: channel block and odorant recognition

Insect olfactory receptors are heteromeric ligand-gated ion channels composed of at least one common subunit (Orco) and at least one subunit that confers odorant specificity. Little is known about how individual subunits contribute to the structure and function of the olfactory receptor complex. We expressed insect olfactory receptors in Xenopus oocytes to investigate 2 functional features, ion channel block and odorant recognition. The sensitivity of Drosophila olfactory receptors to inhibition by ruthenium red, a cation channel blocker, varied widely when different specificity subunits were present, suggesting that the specificity subunits contribute to the structure of the ion pore. Olfactory receptors formed by DmelOr35a and Orco subunits from several different species displayed highly similar odorant response profiles, suggesting that the Orco subunit does not contribute to the structure of the odorant-binding site. We further explored odorant recognition by conducting a detailed examination of the odorant specificity DmelOr67a + DmelOrco, a receptor that responds to aromatic structures. This screen identified agonists, partial agonists, and an antagonist of DmelOr67a + DmelOrco. Our findings favor specific subunit arrangements within the olfactory receptor complex and provide a preliminary odorophore for an olfactory receptor, offering a useful foundation for future exploration of insect olfactory receptor structure.