Abstract
As opposed to humans, insects rely heavily on an acute olfactory system for survival and reproduction. Two major types of olfactory proteins, namely, odorant-binding proteins (OBPs) and odorant receptors (ORs), may contribute to the selectivity and sensitivity of the insects' olfactory system. Here, we aimed at addressing the question whether OBPs highly enriched in the antennae of the southern house mosquito, Culex quinquefasciatus, contribute at least in part to the selective reception of physiologically relevant compounds. Using a fluorescence reporter and a panel of 34 compounds, including oviposition attractants, human-derived attractants, and repellents, we measured binding affinities of CquiOBP1, CquiOBP2, and CquiOBP5. Based on dissociation constants, we surmised that CquiOBP2 is a carrier for the oviposition attractant skatole, whereas CquiOBP1 and CquiOBP5 might transport the oviposition pheromone MOP, a human-derived attractant nonanal, and the insect repellent picardin. Binding of these three ligands to CquiOBP1 was further analyzed by examining the influence of pH on apparent affinity as well as by docking these three ligands into CquiOBP1. Our findings suggest that CquiOBP1 might discriminate MOP from nonanal/picaridin on the basis of the midpoint transition of a pH-dependence conformational change, and that MOP is better accommodated in the binding cavity than the other two ligands. These findings, along with previous experimental evidence suggesting that CquiOBP1 does not detect nonanal in vivo, suggest that OBP selectivity may not be clearly manifested in their dissociation constants.
Highlights
In insects, olfaction is essential for survival and reproduction (Leal, 2012); smell is undoubtedly their most important sensory modality
Our findings suggest that CquiOBP1 might discriminate MOP from nonanal/picaridin on the basis of the midpoint transition of a pH-dependence conformational change, and that MOP is better accommodated in the binding cavity than the other two ligands
We aimed at cloning CquiOBP3, CquiOBP5, CquiOBP2, and CquiOBP11, which are enriched in Culex antennae as indicated by the moderated log fold—a ratio of transcript levels in antennae compared to legs (Leal et al, 2013)
Summary
Olfaction is essential for survival and reproduction (Leal, 2012); smell is undoubtedly their most important sensory modality. For the reception of olfactory signals, be it an insect pheromone or the smell of food spoiled for human consumption, the odorant molecules must be transported from the external environment to receptors embedded in the membrane of olfactory receptor neurons (ORNs; called olfactory sensory neurons, OSN). It is accepted that insect OBPs solubilize odorants (ligands), help transport hydrophobic molecules through the aqueous environment of a lymph surrounding the neurons, and contribute to the sensitivity of the olfactory system (Leal, 2013). If these were the only roles for OBPs, why do the genome of some species have as many as 50 OBP genes? If these were the only roles for OBPs, why do the genome of some species have as many as 50 OBP genes? (Leal, 2013) Theoretically, these roles could be performed with a single OBP as in humans (Sell, 2014)
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