Aphid Odorant-Binding Protein 9 Is Narrowly Tuned to Linear Alcohols and Aldehydes of Sixteen Carbon Atoms

SIMPLE SUMMARY: Odorant-binding proteins (OBPs) mediate chemical communication in insects and can provide a shortcut to deciphering their olfactory code. Although being less specific than olfactory receptors, they are easy to express, purify and characterize. Aphids, in particular, are endowed with a limited repertoire of only 10 OBPs, which are exceptionally well conserved across species. Therefore, using OBPs to study olfaction in aphids appears to be a very attractive strategy. Within our project to functionally characterize all OBPs of the pea aphid, Acyrthosiphon pisum, we have expressed OBP9 and found it to be narrowly tuned to the linear alcohols and aldehydes of 16 carbon atoms. 1-Hexadecanol has been reported as a strong feeding repellant produced by toxic fungi. On the other hand, 16-carbon linear aldehydes are components of several lepidopteran pheromones and could represent warning signals to avoid potential feeding competition. ABSTRACT: Aphid odorant-binding protein 9 is almost exclusively expressed in antennae and is well conserved between different aphid species. In order to investigate its function, we have expressed this protein and measured ligand-binding affinities to a number of common natural compounds. The best ligands are long-chain aldehydes and alcohols, in particular Z9-hexadecenal and Z11-hexadecenal, as well as 1-hexadecanol and Z11-1-hexadecenol. A model of this protein indicated Lys37 as the residue that is likely to establish strong interactions with the ligands, probably a Schiff base with aldehydes and a hydrogen bond with alcohols. Indeed, when we replaced this lysine with a leucine, the mutated protein lost its affinity to both long aldehydes and alcohols, while the binding of other volatiles was unaffected. Long-chain linear alcohols are common products of molds and have been reported as aphid antifeedants. Corresponding aldehydes, instead, are major components of sex pheromones for several species of Lepidoptera. We speculate that aphids might use OBP9 to avoid mold-contaminated plants as well as competition with lepidopteran larvae.