β-Ionone as putative semiochemical suggested by ligand binding on an odorant-binding protein of Hylamorpha elegans and electroantennographic recordings

Currently, odorant-binding proteins (OBPs) are considered the first filter for olfactory information for insects and constitute an interesting target for pest control. Thus, an OBP (HeleOBP) from the scarab beetle Hylamorpha elegans (Burmeister) was identified, and ligand-binding assays based on fluorescence and in silico approaches were performed, followed by a simulated binding assay. Fluorescence binding assays showed slight binding for most of the ligands tested, including host-plant volatiles. A high binding affinity was obtained for β-ionone, a scarab beetle-related compound. However, the binding of its analogue α-ionone was weaker, although it is still considered good. On the other hand, through a three-dimensional model of HeleOBP constructed by homology, molecular docking was carried out with 29 related ligands to the beetle. Results expressed as free binding energy and fit quality (FQ) indicated strong interactions of sesquiterpenes and terpenoids (α- and β-ionone) with HeleOBP as well as some aromatic compounds. Residues such as His102, Tyr105 and Tyr113 seemed to participate in the interactions previously mentioned. Both in silico scores supported the experimental affinity for the strongest ligands. Therefore, the activity of α-ionone, β-ionone and 2-phenyl acetaldehyde at antennal level was studied using electroantenography (EAG). Results showed that the three ligands are electrophysiologically active. However, an aliquot of β-ionone (represented by 3.0 ng) elicited stronger EAG responses in antennae of males than of females. Finally, the role of these ligands as potential semiochemicals for H. elegans is discussed.