Binding characteristics and physicochemical basis of C-minus odorant-binding protein 8 in western flower thrips, Frankliniella occidentalis, with various host vegetable volatiles.

Western flower thrips (WFT), Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), is a globally invasive polyphagous pest that relies on olfaction to sense host plants. Odorant-binding proteins (OBPs) in insect sensillar lymph mediate interactions with plant volatiles. However, the physicochemical mechanisms by which OBPs in F. occidentalis bind to host-plant volatiles, the key amino acid residues involved in these interactions and the structural characteristics of the preferred ligands remain unclear. Here, we cloned a novel OBP, FoccOBP8, which belongs to the C-minus OBP subfamily based on sequence alignment and phylogenetic analyses. Competitive fluorescence binding assays using purified recombinant FoccOBP8 revealed broad binding interactions towards host-plant volatiles. Notably, 4-ethylacetophenone and ethyl isonicotinate showed the strongest binding affinity with dissociation constants (KD) of 2.00 and 2.46 μmol/L, respectively. Thermodynamic analyses indicated the binding processes were dynamic quenching processes and mainly driven by hydrophobic interactions, spontaneously. Molecular docking and the corresponding residue-interaction energy heatmap analyses predicted that the conserved Gln4 residue contributed substantially to the ligand-protein interactions. Molecule clustering and structural analyses predicted that the ligands containing aromatic moieties π-π conjugated with the carbonyl group or carbon-carbon double bond preferentially bound to FoccOBP8. This study elucidates the broad binding profiles of FoccOBP8 with ligands and their physicochemical basis, providing insights into the olfactory-driven polyphagia of WFT and theoretical foundations for developing odour-based attractants or repellents targeting this invasive pest.