Expression patterns and ligand binding characterization of Plus-C odorant-binding protein 14 from Adelphocoris...

Odorant-binding proteins (OBPs) can bind and transport hydrophobic odorants across the sensillum lymph to the olfactory receptors (ORs) and play crucial roles in insect chemosensory systems. Although the ligand spectra of classical OBPs have been extensively characterized, little is known about OBPs in the Plus-C subgroup. Here, we focus on AlinOBP14, a Plus-C OBP from the hemipteran mirid bug pest Adelphocoris lineolatus (Goeze). Quantitative real-time PCR experiments suggest that AlinOBP14 is ubiquitously expressed at different developmental stages but is highly expressed in the adult head, the non-chemosensory organ. Fluorescence-based competitive binding assays show that β-ionone, nerolidol, farnesol and insect juvenile hormone III (JHIII) strongly bind to AlinOBP14. No significant internal binding pocket is predicted by homology modeling. Instead, the long N-terminal and C-terminal regions and parts of several α-helixes form a cupped cavity to accommodate ligands. Molecular docking reveals that the four potential ligands have distinct binding orientations, implying different roles of the N-terminal extension in ligand recognition. This hypothesis is further confirmed via a ligand binding assay in which the recombinant N-terminal mutant AlinOBP14 displays comparable binding affinities for β-ionone and trans, trans-farnesol but decreased binding affinities for nerolidol and JHIII. Thus, our current study is the first to characterize the ligand binding spectra of a Plus-C OBP in hemipteran insect species and reveals that N-terminal extensions could be required for its recognition of putative ligands.