Chemical structure of semiochemicals and key binding sites together determine the olfactory functional modes of odorant-binding protein 2 in Eastern honey bee, Apis cerana

Insects can exhibit flexible olfaction that is sensitive to complex natural chemical environments. Odorant-binding proteins (OBPs) in insects' antennal chemosensilla can act as transporters of plant volatiles and pheromones across the sensillar lymph. Although the physiological functions of OBPs have been widely reported, it is still unclear how OBP binds to ligands with various structures in detail. Here, we further investigated the ligand-binding modes and characteristics of AcerOBP2 from the Eastern honey bee (Apis cerana). The results showed that, as a specific protein distributed below the base of chemosensilla on the antennal surface, AcerOBP2 was strongly bound with the candidate floral volatiles and bee pheromones. By docking analysis and site-directed mutagenesis, four different binding modes were found in the five AcerOBP2 mutants between six ligands. Two key amino acids, Ser123 and Lys51, play a key role in AcerOBP2 binding to odors, depending on the presence or absence of hydrogen bonds. In addition, the binding modes depend on their chemical structures and the binding poses of the diverse ligands. These results not only further prompted the functional basis of the relationship between the chemical structures of odorants and bee OBPs, but also revealed the complexity of the flexible behavioral modes of odor binding in insect olfactory systems.