Expression Profile and Ligand Screening of a Putative Odorant-Binding Protein, AcerOBP6, from the Asian Honeybee

SIMPLE SUMMARY: The olfactory sensillum, which is located in the antenna of insects, is the basic unit of the olfactory organ. Olfactory-related genes are expressed in the sensillum. It is believed that the process of olfaction recognition is mainly mediated by two gene families, odorant binding proteins (OBPs) and olfactory receptors (ORs). The honeybee possesses a large numbers of ORs, but few OBPs. Up to now, the function of OBPs in the honeybee has not yet been fully elucidated. In order to reveal the specific role of OBPs from Apis cerana cerana, we selected an OBP gene, AcerOBP6, which is highly expressed in the antennae of worker bees, acquired a purified protein via a prokaryotic expression system, and analyzed its function using bioinformatics, molecular biology, and electrophysiology. According to the result, AcerOBP6 was a protein with extensive binding affinity, and we speculated that its function was chiefly related to foraging. Overall, this research not only explains the essential role of OBPs in ligand binding, but also provides valuable resources to help researchers further understand the nature and mechanism of the olfactory system. ABSTRACT: Olfaction is essential in some behaviors of honeybee, such as nursing, foraging, attracting a mate, social communication, and kin recognition. OBPs (odorant binding proteins) play a key role in the first step of olfactory perception. Here, we focused on a classic OBP with a PBP-GOBP domain from the Asian honeybee, Apis cerana cerana. Beyond that, the mRNA expression profiles and the binding affinity of AcerOBP6 were researched. According to qRT-PCR analysis, AcerOBP6 transcripts were mainly expressed in the antennae of forager bees. In addition, we found that the expression level of AcerOBP6 was higher than that of AmelOBP6. The fluorescence competitive binding assay indicated that the AcerOBP6 protein had binding affinity with most of the tested odors, including queen pheromone, worker pheromone, and floral volatiles, among which the strongest one was linolenic acid (with a Ki value of 1.67). However, AcerOBP6 was not sensitive to the brood pheromones. A further study based on EAG assay revealed that the antennae had the strongest response to 2-heptanone. The EAG recording values of the selected ligands were all reduced after AcerOBP6 was silenced, with 8 of 14 declining significantly (p < 0.01) given that these odors could specifically bind to AcerOBP6. As revealed in our current study, AcerOBP6 might be a crucial protein involved in olfactory recognition for foraging. Overall, the research provides a foundation for exploring the olfactory mechanism of A. cerana cerana.