Two general-odorant binding proteins in Spodoptera litura are differentially tuned to sex pheromones and plant odorants

Moths have evolved a sensitive and sophisticated olfactory system to sense a variety of semiochemicals from the external environment. In chemosensory processes, the odorant binding protein (OBP) is an essential element for filtering, binding and transporting hydrophobic odorant molecules to the specific receptors. Here focusing on a major sub-class of lepidopteran OBPs, general-odorant binding proteins (GOBPs), we explored the relationship and functional difference between two GOBP members from a noctuid species Spodoptera litura. Using genomic DNA as the template, we demonstrated that SlitGOBP2 and three SlitPBPs are clustered on the same chromosome within a close proximity. qPCR results showed that two SlitGOBPs were primarily expressed in antennae at similar levels between females and males, but GOBP2 displayed much higher expression than GOBP1. Binding studies revealed that both SlitGOBP1 and 2 strongly bound C14-C16 alcohol-pheromone analogs with high affinities (Ki<1.0 μM). However, SlitGOBP2 also strongly bound most acetate- and aldehyde-sex pheromone components and analogs, while SlitGOBP1 could not. For tested plant odorants, SlitGOBP1 showed a relatively broad ligand-binding spectrum with moderate affinities, while SlitGOBP2 was tuned to some compounds with strong binding activities (Ki<5.0 μM). Finally, by molecular docking we explored the differences in protein structures and potential key residues in the binding pockets between the two SlitGOBPs. Taken together, our study strongly suggests that SlitGOBP2 and SlitPBPs evolved by gene duplication events, and two SlitGOBPs have functionally differentiated in odorant recognition.