Two subclasses of odorant-binding proteins in Spodoptera exigua display structural conservation and functional divergence.

Although many studies on lepidopteran pheromone-binding proteins (PBPs)/ general odorant-binding proteins (GOBPs) have been reported, the functional differentiation within and between the two odorant-binding protein (OBP) subclasses is still elusive. Here we conducted a comparative study on three SexiPBPs and two SexiGOBPs in Spodoptera exigua. Results showed that all five SexiPBP/GOBP genes have the same intron numbers and conserved exon/intron splice sites. Reverse transcription PCR results showed that these five SexiPBP/GOBPs were primarily expressed in antennae of both sexes and some were also detected in other tissues. Further, quantitative real-time PCR showed that five SexiPBP/GOBPs had different sex-biased expression patterns, with PBP1 being highly male-biased (5.96-fold difference) and PBP3 slightly female-biased (2.43-fold difference), while PBP2 and two GOBPs were approximately sex-equivalent (the absolute value<1.90-fold difference). Binding assays showed that all three SexiPBPs could bind all six sex pheromone components, but SexiPBP1 had much higher affinities [dissociation constant (Ki ) <1.10 μM] than did the other two SexiPBPs (Ki  >1.20 μM). Very intriguingly, SexiGOBP2 displayed even stronger binding to five sex pheromone components (Ki  <0.40 μM) than SexiPBP1. In contrast, SexiGOBP1 only exhibited weak binding to three alcohol-pheromone components. Similar results were obtained for tested pheromone analogues. In addition, each of SexiPBP/GOBPs selectively bound some plant odorants with considerable affinities (Ki  <10.0 μM). Taken together, of the three SexiPBPs, SexiPBP1 may play the most important role in female sex pheromone reception, and additionally all three SexiPBPs can detect some plant odorants, while SexiGOBP2 may be involved in the detection of female sex pheromones in addition to plant odorants. The results strongly suggest functional differentiation within and between the two OBP sub-classes.