Functional differentiation of pheromone-binding proteins in the common cutworm Spodoptera litura

Pheromone-binding proteins (PBPs), a sub-family of odorant-binding proteins, are thought primarily to bind and transport the sex pheromones in moths. Considering multiple components of sex pheromone and multiple PBP genes exist in a single species, PBPs may contribute to the discrimination of different sex pheromone components. However, so far this discrimination is still unclear. Our previous ligand-binding assays showed that Spodoptera litura PBP1 (SlitPBP1) did not exhibit an obvious binding specificity among different sex pheromone components. In this study, binding specificity of the other two PBPs in S. litura (SlitPBP2 and SlitPBP3) was further investigated. As a result, SlitPBP2 was capable of binding all four sex pheromone components with similar affinities; whereas SlitPBP3 showed very weak binding affinities to them except Z9,E12-14:Ac. Similar results were also obtained from studied pheromone analogs, to which SlitPBP2 showed much stronger affinities than SlitPBP3. However, both SlitPBP2 and SlitPBP3 exhibited overall weaker affinities to sex pheromones and their analogs than SlitPBP1. In addition, quantitative real time PCR showed that three SlitPBP genes exhibited a very different sex-biased expression in adult antenna with male-biased for SlitPBP1 and SlitPBP2 while female-biased for SlitPBP3. Finally, ligand-binding assays indicated that the two SlitPBPs showed a similar pH-dependent conformational change as reported SlitPBP1, but these three SlitPBPs showed different behavior across a pH range or something similar. Taken together, our data suggest that in S. litura PBP1 and PBP2 may play critical roles in the perception of female sex pheromones, but do not show an obvious discriminative ability among different sex pheromone components; whereas PBP3 may have other functions.