Characterization of two odorant binding proteins in Spodoptera exigua reveals functional conservation and difference

As the first biochemical step of olfactory reception and recognition, odorant binding proteins (OBPs) have been demonstrated to be essential. Considering functional diversities of OBPs within a single species, we here extended the characterization of two other OBPs from Spodoptera exigua, belonging to insect Classic OBPs. With a combination of transcriptome and Rapid Amplification of cDNA End (RACE) approaches, two OBP genes in S. exigua were identified, namely SexiOBP1 and OBP7. Expression pattern analysis revealed that both of them exhibited a distinct expression pattern, where OBP1 was broadly and highly expressed in several tissues including antennae of adults whereas OBP7 was abundant only in the antennae of both sexes, strongly indicative of olfactory roles. Further, binding assays showed that the two SexiOBPs shared a common odorant-response spectrum with considerable affinities to host odorants of acetophenone, farnesol and β-ionone (K_i<20μM). Specially, OBP1 could strongly bind an insect attractant β-caryophyllene (K_i=2.76μM) released by maize. Intriguingly, the major sex pheromone of S. exigua, Z9,E12-14:Ac, was the best ligand for OBP7 with K_i value of 7.58μM. Ligand structural analysis revealed that the two SexiOBPs were capable of accommodating different types of ligands in shape and size, possibly implying the plasticity of binding pockets. Ultimately, comparison of binding properties among 10 SexiOBPs including the two OBPs in this study implied a cross-talk in functions, i.e. different OBPs are also suitable to accept some common odorants except for unique ligands. Taken together, this study has provided evidence for their involvements in seeking and orientation of host plants, and meanwhile indicates functional conservation and differences between OBP1 and OBP7 from S. xigua.