Molecular characterization of three Niemann-Pick type C2 proteins in the predatory mite Neoseiulus barkeri Hughes (Acari: Phytoseiidae)

In arthropods, a host of studies focused on insect olfactory systems, where most functional proteins including odorant-binding proteins (OBPs) and chemosensory proteins (CSPs) have been identified. However, in other arthropods such as ticks and mites, genes encoding for OBPs and CSPs are absent. The Niemann-Pick type C2 (NPC2), as a third class of binding proteins, was indicated as potential carriers for semiochemicals and participants in chemical communication. Here, we cloned three full-length NPC2 genes (NbNPC2-1, NbNPC2-2 and NbNPC2-3) from a predatory mite Neoseiulus barkeri Hughes (Acari: Phytoseiidae) and investigated the expression levels of these genes by quantitative real-time PCR. These NPC2 proteins possessed a conserved motif of six cysteines paired in three disulfide bridges with a coincidence to insect OBPs. The 3D molecular modeling of NbNPC2 showed ligand-binding pocket of NbNPC2-1 was composed of a flexible beta-structure that contributed to binding to a wide range of potential semiochemicals. Three NPC2 genes all possessed one signal peptide revealed their specific character as secretory proteins. Three NPC2 genes in N. barkeri displayed the highest expression levels in adult males, suggesting a putative role in detecting of the female sex pheromones and highly specific binding to female sex pheromone. The expression levels were lowest in the stage of eggs and increased with developmental stage growing, indicating that NPC2 genes were continuously expressed in post-embryonic stages. NbNPC2-1 in the stage of adult females was expressed 6 and 4 times higher than that of larva and nymph, suggesting NbNPC2-1 might contribute an important role in odor recognition of adult females to hunt for suitable areas for the oviposition and predation. These results showed that NPC2 genes might be involved in chemical communications of adult males, and that NbNPC2-1 preformed specified olfactory functions in adult females. This study provided a foundation for research to determine the roles of NPC2 proteins acting as odorant carrier and involved in chemical communication in phytoseiid mites.