Crystal structures and binding dynamics of odorant-binding protein 3 from two aphid species Megoura viciae and Nasonovia ribisnigri.

Odorant-binding proteins (OBP) are believed to capture and transport semiochemicals to their olfactory receptors. The OBP structures have been obtained from Diptera (mosquitos and fruitflies), Hymenoptra (honey bee), Lepidoptra (moths), Orthoptera (locust) and Dictyoptera (cockroach), but to date there are no such structures for aphid OBPs. Indeed there are no reports of an OBP 3D structure for any Hemipteran, which comprise around 50,000-80,000 species of aphids, planthoppers, leafhoppers, shield bugs, and other crop pests despite there being more than 300 hemipteran OBPs reported in NCBI GenBank. Here, we report the first aphid OBP crystal structures and examine their molecular interactions with the biological active alarm pheromone components using biochemical and structural approaches. Our study reveals some unique structural features and ligand binding mechanisms: 1) the ligand binding site is not internal; 2) a striking groove is present in the surface of the proteins as a putative binding site; 3) the N-terminus occupies the site more commonly filled by the C-terminus closing off the conventional OBP pocket. The results from fluorescent binding assays, molecular docking and molecular dynamics demonstrate that OBP3 from Megoura viciae Buckton can bind to all four alarm pheromone components. We will demostrate that the differential ligand binding between these very similar OBP3s from the two aphid species is determined mainly by the direct π-π interactions between ligands and the aromatic residues of OBP3s in the binding pocket.