Aphids use chemical cues to locate hosts and find mates. The vetch aphid Megoura viciae feeds exclusively
on the Fabaceae, whereas the currant-lettuce aphid Nasonovia ribisnigri alternates hosts between the
Grossulariaceae and Asteraceae. Both species use alarm pheromones to warn of dangers. For N. ribisnigri this
pheromone is a single component (E)-β-farnesene but M. viciae uses a mixture of (E)-β-farnesene, (-)-α-
pinene, β-pinene, and limonene. Odorant-binding proteins (OBP) are believed to capture and transport such
semiochemicals to their receptors. Here, we report the first aphid OBP crystal structures and examine their
molecular interactions with the alarm pheromone components. Our study reveals some unique structural
features: 1) the lack of internal ligand binding site; 2) a striking groove in the surface of the proteins as a
putative binding site; 3) the N-terminus rather than the C-terminus occupies the site closing off the
conventional OBP pocket. The results from fluorescent binding assays, molecular docking and dynamics
demonstrate that OBP3 from M. viciae can bind to all four alarm pheromone components and 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