Pheromone binding proteins (PBPs) are small helical proteins ( approximately 13-17 kDa) present in several sensory organs from moth and other insect species. They are involved in the transport of pheromones from the sensillar lymph to the olfactory receptors. We report here the crystal structure of a PBP (Amel-ASP1) originating from the honey-bee (Apis mellifera) antennae and expressed as recombinant protein in the yeast Pichia pastoris. Crystals of Amel-ASP1 were obtained at pH 5.5 using the nano-drops technique of crystallization with a novel optimization procedure, and the structure was solved initially with the single-wavelength anomalous diffraction technique using sulfur anomalous dispersion. The structure of Amel-ASP1 has been refined at 1.6-A resolution. Its fold is roughly similar to that of other PBP/odorant binding proteins, presenting six helices and three disulfide bridges. Contrary to the PBPs from Bombyx mori (Sandler, B. H., Nikonova, L., Leal, W. S., and Clardy, J. (2000) Chem. Biol. 7, 143-151) and Leucophea maderae (Lartigue, A., Gruez, A., Spinelli, S., Riviere, S., Brossut, R., Tegoni, M., and Cambillau, C. (2003) J. Biol. Chem. 278, 30213-30218), the extended C terminus folds into the protein and forms a wall of the internal hydrophobic cavity. Its backbone groups establish two hydrogen bonds with a serendipitous ligand, n-butyl-benzene-sulfonamide, an additive used in plastics. This mode of binding might, however, mimic that used by one of the pheromonal blend components and illustrates the binding versatility of PBPs.