Olfactory coding in a compound nose. Coexpression of odorant-binding proteins in Drosophila

Odorant-binding proteins (OBPs) are small, soluble proteins present in the aqueous medium surrounding olfactory receptor neurons. Their function in olfaction is unknown: they have been proposed to facilitate the transit of hydrophobic molecules to olfactory receptors, to deactivate the odorant stimulus, and/or to play a role in chemosensory coding. We have examined the genomic organization and expression patterns of two olfactory-specific genes (OS-E and OS-F) of Drosophila melanogaster, the products of which are members of a protein family in Drosophila sharing sequence similarity with moth OBPs. We found that the OS-E and OS-F transcription units are located < 1 kb apart. They are oriented in the same direction and display a similar intron-exon organization. Expression of both OS-E and OS-F proteins is spatially restricted to the ventrolateral region of the Drosophila antenna. Within this region, both OS-E and OS-F proteins are expressed within two different types of sensory hairs: in most, if not all, sensilla trichodea and in approximately 40% of the interspersed small sensilla basiconica. We consistently observe that OS-E and OS-F are coexpressed, indicating that an individual sensillum can contain more than one odorant-binding protein. This finding has potential implications for the roles of odorant-binding proteins in olfactory coding.