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Coexpression of Two Odorant-Binding Protein Homologs inDrosophila: Implications for Olfactory Coding

Odorant-binding proteins (OBPs) are small soluble proteins present in the aqueous medium surrounding olfactory receptor neurons. Their function in olfaction is still 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. In this study we examine the genomic organization and expression patterns of twoolfactory-specific genes (OS-E and OS-F) ofDrosophila melanogaster, the products of which are members of a protein family in Drosophila sharing sequence similarity with moth OBPs. We show that theOS-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 restricted spatially 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 ∼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. The functional significance of the observed expression pattern and its implications for olfactory coding are discussed.