Olfactory Coding in a Compound Nose: Coexpression of Odorant-Binding Proteins in Drosophilaa

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Abstract
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 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. This finding has potential implications for the roles of odorant‐binding proteins in olfactory coding.