Expression patterns of two putative odorant-binding proteins in the olfactory organs of Drosophila melanogaster have different implications for theirfunctions

The aqueous medium bathing the dendrites of olfactory neurons contains high concentrations of odorant-binding proteins (OBPs) whose role is still uncl ear. OBPs may facilitate interactions between odorants and their membrane-b ound receptors, perhaps by increasing the water solubility of hydrophobic m olecules. Alternatively, OBPs may be involved in the inactivation of odoran ts and other volatile molecules, preventing desensitization and/or protecti ng olfactory neurons from toxic chemicals. We report here novel features of the localization of two putative OBPs, PBPRP2 and PBPRP5, that have import ant and different implications for their role in olfaction. Unlike several other putative OBPs of Drosophila melanogaster that are only found in adult olfactory organs, PBPRP5 is also expressed in the larval olfactory organs, suggesting that it plays a common role in olfaction at both stages. In the adult, PBPRP5 expression is restricted to the sensillum lymph that bathes the olfactory dendrites of a subset of olfactory hairs, the basiconic sensi lla, Since individual basiconic sensilla differ in olfactory specificity, P BPRP5 may be able to bind to and mediate olfactory responses to a wide rang e of odorants. In contrast, PBPRP2 is present in the space immediately belo w the antennal cuticle and in the outer cavity of approximately 30% of the double-walled coeloconic sensilla on the antennal surface. In neither case is PBPRP2 in contact with the dendritic membranes of olfactory neurons, mak ing a carrier function unlikely for this protein. Instead, PBPRP2 may act a s a sink, binding to odorants and other volatile chemicals and limiting the ir interactions with olfactory neurons.