6533b828fe1ef96bd12895e5

RESEARCH PRODUCT

Role of odorant-binding proteins in Drosophila melanogaster chemosensory perception

subject

description

Chemoperception is used by animals to detect nutritive food and avoid toxic compounds. It also allows animals to identify suitable ecological niche and mating partners. Like many other insects, Drosophila melanogaster possesses a very sensitive chemosensory ability and can detect and discriminate a wide panel of semiochemicals. Chemosensory detection is mostly mediated by olfactory and gustatory systems involving several multigene chemoreceptor families. Volatile and non-volatile chemical compounds entering the sensory organ (sensillum) must be solubilized before being transported through the hydrophilic sensillum lymph bathing the dendrites of chemosensory neurons. These perireceptor events involve a family of soluble proteins named odorant-binding proteins (OBPs). Despite the fact that OBPs were initially found in olfactory sensilla, some OBPs are also expressed in gustatory sensilla. While their physiological roles in olfaction and gustation remain unclear, many studies suggest that OBPs transport lipophilic chemicals. The relatively low affinity of OBPs for odorants and their high abundance in the sensillum lymph both suggest that OBPs can bind, solubilize and transport hydrophobic stimuli to the chemoreceptors across the aqueous sensilla lymph. In addition to this broadly accepted “transporter role” hypothesis, OBPs have also been proposed to buffer sudden changes in odorant levels and to be involved in hygroreception. The role of OBP49a was recently shown in taste: this OBP, expressed in the gustatory system, is required to detect some bitter compounds. However, the role of OBPs in perireceptor events remains largely unknown. The main goal of my thesis project consisted to investigate the involvement of OBPs in the smell and taste sensory modalities using a multi-faceted approach in Drosophila melanogaster.My first research axis consisted to better understand the role of OBPs in the perception of food compounds by using both in vitro and in vivo approaches of OBPs expressed in the gustatory appendages of D. melanogaster adults. After identifying by q-PCR the OBPs expressed in gustatory appendages, we produced them using a heterologous yeast expression system. Then, the binding properties of the recombinant purified OBP were investigated. Our binding assay screen revealed that the taste-expressed OBP19b is able to bind some amino acids. The expression of OBP19b was mapped in specific accessory cells in a subset of proboscis sensilla. This OBP was also expressed in the digestive tract and in some internal reproductive organs. The comparison of behavioural and single-taste sensilla responses between transgenic variants and control flies supported our finding that OBP19b is indeed involved in the detection of some amino acids. Finally, the comparison between various dipteran insects of the OBP19b-like protein coding sequence indicates the relatively high conservation of this protein suggesting its critical role in food search. The second research axis of my PhD thesis focused on the olfactory role of OBP28a. OBP28a was previously shown to be highly expressed in the Drosophila antennae and proposed to buffer quantitative odour variations. To better understand the physiological role of this OBP, and in collaboration with different members of the team, we used structural, genetic, biochemical, behavioural and electrophysiological methods to better understand the role of this OBP. OBP28a was first heterologously expressed and purified. The folding of OBP28a was then determined and the protein was crystallized. The study of the binding properties of OBP28a revealed that it can bind floral compounds such as β-ionone. Behavioural and electrophysiological recordings supported the physiological role of OBP28a in β-ionone detection. In summary, this PhD thesis reveals novel roles of two OBPs in perireceptor chemoreception: OBP28a in the detection of floral compounds and OBP19b in the detection of some amino acids.