Odorant-binding proteins (OBPs) are small abundant extracellular proteins belonging to the lipocalin superfamily. They are thought to participate in perireceptor events of odor detection by carrying, deactivating, and/or selecting odorant molecules. Putative human OBP genes (hOBP) have recently been described [Lacazette et al. (2000) Hum. Mol. Genet. 9, 289-301], but the presence of the corresponding proteins remained to be established in the human olfactory mucus. This paper reports the first evidence of such expression in the mucus covering the olfactory cleft, where the sensory olfactory epithelium is located. On the contrary, hOBPs were not observed in the nasal mucus covering the septum and the lower turbinate. To demonstrate the odorant binding activity of these proteins, a corresponding recombinant protein variant, hOBP(IIa)(alpha), was secreted by the yeast Pichia pastoris and thoroughly characterized. It appears as a monomer with one disulfide bond located between C59 and C151, a conservative feature of all other vertebrate OBPs. By measuring the displacement of several fluorescent probes, we show that hOBP(IIa)(alpha) is able to bind numerous odorants of diverse chemical structures, with a higher affinity for aldehydes and large fatty acids. A computed 3D model of hOBP(IIa)(alpha) is proposed and reveals that two lysyl residues of the binding pocket may account for the increased affinity for aldehydes. The relatively limited specificity of hOBP(IIa)(alpha) suggests that other human OBPs are expected to take into account the large diversity of odorant molecules.

Chemosensory proteins (CSPs) are ubiquitous soluble small proteins isolated from sensory organs of a wide range of insect species, which are believed to be involved in chemical communication. We report the cloning of a honeybee CSP gene called ASP3c, as well as the structural and functional characterization of the encoded protein. The protein was heterologously secreted by the yeast Pichia pastoris using the native signal peptide. ASP3c disulfide bonds were assigned after trypsinolysis followed by chromatography and mass spectrometry combined with microsequencing. The pairing (Cys(I)–Cys(II), Cys(III)–Cys(IV)) was found to be identical to that of Schistocerca gregaria CSPs, suggesting that this pattern occurs commonly throughout the insect CSPs. CD measurements revealed that ASP3c mainly consists of α‐helices, like other insect CSPs. Gel filtration analysis showed that ASP3c is monomeric at neutral pH. Using ASA, a fluorescent fatty acid anthroyloxy analogue as a probe, ASP3c was shown to bind specifically to large fatty acids and ester derivatives, which are brood pheromone components, in the micromolar range. It was unable to bind tested general odorants and other tested pheromones (sexual and nonsexual). This is the first report on a natural pheromonal ligand bound by a recombinant CSP with a measured affinity constant.

No abstract