Odorant-binding proteins mutants having novel binding properties

Odorant-binding proteins (OBP) are small soluble proteins present in the nasal mucus covering the olfactory epithelium. Vertebrate OBPs belong to the lipocalin superfamily, whose members share a common scaffold made of 8-stranded β-barrel. This folding pattern defines a central apolar cavity, named calyx, whose role is to bind hydrophobic molecules such as odorants. Although the physiological role of OBPs is not clearly established, they are supposed to carry odorants from the air to olfactory receptors through the aqueous nasal mucus. OBPs have been described in numerous species including pig, rat and human beings. OBPs are broadly tuned and bind a large spectrum of volatile molecules. Interestingly, it has been shown that the three rat OBP subtypes (rOBP1, rOBP2, rOBP3) have different and complementary ligand properties [1], suggesting that OBPs are involved in odorant discrimination. Protein sequence alignment of the three rat OBPs reveals the presence of an amino acid residue located in the binding pocket, which may be important for guiding binding specificity. Using site-directed mutagenesis, we generated variants of rOBP3, in which this amino acid residue has been substituted. Using isothermal titration calorimetry, we found that some substitutions decreased the affinity of rOBP3 towards some odorant molecules while others generated OBPs possessing novel binding properties. Our work gives new elements to understand the binding mechanisms of OBPs and opens the way towards technological applications based on OBP, as odorant biosensors. [1] D. Löbel, M. Jacob, M. Volkner, H. Breer, Odorants of different chemical classes interact with distinct odorant binding protein subtypes, Chem. Senses, 27 (2002) 39-44