Odorant detection using liposome containing olfactory receptor in the SPR system

The binding of odorant to human olfactory receptor (hOR) is the first step of the odor-sensing mechanism in human olfactory system. It is well known that humans recognize and discriminate smell through a signaling cascade initiated by the binding of many different kinds of odorant molecules to approximately 390 different types of hORs but most ORs remain orphans. Hence, the identification of odorant molecules that bind to orphan hORs and the characterization of hORs is required to discover the function of orphan hORs. For this purpose, various heterologous expression systems have been developed but the low expression efficiency of hORs has been an obstacle. Moreover, the cell-based odorant detection system was easily influenced by environmental conditions. In this study, we expressed human olfactory receptor hOR3A1 in an Escherichia coli expression system for high expression efficiency and stable odorant detection. The hOR3A1, which is mainly produced as an inclusion body in E. coli, was partially purified and reconstituted using lipid/detergent mixed micelles for stabilizing the structure of hOR3A1, which is a transmembrane protein. The reconstituted hOR3A1 was immobilized on the gold surface of surface plasmon resonance (SPR), and the function of the odorant detection was analyzed. The results demonstrate that the reconstituted hOR3A1 can detect the specific odorant, helional, in a dose-dependent manner and can also discriminate it from odorants of a similar structure. Such a reconstituted olfactory receptor produced by E. coli expression system, can be useful as an efficient sensing material for the protein-based odorant detection system using various detection devices as well as using SPR.