Although understanding of the olfactory system has progressed at the level of downstream receptor signaling and the wiring of olfactory neurons, the system remains poorly understood at the molecular level of the receptors and their interaction with and recognition of odorant ligands. The structure and functional mechanisms of these receptors still remain a tantalizing enigma, because numerous previous attempts at the large-scale production of functional olfactory receptors (ORs) have not been successful to date. To investigate the elusive biochemistry and molecular mechanisms of olfaction, we have developed a mammalian expression system for the large-scale production and purification of a functional OR protein in milligram quantities. Here, we report the study of human OR17-4 (hOR17-4) purified from a HEK293S tetracycline-inducible system. Scale-up of production yield was achieved through suspension culture in a bioreactor, which enabled the preparation of >10 mg of monomeric hOR17-4 receptor after immunoaffinity and size exclusion chromatography, with expression yields reaching 3 mg/L of culture medium. Several key post-translational modifications were identified using MS, and CD spectroscopy showed the receptor to be Ϸ50% ␣-helix, similar to other recently determined G protein-coupled receptor structures. Detergent-solubilized hOR17-4 specifically bound its known activating odorants lilial and floralozone in vitro, as measured by surface plasmon resonance. The hOR17-4 also recognized specific odorants in heterologous cells as determined by calcium ion mobilization. Our system is feasible for the production of large quantities of OR necessary for structural and functional analyses and research into OR biosensor devices. BiacoreA100 ͉ detergent screen ͉ fos-choline 14 ͉ G protein-coupled receptor purification ͉ membrane protein A nimal noses have evolved the ability to rapidly detect a seemingly infinite array of odors at minute concentrations. The basis of this sensitivity are the olfactory (smell) receptors, a large class of G protein-coupled receptors (GPCRs) that function together combinatorially to allow discrimination between a wide range of volatile and soluble molecules (1, 2). As GPCRs, all olfactory receptors (ORs) are integral membrane proteins with 7 predicted transmembrane domains. To date, crystal structures exist for only 5 GPCR proteins (3). Despite the fact that ORs represent the largest class of known membrane proteins, no detailed structure exists for any OR, because the major obstacle to structural and functional studies on membrane proteins is the notorious difficulty involved in expressing and purifying the large quantities of receptor protein sample required for techniques such as X-ray crystallography. The first crucial step to enable such pivotal biochemical and structural analyses is to engineer systems with the capacity to produce and purify milligram quantities of an OR. hOR17-4 (alternately known as OR1D2) is of particular interest because, in addition to olfactory neurons, it is exp...

High-level production of membrane proteins, particularly of G protein-coupled receptors (GPCRs) in heterologous cell systems encounters a number of difficulties from their inherent hydrophobicity in their transmembrane domains, which frequently cause protein aggregation and cytotoxicity and thus reduce the protein yield. Recent advances in cell-free protein synthesis circumvent those problems to produce membrane proteins with a yield sometimes exceeding the cell-based approach. Here, we report cell-free production of a human olfactory receptor 17-4 (hOR17-4) using the wheat germ extract. Using the simple method, we also successful produced two additional olfactory receptors. To obtain soluble olfactory receptors and to increase yield, we directly added different detergents in varying concentrations to the cell-free reaction. To identify a purification buffer system that maintained the receptor in a nonaggregated form, we developed a method that uses small-volume size-exclusion column chromatography combined with rapid and sensitive dot-blot detection. Different buffer components including salt concentration, various detergents and detergent concentration, and reducing agent and its concentrations were evaluated for their ability to maintain the cell-free produced protein stable and nonaggregated. The purified olfactory receptor displays a typical a ␣-helical CD spectrum. Surface plasmon resonance measurements were used to show binding of a known ligand undecanal to hOR17-4. Our approach to produce a high yield of purified olfactory receptor is a milestone toward obtaining a large quantity of olfactory receptors for designing bionic sensors. Furthermore, this simple approach may be broadly useful not only for other classes of GPCRs but also for other membrane proteins.detergent screen ͉ G protein-coupled receptor purification ͉ membrane protein ͉ odorant interaction ͉ surface plasmon resonance