Engineered odorant receptors illuminate structural principles of odor discrimination.

A central challenge in olfaction is understanding how the olfactory system detects and distinguishes odorants with diverse physicochemical properties and molecular configurations. Vertebrate animals perceive odors via G protein-coupled odorant receptors (ORs). In humans, ~400 ORs enable the sense of smell. The OR family is composed of two major classes: Class I ORs are tuned to carboxylic acids while Class II ORs, representing the vast majority of the human repertoire, respond to a wide variety of odorants. How ORs recognize chemically diverse odorants remains poorly understood. A fundamental bottleneck is the inability to visualize odorant binding to ORs. Here, we uncover fundamental molecular properties of odorant-OR interactions by employing engineered ORs crafted using a consensus protein design strategy. Because such consensus ORs (consORs) are derived from the 17 major subfamilies of human ORs, they provide a template for modeling individual native ORs with high sequence and structural homology. The biochemical tractability of consORs enabled four cryoEM structures of distinct consORs with unique ligand recognition properties. The structure of a Class I consOR, consOR51, showed high structural similarity to the native human receptor OR51E2 and yielded a homology model of a related member of the human OR51 family with high predictive power. Structures of three Class II consORs revealed distinct modes of odorant-binding and activation mechanisms between Class I and Class II ORs. Thus, the structures of consORs lay the groundwork for understanding molecular recognition of odorants by the OR superfamily.

Development of an odorant sensor with a cell-free synthesized olfactory receptor and a graphene field-effect transistor.

Animals sense odorants using olfactory receptors. Many trials have been conducted to develop artificial odorant sensors using olfactory receptors. However, the development has been hindered by the difficulty in obtaining olfactory receptors. In this study, we expressed an olfactory receptor, cOR52, using a wheat germ cell-free synthesis system. The functionality of the expressed cOR52 was confirmed by ligand concentration-dependent interactions with the mini-G protein. The expressed cOR52 was immobilized on a graphene field-effect transistor. The cOR52-modified graphene field-effect transistor exhibited a ligand-specific response between 100 nM and 100 µM. This approach seems to be applicable for other olfactory receptors. Therefore, it will be possible to develop an odorant sensor equipped with various olfactory receptors by this method.