Trafficking of Mammalian Chemosensory Receptors by Receptor‐transporting Proteins

Abstract

Although mammalian odorant receptors (ORs) were identified over 15 years ago, we still do not understand how odorant molecules interact with ORs at a molecular level. Previous studies of mammalian ORs have tested small numbers of ORs against large numbers of odorants. Some fundamental properties of the olfactory system, however, require investigation of a wide panel of diverse ORs with a large number of chemically diverse odorants. Previously, we identified OR accessory proteins, RTP1 and RTP2. They are expressed specifically in olfactory neurons, are associated with OR proteins and facilitate the OR trafficking to the plasma membrane when coexpressed in mammalian cell lines. Using this approach, high-throughput screening using a large repertoire of mammalian ORs is now possible. The activation profiles can be used to develop a predictive model relating physicochemical odorant properties, receptor sequences, and their interactions, enabling us to predict a tested receptor's response to a novel odorant and a novel receptor's response to a tested odorant. This will provide a basis for understanding how structurally diverse odorant molecules activate the mammalian OR repertoire. Similarly, two families of vomeronasal receptors, V1Rs and V2Rs, are also notoriously difficult to functionally express in heterologous cells. However, coexpression of the RTP family members with V1Rs or V2Rs does not seem to facilitate trafficking of the receptor proteins. This suggests that the vomeronasal organ has a unique biosynthetic pathway for membrane proteins.Since mammalian odorant receptors (ORs) were identified over 15 years ago, many chemosensory (olfactory and taste) receptors have been identified. These include ORs and TAARs expressed by the olfactory sensory neurons in the olfactory epithelium, V1Rs and V2R expressed in the vomeronasal organ, T1Rs and T2Rs expressed in the taste buds [1][2][3][4][5][6][7][8][9]. These are seven transmembrane, G-protein coupled receptors. Insects use a completely different set of chemoreceptors, the Ors and the Grs, to detect chemicals [10][11][12][13][14]. These proteins likely function primarily as ligand-gated channels [15,16].The vast majority of chemosensory receptors must form a complex with a partner protein to functionally respond to stimuli. Taste receptors T1R1, T1R2 and T1R3 show little response to taste stimuli when expressed alone in a heterologous system, however coexpression of T1R2 and T1R3 results in the formation of a sweet taste receptor and coexpression of T1R1 and T1R3 results in the formation of an umami (l-amino acid) taste receptor [17][18][19]. Similarly, candidate sour taste receptors PKD2L1 and PKD1L3 need to be coexpressed in the same cells to be efficiently trafficked to the cell surface in HEK293T cells [20]. In the case of V2Rs, a nonclassical MHC class 1b molecules and ß2-microglobulin may form a complex with some V2Rs [21]. Finally, in insect olfaction, all Ors require Or83b as a partner for proper trafficking and function [22].Following this pattern,...

Co-Opting the Unfolded Protein Response to Elicit Olfactory Receptor Feedback

“…ORs may be highly enriched for select peptides acting as Perk ligands. The maintenance of these peptide ligands should aid in the initiation of feedback at the cost of immediate ER export, explaining why ORs, and chemoreceptors in general, tend to be retained in the ER or to be degraded upon heterologous expression, unlike most other GPCRs (Matsunami, 2005; Matsunami et al, 2009; Saito et al, 2004; Zhuang and Matsunami, 2007). The tendency of OR proteins to induce ER-stress may be also amplified by the extreme expression levels of OR genes in OSNs, providing an additional, quantitative trigger for UPR induction by OR proteins.…”

Ultrasensitive Detection of Amines by a Trace Amine-Associated Receptor

“…OSNs likely have specialized mechanisms for proper expression and localization of receptors (Matsunami et al, 2009). The readout of TAAR activation may be more sensitive in OSNs because the receptors are localized at high density in cilia and are coupled to the endogenous signal transduction cascade.…”

The Membrane Proteome of Sensory Cilia to the Depth of Olfactory Receptors

“…In addition to 62 ORs, we also identified factors enhancing OR expression at the surface (REEP1, RPT1, and RTP2) (77,78) and different subunits of PKA attenuating OR activity by phosphorylation (79).…”