Abstract
Olfactory receptors (ORs) are G protein-coupled receptors that detect odorants in the olfactory epithelium, and comprise the largest gene family in the genome. Identification of OR ligands typically requires OR surface expression in heterologous cells; however, ORs rarely traffic to the cell surface when exogenously expressed. Therefore, most ORs are orphan receptors with no known ligands. To date, studies have utilized non-cleavable rhodopsin (Rho) tags and/or chaperones (i.e. Receptor Transporting Protein, RTP1S, Ric8b and Gαolf) to improve surface expression. However, even with these tools, many ORs still fail to reach the cell surface. We used a test set of fifteen ORs to examine the effect of a cleavable leucine-rich signal peptide sequence (Lucy tag) on OR surface expression in HEK293T cells. We report here that the addition of the Lucy tag to the N-terminus increases the number of ORs reaching the cell surface to 7 of the 15 ORs (as compared to 3/15 without Rho or Lucy tags). Moreover, when ORs tagged with both Lucy and Rho were co-expressed with previously reported chaperones (RTP1S, Ric8b and Gαolf), we observed surface expression for all 15 receptors examined. In fact, two-thirds of Lucy-tagged ORs are able to reach the cell surface synergistically with chaperones even when the Rho tag is removed (10/15 ORs), allowing for the potential assessment of OR function with only an 8-amino acid Flag tag on the mature protein. As expected for a signal peptide, the Lucy tag was cleaved from the mature protein and did not alter OR-ligand binding and signaling. Our studies demonstrate that widespread surface expression of ORs can be achieved in HEK293T cells, providing promise for future large-scale deorphanization studies.
Highlights
Olfactory receptors (ORs) are seven transmembrane domain G protein-coupled receptors (GPCRs) that govern the sense of smell in the olfactory epithelium, and comprise the largest gene family in the genome (~1000 or Lucy tags (OR) genes in mice [1] and ~300 [2] in humans)
Non-permeabilized HEK 293T cells were surface labeled with a Flag antibody to detect membrane associated receptor and OR surface expression was scored based on detectable surface immunofluorescence
When combined with RTP1S, Ric8b and Gαolf, we found that all 15 of the Lucy-ORs that we examined successfully trafficked to the surface, providing promise for future deorphanization and other functional studies
Summary
Olfactory receptors (ORs) are seven transmembrane domain G protein-coupled receptors (GPCRs) that govern the sense of smell in the olfactory epithelium, and comprise the largest gene family in the genome (~1000 OR genes in mice [1] and ~300 [2] in humans). This family was first identified over 20 years ago [3], the majority of ORs remain orphan receptors, with no known ligand. Many ORs are still unable to reach the cell surface when heterologously expressed, and remain as orphan receptors
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