Using GEFs to Deorphanize Odorant Receptors

Odorant perception is initiated when odorants activate unique combinations of odorant receptors (ORs) expressed in the cilia of olfactory sensory neurons in the nose (Buck and Axel 1991; Malnic et al. 1999). One of the greatest challenges in the olfactory science is to correlate ORs with their ligands (to deorphanize the ORs). The determination of OR–ligand pairs should reveal how the OR family is used to generate diverse odorant perceptions (Keller et al. 2007, Menashi et al. 2007). However, these studies are complicated by the poor functional expression of the ORs in heterologous cells because they are retained in the endoplasmic reticulum and cannot reach the plasmatic membrane (McClintock and Sammeta 2003). Consequently, a very small number of mammalian ORs have been linked to odorants they recognize to date (Malnic 2007). Methods that enhance OR trafficking to the cell membrane have been successfully used to obtain functional expression of ORs in heterologous cells. The fusion of the 20 N-terminal amino acids of rhodopsin (rho tag) to the N-terminal region of ORs facilitates cell surface expression of the receptors (Krautwurst et al. 1998). Also, coexpression with the receptor transporting proteins (RTPs, Saito et al. 2004) enhances surface expression of ORs in HEK293 cells.