Odorant receptors: a plethora of G-protein-coupled receptors

Odorant receptors (ORs) comprise the largest family of G-protein-coupled re ceptors (GPCRs). They are located in the nasal epithelium, at the ciliated surface of olfactory sensory neurones, where the initial steps of the olfac tory transduction cascade occur. ORs are encoded by a large and diverse mul ti-gene family, which has been characterized in cyclostomes, teleosts, amph ibia, birds and mammals, as well as in Drosophila and Caenorhabditis elegan s. Here, the range of diversity in OR and chemoreceptor structure is examin ed, noting that their functions are fundamentally similar to those of many neurotransmitter or neurohormone receptors. It is argued that ORs have emer ged directly from other GPCRs Independently in many species. According to t his view, there is no structural prerequisite for OR identity and any GPCR has the potential to be or become an OR at a given point in evolution.