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

Odorant receptors (ORs) comprise the largest family of G-protein-coupled receptors (GPCRs). They are located in the nasal epithelium, at the ciliated surface of olfactory sensory neurones, where the initial steps of the olfactory transduction cascade occur. ORs are encoded by a large and diverse multi-gene family, which has been characterized in cyclostomes, teleosts, amphibia, birds and mammals, as well as in Drosophila and Caenorhabditis elegans. Here, the range of diversity in OR and chemoreceptor structure is examined, noting that their functions are fundamentally similar to those of many neurotransmitter or neurohormone receptors. It is argued that ORs have emerged directly from other GPCRs independently in many species. According to this 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.