Sweet success

Receptors on the surface of the tongue's specialized taste bud cells allow us to distinguish the five classes of taste: sweet, sour, bitter, salty and umami. Receptors responding to bitter tastes are members of the G-protein-coupled receptor (GPCR) family, but mammalian sweet receptors have not been found. Two papers, published in the May issues of Nature Genetics and Nature Neuroscience, report the cloning of a candidate taste receptor gene, T1r3, which is linked to the Sac locus that determines mouse sensitivity to sweet tastants. Both groups scoured the human genomic sequence around chromosome 1p36, syntenic to the Sac locus on the distal end of mouse chromosome 4, in search of GPCR genes. They identified one gene that encodes a receptor with seven transmembrane helices and a large extracellular domain, and is similar to T1R1 and T1R2, two 'orphan' GPCRs (with no known ligands) that are expressed on taste cells. T1R3 ...