In this issue (Nature 416, 199–202; 2002), Greg Nelson and colleagues begin to solve the conundrum. They have characterized a mammalian amino-acid taste receptor and find that, surprisingly, this one receptor responds to several of the 20 l-amino acids that are commonly found in proteins. In another clever move by nature, the receptor is entirely unresponsive to their mirror images — the biologically less significant d-amino acids.
The amino-acid receptor is located on the surface of taste cells and is coupled to well-known molecular switches called G proteins. In fact, it consists of two different, taste-specific G-protein-coupled receptors (GPCRs), T1R1 and T1R3. Nelson et al. developed a cell-based assay to identify candidate taste receptors, and found that cells expressing both T1R1 and T1R3, together with a 'promiscuous' G protein that causes the release of calcium from internal cellular stores, responded to the application of different l-amino acids with an increase in intracellular calcium concentration. Moreover, a substance that enhances the tongue's response to l-amino acids — inosine monophosphate — increased the response of the T1R1+3 receptor in vitro and in vivo.
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