Although the receptors for most taste modalities have been identified, the mechanisms by which sour and salty tastes are detected have remained elusive. Ishimaru et al. show that two members of the transient receptor potential (TRP) family, PKD1L3 and PKD2L1, are expressed in particular regions of taste tissue (in situ mRNA hybridization and reverse transcription polymerase chain reaction analysis) and that their expression does not overlap with that of TRPM5, a downstream effector in some forms of taste perception. Antibodies raised against PKD2L1 showed that the protein was present at the taste pore, the predicted location of a taste receptor. Coexpression of tagged versions of the two proteins in human embryonic kidney (HEK) 293 cells showed that both proteins had to be coexpressed for abundant surface expression, and coimmunoprecipitation experiments indicated that the proteins interacted. Electrophysiological analysis and calcium imaging showed that HEK293 cells expressing the two proteins responded to acids, such as citric acid, HCl, and malic acid, with a rapidly inactivating current and with an increase in intracellular calcium concentration. The response did not appear to be mediated solely by a change in pH, because citric acid was more potent at activating the channel than HCl at the same pH.
Y. Ishimaru, H. Inada, M. Kubota, H. Zhuang, M. Tominaga, H. Matsunami, Transient receptor potential family members PKD1L3 and PKD2L1 form a candidate sour taste receptor. Proc. Natl. Acad. Sci. U.S.A. 103, 12569-12574 (2006). [Abstract] [Full Text]