According to the lead scientist on the study, Dr. Hiroaki Matsunami at Duke University Medical Center, this is the first study to define how humans perceive sour taste.
The identification of these proteins, called PKD1L3 and PKD2L1, could lead to ways to manipulate the perception of taste in order to fool the mouth that something sour, such as some children's medicines or health foods, tastes sweet, he said.
Previous research into taste has revealed that the human tongue has about 10,000 taste buds with five taste sensations: sweet, bitter, and umami, which work with a signal through a G-protein coupled receptor; while salty and sour have been proposed to work with ion channels.
The science behind bitter, sweet, and umami tastes has been extensively studied, say the researchers, but understanding of sour and salty taste perception is poor and even confusing.
Matsunami and his team used fluorescent tags to label the subsets of cells that are known to be responsible for bitter, sweet, and umami taste, as well as the subsets of cells that express PKD1L3 and PKD2L1.
The two proteins are said to combine to form "ion channels" that control the flow of calcium ions, in and out of the taste cells. This flow of ions controls the electrical signals that are sent from the cells to the brain in response to various stimuli.
The results are published in the Proceedings of the National Academy of Sciences (Vol. 103, pp. 12569-12574).
The researchers stimulated mouse cells expressing PDK1L3 and PKD2L1 with various taste chemicals to identify which stimuli caused a response in the ion channels. This was achieved by loading the cells with calcium-sensitive fluorescent dyes - one dye glowed green in response to high calcium concentrations, the other that glowed red when the concentration was low.
In the presence of sour acids, the researchers report a change of colour from red to green, which indicated that the ion channel had opened in response to the sour taste.
No colour change was recorded in the presence of salty, sweet, or bitter solutions.
"Our study showed that PKD1L3- and PKD2L1-expressing cells are segregated from bitter, sweet, and umami receptor-expressing taste cells, raising the possibility that a subset of cells may be 'labelled' as sour-sensing cells," wrote lead author Yoshiro Ishimaru.
Ishimaru noted however that the suggestive role for PKD1L3 and PKD2L1 in sour taste perception does not rule the possibility that other receptors may be active in sensing sour tastes.
The next step, said Matsunami in a statement, is to use this finding to screen for chemicals that can block the function of these sour taste cells. The research also could lead to a better understanding of how the sense of taste functions neurologically, he said. "We still do not know what is happening in the brain - that is, exactly how the brain would interpret the signals coming from the tongue to tell the difference between lemons and lemonade," Matsunami said.
Further studies using animals with PKD1L3 and PKD2L1 'knocked out' (the genes will not code for the proteins of the same name) will help to further define the role of these proteins, said the researchers.
Dr. Matsunami told FoodNavigator.com: "In terms of new flavours, one thing that may be possible in the future is to conduct a search from much more candidate compounds, as initial screening for taste enhancers or inhibitors can be done using cell cultures in the laboratory rather than relying on human subjects."
Bitter chemicals are said to be detected by 30 or so so-called T2R receptors. Sugars and sweet flavours are detected by T1R2 and T1R3 receptors, and umami tasting L-amino acids are reported to be detected by T1R1 and T1R3 receptors.
Contrary to popular understanding, taste is not experienced on different parts of the tongue. Though there are small differences in sensation, which can be measured with highly specific instruments, all taste buds, essentially clusters of 50 to 100 cells, can respond to all types of taste.