How Mammals Taste Sour

Nature is reporting (subscribers only) that researchers have identified how mammal tongues taste sour (although they are still not sure how or why they came to be). Taste in mammals has five classifications: sweet, salty, bitter, sour and umami. Until now, however, only the sweet, bitter and umami taste receptors had been identified, and researchers were unsure whether the other two tastes even had specialized receptors.

To find the sour receptor, researchers identified all 10,000 proteins in the mouse genome that exist in cell membranes (and can, therefore, pick up signals from the outside world and transmit them to nerves). This list was narrowed down by eliminating those not found in the tongue (which left 900) and then those known to exist in other taste receptors, leaving a single protein called PKD2L1 as a prime candidate. Genetically engineered mice with no live PKD2L1 cells showed no neural activity in response to sour foods, confirming the find.

Researchers also stumbled across some interesting finds in their search. These sour receptors are also found in the spinal chord, possibly to aid in monitoring the acidity of the nervous system, and there is a "delay between the introduction of acid and the cells firing off a 'sour' signal ... [indicating] something else might be going on inside the mouth to help mammals identify the taste."

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