The same protein in your tongue that allows you to taste sour sweets and citric fruit, also enables you to sense gravity. And it has been with us throughout evolution.
Ion channels are a super-family of proteins that exist in the membranes of pretty-much all cells in the body. They enable our cells to interact and respond with their extracellular environment by allowing the movement of tiny charged ions across their membranes.
One type of ion channel that allows protons (H+ ions) to move across the cell membrane has been shown to be important for taste sensation.
The sour point: the acid-sensitive channel identity
Emily Lyman's group at the University of Southern California have been working on the ionic mechanisms of taste for several years. Their research identified a proton channel that underlies the sour taste experienced when eating or drinking low-pH food and drink, like sour sweets, citric fruit and carbonated drinks. However, they did not know the genetic identity of this proton channel, until now.
Why do things taste sour? - Science on the Web #97 by How Stuff Works
By mining this data they could identify the taste-receptor genes being actively transcribed in the taste cells. This transcriptional analysis of mouse taste receptors enabled identification of the proton channel responsible for sour-taste sensation.
The group then validated the identity of the sour-taste receptor by injecting the DNA from the enriched genes into frog's eggs. This is a routine scientific method of screening, in which the frog's eggs produce the proteins on their surface and the researchers can then explore their function. By increasing the acidity of the solution bathing the frog's eggs the researchers measured the electrical changes in the egg cells.
Cells that had been injected with the Otop gene became electrically more positive as the proton channels expressed on their membranes opened in response to the increased acidity and protons moved inside the cells.
Armed with this genetic and functional evidence, the group identified the proton channel gene as Otop1.
Interestingly, this channel is expressed in cells throughout the body from fat cells to heart cells. It is particularly important for the correct development of otoconia, sensors of gravity and motion in the balance-sensing areas of our inner ears. However, the protein is structurally unrelated to other ion channels, especially other known proton channels in the body. Despite this un-relatedness, the authors state that it must be an important protein as it is evolutionarily conserved in nematode worms and fruit flies.
Tu, Y., Cooper, A. J., Teng, B., Chang, R. B., Artiga, D. J., Turner, H. N., . . . Liman, E. R. (2018). An evolutionarily conserved gene family encodes proton-selective ion channels. Science. doi:10.1126/science.aao3264