How scientist Emily Liman revealed a powerful new kind of taste
Sweet, salty, sour, bitter, umami: Meet ammonium chloride! And as Liman explains, the more we learn about new bodily functions, the more we learn about our bodies—and how to keep them healthy.
In the late 1980s, when Emily Liman was starting her career, scientists were making huge breakthroughs in the field of perception where the mysteries of human senses were being solved. Molecular biologist and neuroscientist Jeremy Nathans had just identified the genes that govern our ability to see color. And biologist Linda Buck, with whom Liman would one day study, was doing groundbreaking research on odor receptors in the nose that would later win her the Nobel Prize. Initially, Liman gravitated toward the study of sight. “I was just taken with looking at rainbows and thinking about color vision,” she recalls. “Do we all experience it the same?” Ultimately, though, she was drawn to study taste, because there were so many unknowns for her to investigate. As Liman says, “It was basically the Wild West.”
At the time, the four established tastes—sweet, sour, salty, and bitter—had just been officially joined by a fifth: savory, or umami. Now Liman’s lab at the University of Southern California is raising the question of whether there may be yet another basic taste. She has discovered that the same protein sensor that detects sour flavors also responds to a different taste: ammonium chloride. It’s the ingredient that gives Nordic salt licorice its astringent, salty-sweet flavor.
“Ammonium is actually super important to be able to detect,” Liman explains. In large amounts, it’s toxic. Humans are unlikely to stumble across large amounts of ammonium—it appears in fertilizer and certain animal waste—but “an animal in its environment may come across something like decaying material and need to avoid it,” she says.
The broader impact of Liman’s discovery extends beyond our system of taste. When we ingest something, cells in our taste buds called sensory receptors send signals to the brain. In 2018, Liman and her team discovered that the receptor for sour taste is a critical protein known as OTOP1, of the otopetrin family of proteins, which also appears in the inner ear and helps form the tiny calcium carbonate crystals we need for balance, and in brown adipose tissue, or the “good fat” that helps your body keep warm. “We were quite shocked,” Liman says of the discovery. She was equally surprised, last year, when her team revealed that OTOP1 is also the receptor for ammonium chloride.
Armed with that knowledge, Liman and her fellow researchers are now trying to understand the structure of OTOP proteins and how to modify them. For instance, levels of OTOP2 in the colon have been linked to “the severity and progression of colon cancer,” Liman says. Greater knowledge of the OTOP proteins could help with the development of therapeutics for disease. “Whenever we find a molecule in the body that has sort of a new function,” she says, we can better “understand how our body works and how we can manipulate it.”
