Human olfactory receptors belong to a large family of proteins known as G protein-coupled receptors (GPCRs). These proteins sit within cell membranes and facilitate a myriad of physiological processes by detecting stimuli ranging from light to hormones.
Over the past two decades, researchers have determined the detailed structure of an increasing number of GPCRs, but not yet of the olfactory receptors within them. To obtain enough receptors for these studies, researchers must produce them in cultured cells. However, when olfactory receptors grow outside of olfactory neurons (their natural habitat), they often refuse to mature properly.
To overcome this problem, Matsunami and Claire de March, researchers in Matsunami’s lab, set out to explore the possibility of genetically altering olfactory receptors to make them more stable and easier to grow in other cells. They teamed up with biochemist Aashish Manglik at UCSF and Christian Billesbølle, a senior scientist in Manglik’s lab.
Although the work was progressing, the team decided to try again to extract the natural receptor. “It could fail like everyone else,” Manlik recalls. “[But] We should try it anyway. “
They boosted their success by targeting an odorant receptor, OR51E2, which is also found in the gut, kidney, prostate and other organs beyond the nose. After careful efforts by Billesbølle, they finally obtained enough OR51E2 for research. They then exposed the receptors to an odor molecule they knew they could detect: propionate, a short fatty acid produced by fermentation.
To generate detailed images of receptors and propionate locked together, an interaction that triggers the firing of sensory neurons, they used cryo-electron microscopy, an advanced imaging technique that captures images of rapidly frozen proteins. snapshot.
The team found that OR51E2 traps propionate inside a small pocket in the structure of an interlocking molecule. When they widened the pocket, the receptor lost its sensitivity to propionate and another small molecule that normally activates it. The tuned receptors preferred larger odor molecules, confirming that the size and chemistry of the binding pocket tuned the receptors to detect only a small subset of molecules.
Structural analysis also revealed a small, flexible loop at the top of the receptor, which locks like a lid on a pocket once an odorant molecule is bound in it. This finding suggests that this highly variable segment of the loop may contribute to our ability to detect different chemicals, Manglik said.
The basic logic of smell
The OR51E2 may have other secrets to share. While this study focused on the pocket that houses propionate, the receptor may possess other binding sites for other odors, or chemical signals it may encounter in tissues other than the nose, the researchers said.
What’s more, the microscope images only show a static structure, but these receptors are actually dynamic, says Nagarajan Vaidehi, a computational chemist at City of Hope’s Beckman Institute who worked on the study. Her team used computer simulations to visualize how OR51E2 might move when it wasn’t frozen.