Water oxidation-driven histidine dioxidation enables probe-free proximity labeling

To understand the challenge faced by scientists in this study, one must first grasp how they attempt to map the invisible landscape inside a living cell. Cells are not empty bags of fluid; they are bustling cities filled with tiny, moving compartments like vesicles, which act as delivery trucks transporting proteins and materials. To see what is inside these trucks and how they move, researchers use a technique called proximity labeling. Imagine a photographer trying to take a picture of a fast-moving object in a dark room. They need a flash to freeze the motion and a way to mark the object so it can be found later. In biology, this "flash" is a chemical reaction that marks nearby proteins, and the "mark" is usually a tag added by a small molecule probe. However, this method has a blind spot. If the probe cannot physically reach a specific compartment because the cell's structure blocks it, that part of the cell remains invisible. This is especially true for dynamic, shifting structures like endosomes and exosomes, where the accessibility of chemical tools is uneven, leading to an incomplete and biased picture of the cell's inner workings.

A team of researchers, led by Tae-Hyuk Kwon at the Ulsan National Institute of Science and Technology and colleagues from Seoul National University and the Broad Institute, has developed a new way to take this photograph that removes the need for the probe to be present during the initial flash. They created a new organic molecule, which they named IDM, that acts as a photocatalyst. When this molecule is exposed to green light inside a living cell, it triggers a chemical reaction that uses water and oxygen—two substances that are everywhere in the cell—to create highly reactive radicals. Unlike previous methods that rely on a specific type of oxygen molecule called singlet oxygen, which creates unstable intermediates that vanish instantly, IDM generates hydroxyl and superoxide radicals. These radicals work together to permanently alter a specific amino acid called histidine, which is found on the surface of many proteins. This alteration, known as dioxidation, changes the histidine into a stable, double-lactam structure that can persist for a long time without falling apart.

The brilliance of this approach lies in the timing. In traditional methods, the chemical probe must be inside the cell at the exact moment the light is turned on to catch the fleeting chemical reaction. If the probe cannot get into a specific vesicle, the proteins inside are never labeled. With the new IDM method, the light is turned on while the cell is alive, and the IDM molecule oxidizes the histidines on nearby proteins, locking them into this stable state. The cell is then broken open, and only after this point is the chemical probe added. Because the histidine has already been permanently changed by the water-based reaction, the probe can attach to it easily in the test tube, regardless of whether it could have reached that spot inside the living cell. This "probe-free" strategy, which the authors call PF-Map, allows them to capture a much wider and more accurate view of the proteins involved in vesicle trafficking.

When the researchers applied this new tool to study how cells move materials, they compared their probe-free method against the traditional probe-dependent approach. Both methods successfully identified well-known markers for exosomes, which are small vesicles that cells release to communicate with one another. However, the probe-free method revealed a hidden layer of the cell's activity that the traditional method missed. It identified 125 additional proteins that are crucial for vesicle trafficking, including specific proteins like Rab5, Rab11, and SEC31A, which help manage the movement and budding of these cellular containers. These proteins were underrepresented in the traditional method, likely because the chemical probes used in that approach could not reach the specific vesicles where these proteins reside. By decoupling the initial oxidation step from the need for a probe to be present in the living cell, the researchers were able to map a more complete and less biased proteome of the cell's transport system.

The study confirms that this new mechanism works through a specific chemical pathway. The researchers used various tests, including mass spectrometry and electron paramagnetic resonance, to prove that IDM generates hydroxyl radicals from water and superoxide radicals from oxygen, rather than the singlet oxygen used by older tools. They demonstrated that these radicals attack the histidine ring, adding oxygen atoms to create a stable structure that does not revert to its original form. This stability is key; it allows the labeled proteins to survive the process of breaking open the cell and preparing them for analysis. Furthermore, the team showed that the IDM molecule itself is safe for cells, causing no significant damage or death to the cells it is used on, and it can be activated by green light, which is gentle enough to avoid harming biological tissue.

In the end, this work offers a new lens for looking at the complex, moving parts of a cell. By using the cell's own water and oxygen to create a permanent mark on proteins, and then adding the detection tag later, scientists can now see proteins that were previously hidden behind barriers of probe accessibility. The researchers found that this method not only confirms known biology but also uncovers a sub-proteome of vesicle trafficking proteins that were previously underestimated. This suggests that the cell's transport network is even more intricate and interconnected than previously thought, with proteins like SEC31A potentially linking different transport pathways in ways that were difficult to detect before. The study establishes a new, minimally biased strategy for spatial proteomics, opening the door to understanding dynamic biological processes in living systems with greater clarity and depth.