Fluorescent Ubiquitination-based Trbl Turnover Indicator (FUTTI): A tool to detect activity of the Tribbles pseudokinase in vivo

Inside every living cell, a constant, quiet battle takes place to maintain order. Cells must decide which proteins to keep and which to discard, a process essential for growth, development, and preventing disease. This decision often relies on a system of molecular tags that mark specific proteins for destruction, sending them to the cell's recycling center. Among the many molecules that help direct this cleanup crew are a family of proteins called Tribbles. These molecules act as adaptors, bridging the gap between a target protein that needs to be removed and the machinery that actually destroys it. While these proteins are vital for normal development, their malfunction is linked to serious conditions, including certain types of cancer, where they may destroy healthy proteins too quickly or fail to remove harmful ones. Understanding exactly how these adaptors work, and what parts of their structure are essential for their job, has been a challenge because scientists lacked a simple way to watch them in action within a living organism.

Researchers at the University of Missouri-Kansas City have developed a new tool to solve this problem, allowing them to see where and when these adaptor proteins are active inside a living fruit fly. They created a sensor that glows with a fluorescent light, which dims whenever the adaptor protein is working correctly. By watching this light, they could map the activity of the adaptor protein in real time and test which parts of the protein are necessary for it to function. Their work revealed that while these proteins share a common structure across many animals, the specific mechanism they use to recruit the destruction machinery in fruit flies is unique and differs significantly from the method used in mammals.

The scientists began by designing a sensor based on a specific target protein known to be controlled by the adaptor. In fruit flies, this target is a protein called Slbo, which plays a key role in the development of the ovary. The researchers identified a small, specific section of the Slbo protein that acts as a signal for destruction, a region that the adaptor protein recognizes and binds to. They took this signal and attached it to a fluorescent protein, creating a hybrid molecule that glows green or red. Under normal conditions, this glowing molecule would accumulate and shine brightly. However, if the adaptor protein was present and active, it would recognize the signal, tag the glowing molecule, and send it to be destroyed, causing the light to fade. The researchers named this sensor FUTTI, a fluorescent indicator of the adaptor's turnover activity.

To test if this sensor worked, the researchers introduced it into fruit flies and observed the glowing patterns in their tissues. In the developing wings of the flies, they found that when the adaptor protein was present in high amounts, the glowing sensor disappeared from those specific cells, confirming that the sensor was being destroyed as intended. Conversely, when they reduced the amount of the adaptor protein, the sensor accumulated and glowed much brighter. This inverse relationship proved that the sensor could accurately report the activity levels of the adaptor protein in different parts of the fly's body. They saw similar results in the ovaries, where the sensor dimmed in specific cells where the adaptor was known to be active, demonstrating that the tool could map activity across different tissues with high precision.

With a working sensor in hand, the team moved on to investigate the specific parts of the adaptor protein that make it work. They knew that the adaptor protein has a central region that binds to the target signal and a tail region that recruits the destruction machinery. To test the importance of the central binding region, they created a version of the adaptor with a single, tiny change in its structure, swapping one amino acid for another. When they expressed this altered protein in the flies, the sensor remained bright and did not disappear. This result showed that the central binding region is essential for the adaptor to recognize its target; without it, the adaptor cannot grab the sensor to destroy it. Interestingly, this broken version of the adaptor still affected other cellular processes, such as cell division, suggesting that different parts of the protein control different functions.

The researchers then turned their attention to the tail of the protein, which is responsible for recruiting the destruction machinery. In mammals, this tail contains a specific sequence that binds to a well-known enzyme called COP1, which helps tag proteins for destruction. The team tested whether this same mechanism worked in fruit flies. First, they tried to see if mammalian versions of the adaptor protein could work in the fly. They expressed mouse versions of the adaptor in the flies, but the sensor remained bright, indicating that the mammalian proteins could not destroy the fly sensor. Even when they added the mouse COP1 enzyme to the mix, the mammalian adaptors still failed to work, suggesting that the mouse proteins and the fly sensor are incompatible.

This led the team to investigate the unique structure of the fruit fly adaptor's tail. They discovered that the fruit fly version lacks the specific binding site for the COP1 enzyme found in mammals. Instead, the fly protein has a longer tail with a different sequence of amino acids. By systematically deleting parts of this tail, they found that a specific region, containing a short sequence of four amino acids, was critical for the protein's function. When they mutated this specific sequence, the adaptor lost its ability to destroy the sensor, and the light remained bright. This finding suggests that the fruit fly adaptor uses a unique mechanism to recruit the destruction machinery, one that does not rely on the COP1 enzyme used by mammals.

The study highlights a fascinating divergence in evolution. While the core function of these adaptor proteins—recognizing a target and marking it for destruction—is conserved across animals, the specific tools they use to do the job have changed. In mammals, the adaptor relies on a specific interaction with the COP1 enzyme. In fruit flies, the adaptor has evolved a different strategy, utilizing a unique motif in its tail to achieve the same result. This discovery not only provides a new tool for scientists to study these proteins in living organisms but also deepens our understanding of how molecular machinery can evolve different solutions to the same biological problem. The FUTTI sensor offers a clear, visual way to track these activities, opening the door to further research into how these proteins influence development and disease in a wide range of species.