Harvard researchers identify midnolin protein's role in degrading short-lived nuclear proteins

Inside cells, certain proteins live only minutes. They are born in the nucleus, do their job—turning genes on or off—and then vanish. For more than forty years, biologists knew these short-lived proteins existed and understood why they mattered. They help the brain wire itself. They marshal the immune system. They control whether cells divide or die. What nobody could explain was how the cell actually got rid of them once their work was done.

Harvard researchers have now solved that puzzle. A team led by Michael Greenberg and Stephen Elledge identified a protein called midnolin that acts as a cellular bouncer, grabbing these short-lived proteins and shoving them directly into the proteasome—the cell's waste disposal system—where they are destroyed. The discovery, published in Science on August 24, reveals a mechanism that operates without the usual molecular tag that marks proteins for destruction. It is, as one of the researchers put it, simple and elegant.

The mystery began with a known fact about how cells break down proteins. Scientists understood that cells could attach a small molecule called ubiquitin to proteins, essentially labeling them as trash. The proteasome would then recognize this label and destroy the marked proteins. But researchers had long noticed something odd: sometimes the proteasome destroyed proteins that had no ubiquitin tag at all. There had to be another way. Xin Gu, a research fellow in neurobiology at Harvard Medical School, became fascinated by transcription factors—proteins that appear suddenly in response to a cellular signal, rush to the nucleus, flip genes on, and then disappear almost immediately. "What struck me in the beginning is that these proteins are extremely unstable," Gu said. "Once they are produced, they carry out their function, and they are quickly degraded afterwards."

To find the mechanism, Gu and Christopher Nardone, a PhD candidate in genetics, started with two well-studied transcription factors: Fos, which is crucial for learning and memory, and EGR1, which controls cell division and survival. Using genetic tools developed in Elledge's lab, they searched for proteins that might be responsible for breaking down these factors. They found midnolin. Then they kept searching and found that midnolin breaks down not just two transcription factors, but hundreds of them across the nucleus. "We were shocked and skeptical," Nardone recalled.

To understand how one protein could target so many different proteins, the team used AlphaFold, a machine learning tool that predicts protein structures, combined with laboratory experiments. They discovered that midnolin contains a region called the Catch domain—two separate sections linked together like mittens on a string. This domain grabs the relatively unstructured regions of other proteins, allowing it to capture many different types of targets. Once midnolin has a protein in its grip, it feeds it directly into the proteasome, where the protein is destroyed. The mechanism requires no ubiquitin tag. It is, in essence, a second pathway for protein destruction that cells have been using all along.

The implications ripple outward. The transcription factors that midnolin degrades control processes fundamental to human health. Fos turns on genes that allow neurons to rewire themselves in response to experience. IRF4 activates genes that help the immune system produce functional B and T cells. When cells have too much or too little of these factors, disease can follow. In multiple myeloma, cancer cells become dependent on IRF4, and too much of it fuels the disease. Imbalances in Fos are linked to problems with learning and memory. Greenberg noted that protein degradation is a critical process, and when it goes wrong, it underlies many neurological, psychiatric, and cancer conditions.

The researchers are now planning structural studies to understand the fine details of how midnolin works. They are also creating mice that lack midnolin to see what role the protein plays in different tissues and at different stages of development. The long-term goal is to develop therapies that could control the levels of specific transcription factors by targeting the midnolin-proteasome pathway. "One of the areas we are actively exploring is how to tune the specificity of the mechanism so it can specifically degrade proteins of interest," Gu said. For now, the discovery is foundational science—a forty-year-old question finally answered. But the door it opens may lead to treatments for diseases where protein imbalance is the root of the problem.