The helminth-derived effector protein HpBARI trimerises to selectively target membrane bound ST2 receptor
This study reveals that the helminth-derived protein HpBARI2 forms a stable trimer that selectively inhibits membrane-bound ST2 signaling by simultaneously blocking IL-33 binding, restricting receptor conformation, and leveraging multivalent interactions to avoid the soluble decoy receptor, offering a blueprint for targeted ST2 therapeutics.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
The immune system acts as a vigilant defense force, constantly scanning the body for invaders like bacteria and parasites. When it detects a threat, it sends out chemical signals to rally its troops. One such signal is a protein called IL-33, which acts like an alarm bell. This alarm is heard by a specific receiver on the surface of immune cells, known as the ST2 receptor. When IL-33 locks onto ST2, it triggers a chain reaction that brings in specialized cells to fight the infection. This process is vital for survival, especially against intestinal worms, but if the alarm rings too loudly or for too long, it can cause severe inflammation, leading to conditions like asthma or allergic reactions. The body has a built-in safety valve: a version of the ST2 receptor that floats freely in the blood, acting as a decoy to soak up excess alarm signals. However, this same decoy can make it difficult for medicines to reach the receptors actually sitting on the cells where they are needed to stop the inflammation.
Scientists have long looked to nature for clues on how to control this system. A microscopic worm that lives in the intestines of mice, called Heligmosomoides polygyrus bakeri, has evolved a sophisticated way to survive. To avoid being expelled by the host's immune system, the worm secretes a protein called HpBARI2. This protein is designed to silence the alarm by binding to the ST2 receptor, effectively telling the immune system to stand down. While researchers knew this protein worked, they did not understand the mechanics of how it managed to block the receptor so effectively, especially when a decoy version of the receptor was floating nearby to interfere. A team of scientists has now solved this mystery by creating a detailed 3D map of the worm protein bound to the human receptor, revealing a clever strategy that could guide the development of new treatments for inflammatory diseases.
Using a powerful imaging technique that freezes molecules in time, the researchers visualized the structure of the worm protein HpBARI2. They discovered that it does not exist as a single unit. Instead, three copies of the protein link together to form a flat, triangular shape. This triangular structure is not just a random clump; it is a precisely engineered machine with three distinct points, or vertices. At each of these three points, the protein grabs onto a single ST2 receptor. The researchers found that the protein holds the receptor in a specific way that physically blocks the alarm signal, IL-33, from ever attaching. Furthermore, the grip is so firm and the shape of the receptor is twisted just enough by the worm protein that the receptor cannot change its shape to recruit the other parts needed to send the signal. It is as if the worm protein not only covers the door but also jams the lock, ensuring the alarm cannot be rung.
The most surprising discovery was how this triangular shape helps the worm protein ignore the decoy receptors in the blood. The researchers tested whether the protein could bind to the receptors floating freely in solution versus those attached to the surface of cells. They found that the single-point versions of the protein, which could only grab one receptor at a time, were easily distracted by the decoys. However, the natural triangular version, with its three grabbing points, behaved differently. Because it could grab three receptors at once, it held on much more tightly to the receptors on the cell surface. This multi-point grip, known as avidity, allowed the protein to stay attached to the cell even when decoys were present in high numbers. The researchers confirmed this by creating a version of the protein that could only grab one receptor; this single-point version failed to block the immune response effectively unless used in massive quantities, whereas the natural three-point version worked at very low concentrations.
This finding suggests that the worm has evolved a solution to a problem that human drug designers have struggled with: how to target a receptor on a cell surface without being blocked by decoys in the blood. The study shows that the key is not necessarily to make a drug that binds tighter to a single receptor, but to design a molecule that can grab multiple receptors at once. By doing so, the drug becomes selective for the cell surface, where it is needed, while ignoring the decoys. The researchers demonstrated that this mechanism allows the worm protein to suppress the immune response potently, even in the presence of high levels of the decoy receptor. This insight provides a new blueprint for creating medicines that can treat conditions like asthma and inflammatory bowel disease by selectively turning off the alarm system on immune cells without being neutralized by the body's own safety mechanisms.
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