Mechanistic insights into Mtb effector EspB-mediated mitochondrial membrane damage

Tuberculosis is an ancient and persistent killer, caused by a bacterium that has mastered the art of hiding inside the very cells meant to destroy it. When the immune system's sentinels, called macrophages, swallow this bacterium, they usually trap it in a small, acidic bubble called a phagosome, where it is meant to be digested. However, the tuberculosis bacterium, known as Mycobacterium tuberculosis, has evolved a sophisticated weapon to escape this fate. It uses a specialized molecular syringe to inject proteins into the host cell, breaking the walls of the trap and allowing the bacteria to roam free. Among these injected proteins is a specific effector called EspB. While scientists knew that EspB could damage the cell's power plants, the mitochondria, and eventually kill the cell, the exact mechanism remained a mystery. It was known that EspB could stick to the fatty membranes of mitochondria, but experiments showed that lipids alone were not enough to explain how the protein caused such severe damage. The question was whether the bacterium needed a specific key to unlock the door to the mitochondria, or if the fatty membrane was sufficient on its own.

A team of researchers at the Indian Institute of Science has now solved this puzzle, revealing that EspB does not act alone. Instead, it functions like a two-part tool that requires both a sticky surface and a specific handle to work. The study demonstrates that EspB targets the mitochondria by grabbing onto two things simultaneously: the fatty lipids that make up the mitochondrial outer membrane and a specific receptor protein called Tom70. Tom70 is a normal part of the cell, usually responsible for helping other proteins enter the mitochondria. The researchers found that EspB hijacks this protein, using it as a docking station to anchor itself firmly to the mitochondrion. Without this connection to Tom70, EspB cannot effectively bind to the mitochondria or cause the damage that leads to cell death.

To uncover this mechanism, the scientists first isolated the EspB protein and the mitochondria from cells. They observed that when EspB was mixed with mitochondria, it clung to the surface, causing the mitochondrial membrane to leak. However, when they treated the mitochondria with enzymes that chewed up the proteins on the surface, EspB lost its grip and could no longer bind effectively. This suggested that a protein receptor was essential for the interaction. To identify this missing piece, the team mixed the proteins with mitochondrial extracts and used a technique called mass spectrometry to analyze the resulting mixture. The analysis pointed directly to Tom70 as the primary partner. Further experiments confirmed that EspB binds tightly to Tom70, with a specific strength measured at a level of 131.5 nanomolar, indicating a strong and specific connection.

The researchers then looked at the structure of this partnership using a powerful imaging technique called cryo-electron microscopy, which allows scientists to see molecules at near-atomic resolution. They found that EspB forms a ring-like structure made of seven parts, and when it attaches to Tom70, the receptor sits at the bottom of this ring, partially blocking the central hole. The images showed that the connection is not rigid; the Tom70 protein can shift its position slightly, suggesting a flexible but firm attachment. By building a detailed model of how these two molecules fit together, the team identified two specific spots on the EspB protein, located at positions 69 and 179, that act as the primary contact points. These spots form chemical bonds with the Tom70 protein, locking the two together.

To prove that these specific contact points were critical, the scientists created mutant versions of the EspB protein where these two spots were altered. When they tested these mutants, they found that the altered EspB could no longer bind to Tom70. In living yeast cells, which serve as a model for human cells, the normal EspB protein caused the cells to die by triggering a process similar to apoptosis, or programmed cell death. This death was accompanied by a collapse in the mitochondria's ability to produce energy and a loss of the electrical charge across their membranes. However, when the cells were exposed to the mutant EspB that could not bind to Tom70, the mitochondria remained healthy, and the cells survived. This confirmed that the interaction with Tom70 is not just a side effect but the essential step that allows EspB to destroy the mitochondria.

The findings suggest a clear sequence of events during an infection. Once the tuberculosis bacterium escapes the initial trap inside the macrophage, it releases EspB into the cell's interior. This protein then seeks out the mitochondria, using the Tom70 receptor to anchor itself. Once attached, the complex disrupts the mitochondrial membrane, stopping the production of energy and triggering the cell's self-destruct mechanism. This cell death weakens the immune system's defenses and helps the bacteria spread. The study rules out the idea that EspB acts solely by interacting with the fatty lipids of the membrane; without the Tom70 receptor, the damage does not occur. While the researchers could not determine the exact atomic structure of the entire complex due to the flexibility of the proteins, the evidence from multiple experiments leaves little doubt that this partnership is the key to the bacterium's deadly strategy. By understanding exactly how EspB latches onto Tom70, scientists now have a clearer picture of how tuberculosis disables the human immune system, potentially opening new avenues for treatments that could block this specific interaction and stop the bacteria in its tracks.