A new study led by the Shalek Lab at the Ragon Institute of Mass General Brigham, MIT, and Harvard, as well as the Broad Institute, published in Science, shows how a protein called LAT coordinates the many signals that drive T cell activation. The findings reveal that LAT does not switch these signals on separately. Instead, it holds them in balance, so that disrupting one part of the protein affects the entire response.

T cells are central components of the immune system. When a T cell detects a threat, a receptor on its surface sets off a cascade of signals inside the cell that activate genes, trigger cell division, and produce the molecules needed to fight infection. LAT sits at the center of this cascade, gathering signaling partners together and relaying the initial signal into several pathways at once.

LAT is also largely “disordered,” meaning much of the protein does not fold into a fixed shape. Disordered proteins make up about half of all human proteins, and often serve as flexible hubs that organize other proteins, but their lack of structure has made them difficult to study.

To understand this, the team, co-led by postdoctoral fellow Adam Rubin and graduate student Tyler Dao, developed a genetic screening approach that links specific parts of LAT to their effects on T cell activation. The researchers built a library of 132 LAT variants, each carrying small mutations across the length of the protein, and introduced them into T cells. They then activated the cells and measured the response of each variant at the single-cell level.

The screen identified many important regions across LAT, including several with no previously known role. The team also found that the overall pattern of electrical charge along the protein, rather than any single position, matters for its function.

The most striking finding was that mutations anywhere in LAT tended to weaken all downstream signaling pathways together, rather than affecting them one at a time. Using several additional experiments from the toolkits of molecular biology and high-resolution imaging, the researchers showed that LAT’s partner proteins depend on one another to bind and thus trigger further signals. Disrupting one partner indirectly weakens the others, which keeps signaling outputs in balance. Other studies have shown that imbalanced signaling can lead to dysfunctional immune responses.

Beyond T cell biology, the approach described in the study offers a general framework for studying how the sequences of complex proteins give rise to their functions, including the many disordered proteins that coordinate activities across the cell.