Mapping STING Uncovers How a Key Immune Protein Regulates Multiple Signals

The immune protein STING (stimulator of interferon genes) serves as a molecular alarm inside cells, detecting danger from infections, damaged DNA, and cancer. When it activates at the right time, it helps protect the body. When it activates too easily or fails to respond, it can contribute to disease. Despite its importance, how different parts of STING control its various activities has remained incompletely understood.

A team led by Andrea Ablasser at EPFL has built the first comprehensive functional map of human STING by systematically testing the effect of nearly every possible single amino acid substitution in the protein. The study, published in Nature, reveals how specific regions regulate immune signaling and how small genetic changes can alter protein behavior.

The researchers used deep mutational scanning, generating thousands of STING variants, each carrying a different single amino acid change, and measuring their effects in living cells. The screen focused on two key activities: type I interferon signaling, which helps coordinate antiviral immune defense, and non-canonical autophagy, a process that helps cells respond to stress and infection.

The results showed that STING activity is controlled by many regions distributed throughout the protein. Some mutations activated STING even without its normal trigger, while others weakened its response to cGAMP, the molecule that switches STING on when cells detect misplaced DNA. The screen also identified previously unknown regions that help keep STING inactive under normal conditions.

To understand the structural basis for these effects, the team used cryo-electron microscopy to examine several hyperactive STING variants. Different mutations pushed STING into distinct signaling states: some promoted the formation of large filament-like assemblies associated with activation, while others altered molecular interactions within the protein.

STING did not behave as a simple on-off switch. One mutation preserved interferon signaling while strongly impairing a hallmark of non-canonical autophagy, evidence that these two functions rely on distinct molecular mechanisms and cellular locations.

The researchers then compared their map with human genetic databases. The data helped identify previously unrecognized variants that enhance STING activity, provided evidence that a rare variant found in a patient with inflammatory lung disease can drive excessive immune signaling, and showed that cancer-associated STING mutations tend to reduce activity, suggesting some tumors may benefit from weakening this pathway.