The immune system depends on molecular alarms that detect danger inside cells. One of these alarms is STING, short for "stimulator of interferon genes". STING helps cells respond to 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 activate, it can contribute to disease.
STING controls several immune responses, including the production of type I interferons, antiviral molecules that help coordinate immune defense. It also regulates inflammatory signals and a process called non-canonical autophagy, which helps cells respond to stress and infection. Despite its importance, scientists still don't completely understand how different parts of STING control these activities.
A team led by Andrea Ablasser at EPFL has now created the first comprehensive map of how nearly every possible single amino acid change affects human STING. Published in Nature, the study reveals how different regions of the protein regulate immune signaling and how small genetic changes can alter its behavior.
The researchers used a technique called "deep mutational scanning": They generated thousands of STING variants, each carrying a different single amino acid substitution, and tested their effects in living cells. This allowed them to measure how each variant influenced interferon signaling and non-canonical autophagy.
Screening the mutations revealed 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 natural molecule that switches STING on after cells detect misplaced DNA.
Using this information, the researchers were able to identify both known regulatory sites and previously unknown regions that help keep STING inactive under normal conditions.
To understand how these mutations work, the team used cryo-electron microscopy to determine the structures of several hyperactive STING variants. This revealed that different mutations can push STING into different signaling states: some promote the formation of large filament-like assemblies associated with activation, while others alter key molecular interactions within the protein.
The study also found that STING is not a simple on-off switch. Some mutations selectively affected specific immune responses while leaving others intact. One mutation preserved interferon signaling but strongly impaired a hallmark of non-canonical autophagy. In short, STING's different functions rely on distinct molecular mechanisms and cellular locations.
The researchers then compared their functional map with human genetic databases. Their data helped identify previously unrecognized variants that enhance STING activity and provided evidence that a rare STING variant found in a patient with inflammatory lung disease can drive excessive immune signaling. They also found that cancer-associated STING mutations tend to reduce STING activity, suggesting that some tumors may benefit from weakening this pathway.
The work provides a resource for interpreting genetic variants found in patients and offers new insight into how immune signaling is regulated. More broadly, it shows how large-scale mutational mapping can reveal the molecular rules that govern complex proteins involved in human health and disease.
Other contributors
- Hospices Civils de Lyon
- Université Claude Bernard Lyon
- Hôpital Louis Pradel
- Assistance Publique Hôpitaux de Paris (AP-HP)
- Université Paris Cité
- INSERM
- EPFL Laboratory of Protein Design and Immunoengineering
- EPFL Institute for Cancer Research (ISREC)
