The Mechanism of DNA Sensing by AIM2-Like Receptors

AIM2-Like Receptors are a family of nuclear and cytosolic foreign-DNA sensors consisting of an N-terminal PYD and one or two C-terminal DNA-binding HIN200 domains. AIM2 is a cytosolic sensor which forms a supramolecular structure known as the inflammasome, while IFI16 resides in both the nucleus and cytoplasm, where it can associate with ASC and form its own inflammasome, as well as lead to activation of the interferon pathway by parallel means. Persistent questions in innate immunology remain of how such sensors respond to foreign DNA while remaining silent towards host DNA, as well as their role in activating downstream effectors. Using fluorescence anisotropy, FRET, EMSA, and various mutational studies, it is shown here that the PYD of IFI16 plays a positive, cooperative role in DNA binding, allowing it to oligomerize in a length-dependent manner. Disruption of the non-DNA-binding PYD in turn disrupts DNA binding, demonstrating that oligomerization and DNA binding are coupled events. By fluorescence anisotropy, FRET, EMSA, electron microscopy, and mutagenesis studies, a model which doesn’t invoke the previous autoinhibition is put forth to explain the behavior and activation of AIM2. Results also give clues into how AIM2 may then recruit the next member of the downstream pathway, ASC. Finally, using time-dependent FRET assays as well as competition anisotropy experiments, it is demonstrated that nucleosomes act as effective barriers to IFI16 DNA binding and oligomerization. The length-dependent rates of FRET signal also support a model in which IFI16 uses one-dimensional diffusion as an efficient means of oligomerization. Taken together, these results show that the AIM2-Like Receptor family share many common characteristics that give insight into their roles in host defense and autoimmune disorders