Whether host DNA receptors have any capacity to distinguish self from nonself at the molecular level is an outstanding question in the innate immunity of mammals. Here, by using quantitative assays and electron microscopy, we show that cooperatively assembling into filaments on dsDNA may serve as an integral mechanism by which human IFN-inducible protein-16 (IFI16) engages foreign DNA. IFI16 is essential for defense against a number of different pathogens, and its aberrant activity is also implicated in several autoimmune disorders, such as Sjögren syndrome. IFI16 cooperatively binds dsDNA in a length-dependent manner and clusters into distinct protein filaments even in the presence of excess dsDNA. Consequently, the assembled IFI16·dsDNA oligomers are clearly different from the conventional noninteracting entities resembling beads on a string. The isolated DNA-binding domains of IFI16 engage dsDNA without forming filaments and with weak affinity, and it is the non-DNA-binding pyrin domain of IFI16 that drives the cooperative filament assembly. The surface residues on the pyrin domain that mediate the cooperative DNA binding are conserved, suggesting that related receptors use a common mechanism. These results suggest that IFI16 clusters into signaling foci in a switch-like manner and that it is capable of using the size of naked dsDNA as a molecular ruler to distinguish self from nonself.cooperative filament formation | inflammasome R ecognition of foreign intracellular DNA is a widely conserved defense mechanism by which the innate immune system of mammals detects and responds to invading pathogens (1, 2). Using such a universal molecule as a major danger signal for detecting pathogens must be regulated in a stringent yet efficient manner. However, only a few deciding factors are known for the host innate immune system to selectively engage foreign DNA (i.e., nonself-DNA) while minimizing interactions with self-DNA, which include the compartmentalization of mammalian cells and the size of foreign DNA. These features, however, only raise more questions than provide answers. For example, the footprints of intracellular DNA receptors usually fall below 20 bp, and yet a long foreign DNA fragment [e.g., poly(dA:dT); ≥1,000 bp] is required to induce a robust innate immune response even in a normally DNA-free environment like the cytoplasm (1, 2). On the other hand, foreign DNA-sensing pathways also exist in the host nucleus in which DNA receptors must not respond to abundant self-DNA to prevent spurious activities (1, 2). Indeed, one of the major unresolved questions in understanding the DNA-sensing pathways of mammals is whether the host intracellular DNA receptors have any capacity to distinguish self-from nonself-DNA at the molecular level (2).Human IFN inducible protein-16 (IFI16) is an intracellular DNA receptor of innate immunity that belongs to the family of absent-in-melanoma-2 (AIM2)-like receptors (ALRs) (1-4). IFI16 senses DNA from invading pathogens in both the nucleus and cytoplasm [e.g., vaccinia virus...

AIM2 recognizes foreign dsDNA and assembles into the inflammasome, a filamentous supramolecular signalling platform required to launch innate immune responses. We show here that the pyrin domain of AIM2 (AIM2PYD) drives both filament formation and dsDNA binding. In addition, the dsDNA-binding domain of AIM2 also oligomerizes and assists in filament formation. The ability to oligomerize is critical for binding dsDNA, and in turn permits the size of dsDNA to regulate the assembly of the AIM2 polymers. The AIM2PYD oligomers define the filamentous structure, and the helical symmetry of the AIM2PYD filament is consistent with the filament assembled by the PYD of the downstream adaptor ASC. Our results suggest that the role of AIM2PYD is not autoinhibitory, but generating a structural template by coupling ligand binding and oligomerization is a key signal transduction mechanism in the AIM2 inflammasome.

LRP5 and LRP6 are Wnt co-receptors essential for Wnt/β-catenin signaling. DKK1 inhibits Wnt signaling by interacting with the extracellular domain of LRP5/6, and is a drug target for multiple diseases. Here we present the crystal structures of the first and second halves of LRP6’s four propeller–EGF pairs (LRP6-E1E2 and LRP6-E3E4), and a LRP6-E3E4/DKK1 complex. Combined with EM analysis, these data demonstrate that LRP6-E1E2 and LRP6-E3E4 form two rigid structural blocks, with a short intervening hinge that restrains their relative orientation. DKK1c interacts with the top surface of the LRP6-E3 YWTD propeller, and likely also that of the LRP6-E1 propeller due to structural similarity, through conserved hydrophobic patches buttressed by a network of salt bridges and hydrogen bonds. Our work provides key insights for understanding LRP5/6 structure and the interaction of LRP5/6 with DKK, as well as for drug discovery.