The molecular basis for the severity and rapid spread of the COVID-19 disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is largely unknown. ORF8 is a rapidly evolving accessory protein that has been proposed to interfere with immune responses. The crystal structure of SARS-CoV-2 ORF8 was determined at 2.04-Å resolution by X-ray crystallography. The structure reveals a ∼60-residue core similar to SARS-CoV-2 ORF7a, with the addition of two dimerization interfaces unique to SARS-CoV-2 ORF8. A covalent disulfide-linked dimer is formed through an N-terminal sequence specific to SARS-CoV-2, while a separate noncovalent interface is formed by another SARS-CoV-2−specific sequence, 73YIDI76. Together, the presence of these interfaces shows how SARS-CoV-2 ORF8 can form unique large-scale assemblies not possible for SARS-CoV, potentially mediating unique immune suppression and evasion activities.

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...

DNA repair via homologous recombination (HR) is indispensable for genome integrity and cell survival but if unrestrained can result in undesired chromosomal rearrangements. The regulatory mechanisms of HR are not fully understood. Cyclic GMP‐AMP synthase (cGAS) is best known as a cytosolic innate immune sensor critical for the outcome of infections, inflammatory diseases, and cancer. Here, we report that cGAS is primarily a chromatin‐bound protein that inhibits DNA repair by HR, thereby accelerating genome destabilization, micronucleus generation, and cell death under conditions of genomic stress. This function is independent of the canonical STING‐dependent innate immune activation and is physiologically relevant for irradiation‐induced depletion of bone marrow cells in mice. Mechanistically, we demonstrate that inhibition of HR repair by cGAS is linked to its ability to self‐oligomerize, causing compaction of bound template dsDNA into a higher‐ordered state less amenable to strand invasion by RAD51‐coated ssDNA filaments. This previously unknown role of cGAS has implications for understanding its involvement in genome instability‐associated disorders including cancer.