The Cooperative Assembly of IFI16 Filaments on dsDNA Provides Insights into Host Defense Strategy

Whether host DNA-receptors have any capacity to distinguish self from nonself at the molecular level is one of the foremost questions in the innate immunity of mammals. By using quantitative assays and electron microscopy, we show that cooperatively assembling into filaments on dsDNA may serve an integral mechanism by which human interferon 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 Sjogren's 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 previously proposed 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 (PYD) of IFI16 that drives the cooperative filament assembly. The surface residues on the PYD that mediate the cooperative DNA-binding are conserved, suggesting that related receptors use a common mechanism. These results suggest that IFI16 clusters into signaling complexes in a switch-like manner, and that it may use the size of naked dsDNA as molecular ruler to distinguish self from nonself.