Structures of the HIN Domain:DNA Complexes Reveal Ligand Binding and Activation Mechanisms of the AIM2 Inflammasome and IFI16 Receptor

Abstract

SUMMARY Recognition of DNA by the innate immune system is central to anti-viral and anti-bacterial defenses, as well as an important contributor to autoimmune diseases involving self DNA. AIM2 (absent in melanoma 2) and IFI16 (interferon-inducible protein 16) have been identified as DNA receptors that induce inflammasome formation and interferon production, respectively. Here we present the crystal structures of their HIN domains in complex with double-stranded (ds) DNA. Non-sequence specific DNA recognition is accomplished through electrostatic attraction between the positively charged HIN domain residues and the dsDNA sugar-phosphate backbone. An intramolecular complex of the AIM2 Pyrin and HIN domains in an autoinhibited state is liberated by DNA binding, which may facilitate the assembly of inflammasomes along the DNA staircase. These findings provide novel mechanistic insights into dsDNA as the activation trigger and oligomerization platform for the assembly of large innate signaling complexes such as the inflammasomes.

The emerging role of human PYHIN proteins in innate immunity: Implications for health and disease

“…In order to bind stimulatory dsDNA, the HIN domains of IFI16 associate with both DNA strands across both the major and minor strands, in keeping with the requirement of dsDNA rather than ssDNA for the initiation of signalling from IFI16 [2,41]. While both HIN domains of IFI16 can associate with dsDNA, the HINb domain possesses a stronger DNA binding affinity [2,41]. In contrast to AIM2, DNA recognition by IFI16 can occur in both the cytosolic and nuclear compartments.…”

“…On the contrary, dsDNA sensing by IFI16 is length dependent [2], with dsDNA fragments of 150 base pairs reportedly being favourable for the formation of an optimal binding cluster of approximately ten IFI16 protomers [47]. In order to bind stimulatory dsDNA, the HIN domains of IFI16 associate with both DNA strands across both the major and minor strands, in keeping with the requirement of dsDNA rather than ssDNA for the initiation of signalling from IFI16 [2,41]. While both HIN domains of IFI16 can associate with dsDNA, the HINb domain possesses a stronger DNA binding affinity [2,41].…”

“…The crystal structure of the dsDNA bound AIM2 HIN domain has revealed that AIM2 recognises DNA in a nonsequence specific manner via electrostatic attraction between positively charged HIN domain residues and the dsDNA sugarphosphate backbone [41]. Importantly, these findings provide a rationale for the permissive nature of DNA binding by AIM2.…”

“…Unlike the DNA sensor TLR9, DNA sensing by IFI16 is independent of GC content and sequence [2]. As with AIM2, the sequence independent DNA binding by IFI16 is achieved by electrostatic attraction between positively charged HIN domain residues and the dsDNA sugar phosphate backbone [41]. This allows IFI16 to detect dsDNA from many sources including Kaposi's sarcoma-related herpesvirus (KSHV) [11], Epstein-Barr virus (EBV) [44], herpes simplex virus 1 (HSV-1) [45] and human immunodeficiency virus (HIV-1) [46].…”

Innate immune activation of NFκB and its antagonism by poxviruses

“…Absent in melanoma 2 (AIM2) is a PYRIN and HIN200 domaincontaining protein which forms an inflammasome that is activated by cytosolic DNA [91]. The HIN200 domain of AIM2 directly binds to dsDNA in the cytosol [92], leading to the recruitment of apoptotic speck protein (ASC) via the PYRIN domain, and subsequent recruitment and processing of pro-caspase 1 into caspase 1 [91] (Fig. 2).…”