Inflammasomes are key signalling platforms that detect pathogenic microorganisms and sterile stressors, and that activate the highly pro-inflammatory cytokines interleukin-1β (IL-1β) and IL-18. In this Review, we discuss the complex regulatory mechanisms that facilitate a balanced but effective inflammasome-mediated immune response, and we highlight the similarities to another molecular signalling platform — the apoptosome — that monitors cellular health. Extracellular regulatory mechanisms are discussed, as well as the intracellular control of inflammasome assembly, for example, via ion fluxes, free radicals and autophagy

The detection of intracellular microbial DNA is critical to an appropriate innate immune response, however current knowledge on how such DNA is sensed is limited. Here we identify IFI16, a PYHIN protein, as an intracellular DNA sensor mediating interferon-β (IFNβ)-induction. IFI16 directly associated with IFNβ-inducing viral DNA motifs. STING, a critical mediator of IFNβ responses to DNA, was recruited to IFI16 after DNA stimulation. Reduction of expression of IFI16, or its murine ortholog p204, by RNA interference inhibited DNA- and herpes simplex virus (HSV)-1-induced gene induction and IRF3 and NFκB activation. IFI16/p204 is the first PYHIN protein shown to be involved in IFNβ induction, and thus together with AIM2, a PYHIN protein that senses DNA for caspase 1 activation, is part of a new family of innate DNA sensors which we term AIM2-like receptors (ALRs).

Integrins are important adhesion receptors in all Metazoa that transmit conformational change bidirectionally across the membrane. Integrin α and β subunits form a head and two long legs in the ectodomain and span the membrane. Here, we define with crystal structures the atomic basis for allosteric regulation of the conformation and affinity for ligand of the integrin ectodomain, and how fibrinogen-mimetic therapeutics bind to platelet integrin α IIb b β3 . Allostery in the β 3 I domain alters three metal binding sites, associated loops and a α1-and α7-helices. Piston-like displacement of the a 7-helix causes a 62° reorientation between the β 3 I and hybrid domains. Transmission through the rigidly connected plexin/semaphorin/integrin (PSI) domain in the upper β 3 leg causes a 70Å separation between the knees of the α and β legs. Allostery in the head thus disrupts interaction between the legs in a previously described low-affinity bent integrin conformation, and leg extension positions the high-affinity head far above the cell surface.Integrins are adhesion receptors that transmit signals bidirectionally across the plasma membrane 1-4 . Rearrangements in integrin extracellular, transmembrane and cytoplasmic domains underlie diverse biological processes, including cell migration, morpho-genesis, immune responses and vascular haemostasis. The platelet-specific integrin α IIb β 3 is important in both the arrest of bleeding at sites of vascular injury and pathological thrombosis leading to heart attacks and stroke. Loss of the vascular endothelium results in platelet deposition, and receptors for collagen, thrombin and other agonists then initiate

The complete ectodomain of integrin αIIbβ3 reveals a bent, closed, low-affinity conformation, the β-knee, and a mechanism for linking cytoskeleton attachment to high affinity for ligand. Ca and Mg ions in the recognition site, including the synergistic metal ion binding site (SyMBS), are loaded prior to ligand binding. Electrophilicity of the ligand-binding Mg ion is increased in the open conformation. The β3 knee passes between the β3-PSI and αIIb-knob to bury the lower β-leg in a cleft, from which it is released for extension. Different integrin molecules in crystals and EM reveal breathing that appears on pathway to extension. Tensile force applied to the extended ligand-receptor complex stabilizes the closed, low-affinity conformation. By contrast, an additional lateral force applied to the β subunit to mimic attachment to moving actin filaments stabilizes the open, high-affinity conformation. This mechanism propagates allostery over long distances and couples cytoskeleton attachment of integrins to their high affinity state.

The structure of the I domain of integrin alpha L beta 2 bound to the Ig superfamily ligand ICAM-1 reveals the open ligand binding conformation and the first example of an integrin-IgSF interface. The I domain Mg2+ directly coordinates Glu-34 of ICAM-1, and a dramatic swing of I domain residue Glu-241 enables a critical salt bridge. Liganded and unliganded structures for both high- and intermediate-affinity mutant I domains reveal that ligand binding can induce conformational change in the alpha L I domain and that allosteric signals can convert the closed conformation to intermediate or open conformations without ligand binding. Pulling down on the C-terminal alpha 7 helix with introduced disulfide bonds ratchets the beta 6-alpha 7 loop into three different positions in the closed, intermediate, and open conformations, with a progressive increase in affinity.

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.

Chromosome segregation during mitosis requires assembly of the kinetochore complex at the centromere. Key to kinetochore assembly is the specific recognition of the histone variant CENP-A in the centromeric nucleosome by centromere protein C (CENP-C). We have defined the determinants of this recognition mechanism and discovered that CENP-C binds a hydrophobic region in the CENP-A tail and docks onto the acidic patch of histone H2A/H2B. We further find that the more broadly conserved CENP-C motif uses the same mechanism for CENP-A nucleosome recognition. Our findings reveal a conserved mechanism for protein recruitment to centromeres and a histone recognition mode whereby a disordered peptide binds the histone tail through nucleosome-docking-facilitated hydrophobic interactions.

Summary Linker histones bind to the nucleosome and regulate the structure of chromatin and gene expression. Despite more than three decades of effort, structural basis of nucleosome recognition by linker histones remains elusive. Here, we report the crystal structure of the globular domain of chicken linker histone H5 in complex with the nucleosome at 3.5 Å resolution, which is validated using nuclear magnetic resonance spectroscopy. The globular domain sits on the dyad of the nucleosome and interacts with both DNA linkers. Our structure integrates results from mutation analyses, previous cross-linking and fluorescence recovery after photobleach experiments, and helps resolve the long debate on structural mechanisms of nucleosome recognition by linker histones. The on-dyad binding mode of the H5 globular domain is different from the recently reported off-dyad binding mode of Drosophila linker histone H1. We demonstrate that linker histones with different binding modes could fold chromatin to form distinct higher-order structures.