The activation of NF-kappaB and IKK requires an upstream kinase complex consisting of TAK1 and adaptor proteins such as TAB1, TAB2, or TAB3. TAK1 is in turn activated by TRAF6, a RING domain ubiquitin ligase that facilitates the synthesis of lysine 63-linked polyubiquitin chains. Here we present evidence that TAB2 and TAB3 are receptors that bind preferentially to lysine 63-linked polyubiquitin chains through a highly conserved zinc finger (ZnF) domain. Mutations of the ZnF domain abolish the ability of TAB2 and TAB3 to bind polyubiquitin chains, as well as their ability to activate TAK1 and IKK. Significantly, replacement of the ZnF domain with a heterologous ubiquitin binding domain restored the ability of TAB2 and TAB3 to activate TAK1 and IKK. We also show that TAB2 binds to polyubiquitinated RIP following TNFalpha stimulation. These results indicate that polyubiquitin binding domains represent a new class of signaling domains that regulate protein kinase activity through a nonproteolytic mechanism.

The innate immune system deploys a variety of sensors to detect signs of infection. Nucleic acids represent a major class of pathogen signatures that can trigger robust immune responses. The presence of DNA in the cytoplasm of mammalian cells is a danger signal that activates innate immune responses; however, how cytosolic DNA triggers these responses remained unclear until recently. In this review, we focus on the mechanism of DNA sensing by the newly discovered cGAS-cGAMP-STING pathway and highlight recent progress in dissecting the in vivo functions of this pathway in immune defense as well as autoimmunity.

Summary Type-I interferons (IFNs) are important for antiviral and autoimmune responses. Retinoic acid-induced gene I (RIG-I) and mitochondrial antiviral signaling (MAVS) proteins mediate IFN production in response to cytosolic double-stranded RNA or single-stranded RNA containing 5′-triphosphate (5′-ppp). Cytosolic B-form double-stranded DNA, such as poly(dA-dT)·poly(dA-dT) [poly(dA-dT)], can also induce IFN-β, but the underlying mechanism is unknown. Here we show that the cytosolic poly(dA-dT) DNA is converted into 5′-ppp RNA to induce IFN-β through the RIG-I pathway. Biochemical purification led to the identification of DNA-dependent RNA polymerase III (Pol-III) as the enzyme responsible for synthesizing 5′-ppp RNA from the poly(dA-dT) template. Inhibition of RNA Pol-III prevents IFN-β induction by transfection of DNA or infection with DNA viruses. Furthermore, Pol-III inhibition abrogates IFN-β induction by the intracellular bacterium Legionella pneumophila and promotes the bacterial growth. These results suggest that RNA Pol-III is a cytosolic DNA sensor involved in innate immune responses.

Ubiquitination is catalyzed by a cascade of enzymes consisting of E1, E2, and E3. We report here the identification of an E1-like protein, termed E1-L2, that activates both ubiquitin and another ubiquitin-like protein, FAT10. Interestingly, E1-L2 can transfer ubiquitin to Ubc5 and Ubc13, but not Ubc3 and E2-25K, suggesting that E1-L2 may be specialized in a subset of ubiquitination reactions. E1-L2 forms a thioester with FAT10 in vitro, and this reaction requires the active-site cysteine of E1-L2 and the C-terminal diglycine motif of FAT10. Furthermore, endogenous FAT10 forms a thioester with E1-L2 in cells stimulated with tumor necrosis factor-alpha (TNFalpha) and interferon-gamma (IFNgamma), which induce FAT10 expression. Silencing of E1-L2 expression by RNAi blocks the formation of FAT10 conjugates in cells. Deletion of E1-L2 in mice caused embryonic lethality, suggesting that E1-L2 plays an important role in embryogenesis.

Summary Bromodomain-containing protein 7 (BRD7) is a member of the bromodomain-containing protein family that is known to play role as tumor suppressors. Here, we show that BRD7 is a component of the unfolded protein response (UPR) signaling through its ability to regulate X-box binding protein1 (XBP1) nuclear translocation. BRD7 interacts with the regulatory subunits of phosphatidyl-inositol3-kinase (PI3K) and increases the nuclear translocation of both p85α/β and XBP1s. Deficiency of BRD7 blocks the nuclear translocation of XBP1s. Furthermore, our in vivo studies have shown that BRD7 protein levels are reduced in the liver of obese mice, and reinstating BRD7 levels in the liver restores XBP1s nuclear translocation, improves glucose homeostasis, and ultimately reduces the blood glucose levels in the obese and diabetic mouse models.

RIG-I-like receptors (RLRs) activate host innate immune responses against virus infection through recruiting the mitochondrial adaptor protein MAVS (also known as IPS1, VISA, or CARDIF). Here we show that MAVS also plays a pivotal role in maintaining intestinal homeostasis. We found that MAVS knockout mice developed more severe mortality and morbidity than WT animals in an experimental model of colitis. Bone marrow transplantation experiments revealed that MAVS in cells of nonhematopoietic origin plays a dominant role in the protection against colitis. Importantly, RNA species derived from intestinal commensal bacteria activate the RIG-I-MAVS pathway to induce the production of multiple cytokines and antimicrobial peptides, including IFN-β and RegIIIγ. These results unveil a previously unexplored role of MAVS in monitoring intestinal commensal bacteria and maintaining tissue homeostasis.

Pannexin 1 (Panx1) is a membrane channel implicated in numerous physiological and pathophysiological processes via its ability to support release of ATP and other cellular metabolites for local intercellular signaling. However, to date, there has been no direct demonstration of large molecule permeation via the Panx1 channel itself, and thus the permselectivity of Panx1 for different molecules remains unknown. To address this, we expressed, purified, and reconstituted Panx1 into proteoliposomes and demonstrated that channel activation by caspase cleavage yields a dye-permeable pore that favors flux of anionic, large-molecule permeants (up to ~1 kDa). Large cationic molecules can also permeate the channel, albeit at a much lower rate. We further show that Panx1 channels provide a molecular pathway for flux of ATP and other anionic (glutamate) and cationic signaling metabolites (spermidine). These results verify large molecule permeation directly through caspase-activated Panx1 channels that can support their many physiological roles.

1551 4.2. Tumor Necrosis Factor Receptor (TNFR) 1553 4.3. T-Cell Receptors (TCRs) 1553 4.4. NOD-like Receptors (NLRs) 1554 4.5. RIG-I-like Receptors (RLRs) 1555 4.6. DNA Damage 1555 4.7. Human T-Cell Leukemia Virus-1 (HTLV-1) Tax Protein 1555 5. Negative Regulation of Protein Kinases by Deubiquitination Enzymes