Fluorescence-based sensors to monitor localization and functions of linear and K63-linked ubiquitin chains in cells

Versatile roles of k63-linked ubiquitin chains in trafficking

Modification by Lys63-linked ubiquitin (UbK63) chains is the second most abundant form of ubiquitylation. In addition to their role in DNA repair or kinase activation, UbK63 chains interfere with multiple steps of intracellular trafficking. UbK63 chains decorate many plasma membrane proteins, providing a signal that is often, but not always, required for their internalization. In yeast, plants, worms and mammals, this same modification appears to be critical for efficient sorting to multivesicular bodies and subsequent lysosomal degradation. UbK63 chains are also one of the modifications involved in various forms of autophagy (mitophagy, xenophagy, or aggrephagy). Here, in the context of trafficking, we report recent structural studies investigating UbK63 chains assembly by various E2/E3 pairs, disassembly by deubiquitylases, and specifically recognition as sorting signals by receptors carrying Ub-binding domains, often acting in tandem. In addition, we address emerging and unanticipated roles of UbK63 chains in various recycling pathways that function by activating nucleators required for actin polymerization, as well as in the transient recruitment of signaling molecules at the plasma or ER membrane. In this review, we describe recent advances that converge to elucidate the mechanisms underlying the wealth of trafficking functions of UbK63 chains.

Quantitative proteomics reveal a feedforward mechanism for mitochondrial PARKIN translocation and ubiquitin chain synthesis

Phosphorylation is often used to promote protein ubiquitylation, yet we rarely understand quantitatively how ligase activation and ubiquitin (UB) chain assembly are integrated with phosphoregulation. Here we employ quantitative proteomics and live-cell imaging to dissect individual steps in the PINK1 kinase-PARKIN UB ligase mitochondrial control pathway disrupted in Parkinson's disease. PINK1 plays a dual role by phosphorylating PARKIN on its UB-like domain and poly-UB chains on mitochondria. PARKIN activation by PINK1 produces canonical and noncanonical UB chains on mitochondria, and PARKIN-dependent chain assembly is required for accumulation of poly-phospho-UB (poly-p-UB) on mitochondria. In vitro, PINK1 directly activates PARKIN's ability to assemble canonical and noncanonical UB chains and promotes association of PARKIN with both p-UB and poly-p-UB. Our data reveal a feedforward mechanism that explains how PINK1 phosphorylation of both PARKIN and poly-UB chains synthesized by PARKIN drives a program of PARKIN recruitment and mitochondrial ubiquitylation in response to mitochondrial damage.

Lysine 63-linked polyubiquitination is dispensable for Parkin-mediated mitophagy

PINK1/Parkin-mediated mitophagy is thought to ensure mitochondrial quality control in neurons as well as other cells. Upon the loss of mitochondrial membrane potential (ΔΨm), Lys-63-linked polyubiquitin chains accumulate on the mitochondrial outer membrane in a Parkin-dependent manner. However, the physiological significance of Lys-63-linked polyubiquitination during mitophagy is not fully understood. Here, we report that the suppression of Lys-63-linked polyubiquitination through the removal of Ubc13 activity essentially affects neither PINK1 activation nor the degradation of depolarized mitochondria. Moreover, the inactivation of Ubc13 did not modulate the mitochondrial phenotypes of PINK1 knockdown Drosophila. Our data indicate that the formation of Lys-63-linked polyubiquitin chains on depolarized mitochondria is not a key factor for the PINK1-Parkin pathway as was once thought.

USP8 regulates mitophagy by removing K6-linked ubiquitin conjugates from parkin

Mutations in the Park2 gene, encoding the E3 ubiquitin-ligase parkin, are responsible for a familial form of Parkinson's disease (PD). Parkin-mediated ubiquitination is critical for the efficient elimination of depolarized dysfunctional mitochondria by autophagy (mitophagy). As damaged mitochondria are a major source of toxic reactive oxygen species within the cell, this pathway is believed to be highly relevant to the pathogenesis of PD. Little is known about how parkin-mediated ubiquitination is regulated during mitophagy or about the nature of the ubiquitin conjugates involved. We report here that USP8/UBPY, a deubiquitinating enzyme not previously implicated in mitochondrial quality control, is critical for parkin-mediated mitophagy. USP8 preferentially removes non-canonical K6-linked ubiquitin chains from parkin, a process required for the efficient recruitment of parkin to depolarized mitochondria and for their subsequent elimination by mitophagy. This work uncovers a novel role for USP8-mediated deubiquitination of K6-linked ubiquitin conjugates from parkin in mitochondrial quality control.

Cargo recognition and trafficking in selective autophagy

Selective autophagy is a quality control pathway through which cellular components are sequestered into double-membrane vesicles and delivered to specific intracellular compartments. This process requires autophagy receptors that link cargo to growing autophagosomal membranes. Selective autophagy is also implicated in various membrane trafficking events. Here we discuss the current view on how cargo selection and transport are achieved during selective autophagy, and point out molecular mechanisms that are congruent between autophagy and vesicle trafficking pathways.

Autophagy in antimicrobial immunity

Autophagy plays a key role in cellular homeostasis, responding to various environmental stresses. In particular, pathogen invasion leads to rapid induction of autophagy, which is critical for both innate and adaptive immune responses. In this review, we focus on the emerging molecular mechanisms of pathogen elimination by autophagy (a process known as xenophagy) and on the strategies developed by pathogens to subvert autophagy. We also address other functions of autophagy proteins in restricting pathogen invasion, independent of the formation of a canonical double-membrane autophagosome.

Facile synthesis of native and protease-resistant ubiquitylated peptides

The reversible post-translational modification of eukaryotic proteins by ubiquitin regulates key cellular processes including protein degradation and gene transcription. Studies of the mechanistic roles for protein ubiquitylation require quantities of homogenously modified substrates that are typically inaccessible from natural sources or by enzymatic ubiquitylation in vitro. Therefore, we developed a facile and scalable methodology for site-specific chemical ubiquitylation. Our semisynthetic strategy utilized a temporary ligation auxiliary, 2-(aminooxy)ethanethiol, to direct ubiquitylation to specific lysine residues in peptide substrates. Mild reductive removal of the auxiliary after ligation yielded ubiquitylated peptides with the native isopeptide linkage. Alternatively, retention of the ligation auxiliary yielded protease-resistant analogues of ubiquitylated peptides. Importantly, our strategy was fully compatible with the presence of protein thiol groups, as demonstrated by the synthesis of peptides modified by the human small ubiquitin-related modifier 3 protein.

Selective autophagy

During the last decade it has become evident that autophagy is not simply a non-selective bulk degradation pathway for intracellular components. On the contrary, the discovery and characterization of autophagy receptors which target specific cargo for lysosomal degradation by interaction with ATG8 (autophagy-related protein 8)/LC3 (light-chain 3) has accelerated our understanding of selective autophagy. A number of autophagy receptors have been identified which specifically mediate the selective autophagosomal degradation of a variety of cargoes including protein aggregates, signalling complexes, midbody rings, mitochondria and bacterial pathogens. In the present chapter, we discuss these autophagy receptors, their binding to ATG8/LC3 proteins and how they act in ubiquitin-mediated selective autophagy of intracellular bacteria (xenophagy) and protein aggregates (aggrephagy).

Photonic crystal enhancement of a homogeneous fluorescent assay using submicron fluid channels fabricated by E-jet patterning

We demonstrate the enhancement of a liquid-based homogenous fluorescence assay using the resonant electric fields from a photonic crystal (PC) surface. Because evanescent fields are confined to the liquid volume nearest to the photonic crystal, we developed a simple approach for integrating a PC fabricated on a silicon substrate within a fluid channel with submicron height, using electrohydrodynamic jet (e-jet) printing of a light-curable epoxy adhesive to define the fluid channel pattern. The PC is excited by a custom-designed compact instrument that illuminates the PC with collimated light that precisely matches the resonant coupling condition when the PC is covered with aqueous media. Using a molecular beacon nucleic acid fluorescence resonant energy transfer (FRET) probe for a specific miRNA sequence, we demonstrate an 8× enhancement of the fluorescence emission signal, compared to performing the same assay without exciting resonance in the PC detecting a miRNA sequence at a concentration of 62 nM from a liquid volume of only ∼20 nL. The approach may be utilized for any liquid-based fluorescence assay for applications in point-of-care diagnostics, environmental monitoring, or pathogen detection.

Measuring activity in the ubiquitin-proteasome system: from large scale discoveries to single cells analysis

The ubiquitin-proteasome system (UPS) is the primary pathway responsible for the recognition and degradation of misfolded, damaged, or tightly regulated proteins in addition to performing essential roles in DNA repair, cell cycle regulation, cell migration, and the immune response. While traditional biochemical techniques have proven useful in the identification of key proteins involved in this pathway, the implementation of novel reporters responsible for measuring enzymatic activity of the UPS has provided valuable insight into the effectiveness of therapeutics and role of the UPS in various human diseases such as multiple myeloma and Huntington's disease. These reporters, usually consisting of a recognition sequence fused to an analytical handle, are designed to specifically evaluate enzymatic activity of certain members of the UPS including the proteasome, E3 ubiquitin ligases, and deubiquitinating enzymes. This review highlights the more commonly used reporters employed in a variety of scenarios ranging from high-throughput screening of novel inhibitors to single cell microscopy techniques measuring E3 ligase or proteasome activity. Finally, a recent study is presented highlighting the development of a novel degron-based substrate designed to overcome the limitations of current reporting techniques in measuring E3 ligase and proteasome activity in patient samples.

Cytoplasmic access by intracellular bacterial pathogens

Entry into host cells is a strategy widely used by bacterial pathogens, after which they either remain within membrane-bound compartments or rupture the endocytic vacuole to reach the cytoplasm. During recent years, cytoplasmic access has been documented for an increasing number of pathogens. Here we review how classical cytoplasmic bacterial pathogens rupture their endocytic vacuoles as well as the mechanisms used to accomplish this task by bacterial species for which host cytoplasmic localization has only recently been identified. We also discuss the consequences for pathogenesis resulting from this change in intracellular localization, with a particular focus on the role of the host. What emerges is that cytoplasmic access plays an important role in the pathophysiology of an increasing number of intracellular bacterial pathogens.

Autophagy and cellular immune responses

Autophagy constitutes a mechanism for the sequestration and lysosomal degradation of various cytoplasmic structures, including damaged organelles and invading microorganisms. Autophagy not only represents an essential cell-intrinsic mechanism to protect against internal and external stress conditions but also shapes cellular immunity. Recent evidence indicates that autophagic responses in antigen-donor cells affect the release of several cytokines and "danger signals." Thus, especially when it precedes cell death, autophagy alerts innate immune effectors to elicit cognate immune responses. Autophagy is also important for the differentiation, survival, and activation of myeloid and lymphoid cells. Accordingly, inherited mutations in autophagy-relevant genes are associated with immune diseases, whereas oncogenesis-associated autophagic defects promote the escape of developing tumors from immunosurveillance. Here, we discuss the regulation of autophagy in the course of cellular immune responses and emphasize its impact on the immunogenicity of antigen-donor cells and on the activity of antigen-presenting cells and T lymphocytes.

Recruitment of the autophagic machinery to endosomes during infection is mediated by ubiquitin

After bacterial invasion, ubiquitin is conjugated to host endosomal proteins and recognized by the autophagic machinery independent of LC3.

Although ubiquitin is thought to be important for the autophagic sequestration of invading bacteria (also called xenophagy), its precise role remains largely enigmatic. Here we determined how ubiquitin is involved in this process. After invasion, ubiquitin is conjugated to host cellular proteins in endosomes that contain Salmonella or transfection reagent–coated latex (polystyrene) beads, which mimic invading bacteria. Ubiquitin is recognized by the autophagic machinery independently of the LC3–ubiquitin interaction through adaptor proteins, including a direct interaction between ubiquitin and Atg16L1. To ensure that invading pathogens are captured and degraded, Atg16L1 targeting is secured by two backup systems that anchor Atg16L1 to ubiquitin-decorated endosomes. Thus, we reveal that ubiquitin is a pivotal molecule that connects bacteria-containing endosomes with the autophagic machinery upstream of LC3.

OTULIN restricts Met1-linked ubiquitination to control innate immune signaling

Conjugation of Met1-linked polyubiquitin (Met1-Ub) by the linear ubiquitin chain assembly complex (LUBAC) is an important regulatory modification in innate immune signaling. So far, only few Met1-Ub substrates have been described, and the regulatory mechanisms have remained elusive. We recently identified that the ovarian tumor (OTU) family deubiquitinase OTULIN specifically disassembles Met1-Ub. Here, we report that OTULIN is critical for limiting Met1-Ub accumulation after nucleotide-oligomerization domain-containing protein 2 (NOD2) stimulation, and that OTULIN depletion augments signaling downstream of NOD2. Affinity purification of Met1-Ub followed by quantitative proteomics uncovered RIPK2 as the predominant NOD2-regulated substrate. Accordingly, Met1-Ub on RIPK2 was largely inhibited by overexpressing OTULIN and was increased by OTULIN depletion. Intriguingly, OTULIN-depleted cells spontaneously accumulated Met1-Ub on LUBAC components, and NOD2 or TNFR1 stimulation led to extensive Met1-Ub accumulation on receptor complex components. We propose that OTULIN restricts Met1-Ub formation after immune receptor stimulation to prevent unwarranted proinflammatory signaling.

Ubiquitination-induced fluorescence complementation (UiFC) for detection of K48 ubiquitin chains in vitro and in live cells

Proteins can be modified with eight homogenous ubiquitin chains linked by an isopeptide bond between the C-terminus of one ubiquitin and an amine from one of the seven lysines or the N-terminal methionine of the next ubiquitin. These topologically distinct ubiquitin chains signal for many essential cellular functions, such as protein degradation, cell cycle progression, DNA repair, and signal transduction. The lysine 48 (K48)-linked ubiquitin chain is one of the most abundant chains and a major proteasome-targeting signal in cells. Despite recent advancements in imaging linkage-specific polyubiquitin chains, no tool is available for imaging K48 chains in live cells. Here we report on a ubiquitination-induced fluorescence complementation (UiFC) assay for detecting K48 ubiquitin chains in vitro and in live cells. For this assay, two nonfluorescent fragments of a fluorescent protein were fused to the ubiquitin-interacting motifs (UIMs) of epsin1 protein. Upon simultaneous binding to a ubiquitin chain, the nonfluorescent fragments of the two fusion proteins are brought in close proximity to reconstitute fluorescence. When used in vitro, UiFC preferentially detected K48 ubiquitin chains with excellent signal-to-noise ratio. Time-lapse imaging revealed that UiFC is capable of monitoring increases in polyubiquitination induced by treatment with proteasome inhibitor, by agents that induce stress, and during mitophagy in live cells.

Selective monitoring of ubiquitin signals with genetically encoded ubiquitin chain-specific sensors

Despite intensive research, there is a distinct lack of methodology for visualizing endogenous ubiquitination in living cells. In this protocol, we describe how unique properties of ubiquitin (Ub)-binding domains (UBDs) can be used to selectively detect, visualize and inhibit Ub-dependent processes in mammalian cells. The procedure deals with designing and validating the binding selectivity of GFP-tagged K63- and linear-linked sensors (TAB2 NZF and NEMO UBAN, respectively) in vitro. We describe how these moieties can be used to inhibit tumor necrosis factor (TNF)-mediated NF-κB signaling and to detect ubiquitinated cytosolic Salmonella in living cells, emphasizing a more flexible use compared with chain-specific antibodies. These chain-specific sensors can be used to detect Ub-like or autophagy-related modifiers and, in combination with mass spectrometry, to identify new Ub targets. These Ub (-like) sensors can be designed, constructed and tested in ~2-3 weeks.

OTULIN antagonizes LUBAC signaling by specifically hydrolyzing Met1-linked polyubiquitin

The linear ubiquitin (Ub) chain assembly complex (LUBAC) is an E3 ligase that specifically assembles Met1-linked (also known as linear) Ub chains that regulate nuclear factor κB (NF-κB) signaling. Deubiquitinases (DUBs) are key regulators of Ub signaling, but a dedicated DUB for Met1 linkages has not been identified. Here, we reveal a previously unannotated human DUB, OTULIN (also known as FAM105B), which is exquisitely specific for Met1 linkages. Crystal structures of the OTULIN catalytic domain in complex with diubiquitin reveal Met1-specific Ub-binding sites and a mechanism of substrate-assisted catalysis in which the proximal Ub activates the catalytic triad of the protease. Mutation of Ub Glu16 inhibits OTULIN activity by reducing kcat 240-fold. OTULIN overexpression or knockdown affects NF-κB responses to LUBAC, TNFα, and poly(I:C) and sensitizes cells to TNFα-induced cell death. We show that OTULIN binds LUBAC and that overexpression of OTULIN prevents TNFα-induced NEMO association with ubiquitinated RIPK1. Our data suggest that OTULIN regulates Met1-polyUb signaling.

The LRR and RING domain protein LRSAM1 is an E3 ligase crucial for ubiquitin-dependent autophagy of intracellular Salmonella Typhimurium

Several species of pathogenic bacteria replicate within an intracellular vacuolar niche. Bacteria that escape into the cytosol are captured by the autophagic pathway and targeted for lysosomal degradation, representing a defense against bacterial exploitation of the host cytosol. Autophagic capture of Salmonella Typhimurium occurs predominantly via generation of a polyubiquitin signal around cytosolic bacteria, binding of adaptor proteins, and recruitment of autophagic machinery. However, the components mediating bacterial target selection and ubiquitination remain obscure. We identify LRSAM1 as the E3 ligase responsible for anti-Salmonella autophagy-associated ubiquitination. LRSAM1 localizes to several intracellular bacterial pathogens and generates the bacteria-associated ubiquitin signal; these functions require LRSAM1's leucine-rich repeat and RING domains, respectively. Using cells from LRSAM1-deficient individuals, we confirm that LRSAM1 is required for ubiquitination associated with intracellular bacteria but dispensable for ubiquitination of aggregated proteins. LRSAM1 is therefore a bacterial recognition protein and ubiquitin ligase that defends the cytoplasm from invasive pathogens.

LRSAM1, an E3 Ubiquitin ligase with a sense for bacteria

Antimicrobial autophagy is a host cellular process that captures and delivers intracellular parasites to lysosomes following their targeting as cargo via ubiquitination. Huett et al. (2012) show that the LRR- and RING-domain-containing E3 ubiquitin ligase LRSAM1 recognizes various bacteria and generates a ubiquitin signal that initiates the autophagic cascade.

Using the ubiquitin-modified proteome to monitor protein homeostasis function

The ubiquitin system is essential for the maintenance of proper protein homeostasis function across eukaryotic species. Although the general enzymatic architecture for adding and removing ubiquitin from substrates is well defined, methods for the comprehensive investigation of cellular ubiquitylation targets have just started to emerge. Recent advances in ubiquitin-modified peptide enrichment have greatly increased the number of identified endogenous ubiquitylation targets, as well as the number of sites of ubiquitin attachment within these substrates. Herein we evaluate current strategies using mass-spectrometry-based proteomics to characterize ubiquitin and ubiquitin-like modifications. Using existing data, we describe the characteristics of the ubiquitin-modified proteome and discuss strategies for the biological interpretation of existing and future ubiquitin-based proteomic studies.

The role of 'eat-me' signals and autophagy cargo receptors in innate immunity

Selective autophagy is an important effector mechanism of cell autonomous immunity, in particular against invasive bacterial species. Anti-bacterial autophagy is activated by rupture of bacteria-containing vacuoles and exposure of bacteria to the cytosol. The autophagy cargo receptors p62, NDP52 and Optineurin detect incoming bacteria that have become associated with specific 'eat-me' signals such as Galectin-8 and poly-ubiquitin and feed them into the autophagy pathway via interactions with phagophore-associated ATG8-like proteins. Here we review recent progress in the field regarding the origin of bacteria-associated 'eat-me' signals, the specific roles of individual cargo receptors and how disrupting cargo receptor function may be important for bacterial evasion of autophagy.

The Colossus of ubiquitylation: decrypting a cellular code

Ubiquitylation is an essential posttranslational modification that can regulate the stability, activity, and localization of thousands of proteins. The reversible attachment of ubiquitin as well as interpretation of the ubiquitin signal depends on dynamic protein networks that are challenging to analyze. In this perspective, we discuss tools of the trade that have recently been developed to dissect mechanisms of ubiquitin-dependent signaling, thereby revealing the critical features of an important cellular code.

Substrate recognition in selective autophagy and the ubiquitin-proteasome system

Dynamic protein turnover through regulated protein synthesis and degradation ensures cellular growth, proliferation, differentiation and adaptation. Eukaryotic cells utilize two mechanistically distinct but largely complementary systems - the 26S proteasome and the lysosome (or vacuole in yeast and plants) - to effectively target a wide range of proteins for degradation. The concerted action of the ubiquitination machinery and the 26S proteasome ensures the targeted and tightly regulated degradation of a subset of commonly short-lived cellular proteins. Autophagy is a distinct degradation pathway, which transports a highly heterogeneous set of cargos in dedicated vesicles, called autophagosomes, to the lysosome. There the cargo becomes degraded and its molecular building blocks are recycled. While general autophagy randomly engulfs portions of the cytosol, selective autophagy employs dedicated cargo adaptors to specifically enrich the forming autophagosomes for a certain type of cargo as a response to various intra- or extracellular signals. Selective autophagy targets a wide range of cargos including long-lived proteins and protein complexes, organelles, protein aggregates and even intracellular microbes. In this review we summarize available data on cargo recognition mechanisms operating in selective autophagy and the ubiquitin-proteasome system (UPS), and emphasize their differences and common themes. Moreover, we derive general regulatory principles underlying cargo recognition in selective autophagy, and describe the system-wide crosstalk between these two cellular protein degradation systems. This article is part of a Special Issue entitled: Ubiquitin-Proteasome System. Guest Editors: Thomas Sommer and Dieter H. Wolf.

Regulation of NF-κB by ubiquitination

The nuclear factor κ enhancer binding protein (NF-κB) family of transcription factors regulates the expression of a large array of genes involved in diverse cellular processes including inflammation, immunity and cell survival. Activation of NF-κB requires ubiquitination, a highly conserved and versatile modification that can regulate cell signaling through both proteasome dependent and independent mechanisms. Studies in the past few years have provided new insights into the mechanisms underlying regulation of NF-κB by ubiquitination, including the involvement of multiple linkages of ubiquitin, the essential role of ubiquitin binding, and the function of unanchored polyubiquitin chains. In this review, we will focus on recent advances in understanding the role of ubiquitination in NF-κB regulation in various pathways.

Goliath family E3 ligases regulate the recycling endosome pathway via VAMP3 ubiquitylation

Diverse cellular processes depend on endocytosis, intracellular vesicle trafficking, sorting and exocytosis, processes regulated post-transcriptionally by modifications such as phosphorylation and ubiquitylation. In addition to sorting to the lysosome, cargo is recycled to the plasma membrane via recycling endosomes. Here, we describe a role of the goliath gene family of protease-associated (PA) domain E3 ligases in regulating recycling endosome trafficking. The two Drosophila members of this family--Goliath and Godzilla(CG10277)--are located on endosomes, and both ectopic expression and loss-of-function lead to the accumulation of Rab5-positive giant endosomes. Furthermore, the human homologue RNF167 exhibits similar behaviour. We show that the soluble N-ethylmaleimide-sensitive fusion attachment protein receptor (SNARE) protein VAMP3 is a target of these ubiquitin ligases, and that recycling endosome trafficking is abrogated in response to their activity. Furthermore, mutation of the Godzilla ubiquitylation target lysines on VAMP3 abrogates the formation of enlarged endosomes induced by either Godzilla or RNF167. Thus, Goliath ubiquitin ligases play a novel role in regulating recycling endosome trafficking via ubiquitylation of the VAMP3 SNARE protein.

RING finger protein 11 targets TBK1/IKKi kinases to inhibit antiviral signaling

A key feature of the innate antiviral immune response is a rapid nonspecific response to virus infection largely mediated by the induction and extracellular secretion of type I interferons (IFNs) that restrict virus replication. Cytoplasmic sensors such as RIG-I recognize viral RNA and trigger antiviral signaling pathways that upregulate IFN transcription. However, it remains largely unknown how antiviral signaling is negatively regulated to maintain homeostasis after the elimination of virus. In this report, we have identified the RING domain-containing protein RING finger 11 (RNF11) as a novel negative regulator of innate antiviral signaling. Overexpression of RNF11 downregulated IFN-β expression and enhanced viral replication whereas siRNA-mediated knockdown of RNF11 suppressed viral replication. RNF11 interacted with the noncanonical IKK kinases TBK1/IKKi and attenuated their Lys63-linked polyubiquitination by blocking interactions with the E3 ligase TRAF3. The inhibitory function of RNF11 was dependent on the ubiquitin-binding adaptor molecule TAX1BP1 which was required for RNF11 to target TBK1/IKKi. Collectively, these results indicate that RNF11 functions together with TAX1BP1 to restrict antiviral signaling and IFN-β production.