HOIL-1 ubiquitin ligase activity targets unbranched glucosaccharides and is required to prevent polyglucosan accumulation

The RBR E3 ubiquitin ligase HOIL-1 can ubiquitinate diverse non-protein substrates in vitro

HOIL-1 is a RING-between-RING-family E3 ubiquitin ligase and a component of the linear ubiquitin chain assembly complex. Although most E3 ubiquitin ligases conjugate ubiquitin to protein lysine sidechains, HOIL-1 has also been reported to ubiquitinate hydroxyl groups in protein serine and threonine sidechains and glucosaccharides, such as glycogen and its building block maltose, in vitro. However, HOIL-1 substrate specificity is currently poorly defined. Here, we show that HOIL-1 is unable to ubiquitinate lysine but can efficiently ubiquitinate serine and a variety of model and physiologically relevant di- and monosaccharides in vitro. We identify a critical catalytic histidine residue, His510, in the flexible catalytic site of HOIL-1 that enables this O-linked ubiquitination and prohibits ubiquitin discharge onto lysine sidechains. We use HOIL-1's in vitro non-proteinaceous ubiquitination activity to produce preparative amounts of different ubiquitinated saccharides that can be used as tool compounds and standards in the rapidly emerging field of non-proteinaceous ubiquitination. Finally, we report an engineered, constitutively active HOIL-1 variant that simplifies in vitro generation of ubiquitinated saccharides.

ATP functions as a pathogen-associated molecular pattern to activate the E3 ubiquitin ligase RNF213

The giant E3 ubiquitin ligase RNF213 is a conserved component of mammalian cell-autonomous immunity, limiting the replication of bacteria, viruses and parasites. To understand how RNF213 reacts to these unrelated pathogens, we employ chemical and structural biology to find that ATP binding to its ATPases Associated with diverse cellular Activities (AAA) core activates its E3 function. We develop methodology for proteome-wide E3 activity profiling inside living cells, revealing that RNF213 undergoes a reversible switch in E3 activity in response to cellular ATP abundance. Interferon stimulation of macrophages raises intracellular ATP levels and primes RNF213 E3 activity, while glycolysis inhibition depletes ATP and downregulates E3 activity. These data imply that ATP bears hallmarks of a danger/pathogen associated molecular pattern, coordinating cell-autonomous defence. Furthermore, quantitative labelling of RNF213 with E3-activity probes enabled us to identify the catalytic cysteine required for substrate ubiquitination and obtain a cryo-EM structure of the RNF213-E2-ubiquitin conjugation enzyme transfer intermediate, illuminating an unannotated E2 docking site. Together, our data demonstrate that RNF213 represents a new class of ATP-dependent E3 enzyme, employing distinct catalytic and regulatory mechanisms adapted to its specialised role in the broad defence against intracellular pathogens.

Linear ubiquitination at damaged lysosomes induces local NFKB activation and controls cell survival

Lysosomes are the major cellular organelles responsible for nutrient recycling and degradation of cellular material. Maintenance of lysosomal integrity is essential for cellular homeostasis and lysosomal membrane permeabilization (LMP) sensitizes toward cell death. Damaged lysosomes are repaired or degraded via lysophagy, during which glycans, exposed on ruptured lysosomal membranes, are recognized by galectins leading to K48- and K63-linked poly-ubiquitination (poly-Ub) of lysosomal proteins followed by recruitment of the macroautophagic/autophagic machinery and degradation. Linear (M1) poly-Ub, catalyzed by the linear ubiquitin chain assembly complex (LUBAC) E3 ligase and removed by OTULIN (OTU deubiquitinase with linear linkage specificity) exerts important functions in immune signaling and cell survival, but the role of M1 poly-Ub in lysosomal homeostasis remains unexplored. Here, we demonstrate that L-leucyl-leucine methyl ester (LLOMe)-damaged lysosomes accumulate M1 poly-Ub in an OTULIN- and K63 Ub-dependent manner. LMP-induced M1 poly-Ub at damaged lysosomes contributes to lysosome degradation, recruits the NFKB (nuclear factor kappa B) modulator IKBKG/NEMO and locally activates the inhibitor of NFKB kinase (IKK) complex to trigger NFKB activation. Inhibition of lysosomal degradation enhances LMP- and OTULIN-regulated cell death, indicating pro-survival functions of M1 poly-Ub during LMP and potentially lysophagy. Finally, we demonstrate that M1 poly-Ub also occurs at damaged lysosomes in primary mouse neurons and induced pluripotent stem cell-derived primary human dopaminergic neurons. Our results reveal novel functions of M1 poly-Ub during lysosomal homeostasis, LMP and degradation of damaged lysosomes, with important implications for NFKB signaling, inflammation and cell death.Abbreviation: ATG: autophagy related; BafA1: bafilomycin A1; CALCOCO2/NDP52: calcium binding and coiled-coil domain 2; CRISPR: clustered regularly interspaced short palindromic repeats; CHUK/IKKA: component of inhibitor of nuclear factor kappa B kinase complex; CUL4A-DDB1-WDFY1: cullin 4A-damage specific DNA binding protein 1-WD repeat and FYVE domain containing 1; DGCs: degradative compartments; DIV: days in vitro; DUB: deubiquitinase/deubiquitinating enzyme; ELDR: endo-lysosomal damage response; ESCRT: endosomal sorting complex required for transport; FBXO27: F-box protein 27; GBM: glioblastoma multiforme; IKBKB/IKKB: inhibitor of nuclear factor kappa B kinase subunit beta; IKBKG/NEMO: inhibitor of nuclear factor kappa B kinase regulatory subunit gamma; IKK: inhibitor of NFKB kinase; iPSC: induced pluripotent stem cell; KBTBD7: kelch repeat and BTB domain containing 7; KO: knockout; LAMP1: lysosomal associated membrane protein 1; LCD: lysosomal cell death; LGALS: galectin; LMP: lysosomal membrane permeabilization; LLOMe: L-leucyl-leucine methyl ester; LOP: loperamide; LUBAC: linear ubiquitin chain assembly complex; LRSAM1: leucine rich repeat and sterile alpha motif containing 1; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MTOR: mechanistic target of rapamycin kinase; MTORC1: MTOR complex 1; NBR1: NBR1 autophagy cargo receptor; NFKB/NF-κB: nuclear factor kappa B; NFKBIA/IĸBα: nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor alpha; OPTN: optineurin; ORAS: OTULIN-related autoinflammatory syndrome; OTULIN: OTU deubiquitinase with linear linkage specificity; RING: really interesting new gene; RBR: RING-in-between-RING; PLAA: phospholipase A2 activating protein; RBCK1/HOIL-1: RANBP2-type and C3HC4-type zinc finger containing 1; RNF31/HOIP: ring finger protein 31; SHARPIN: SHANK associated RH domain interactor; SQSTM1/p62: sequestosome 1; SR-SIM: super-resolution-structured illumination microscopy; TAX1BP1: Tax1 binding protein 1; TBK1: TANK binding kinase 1; TH: tyrosine hydroxylase; TNF/TNFα: tumor necrosis factor; TNFRSF1A/TNFR1-SC: TNF receptor superfamily member 1A signaling complex; TRIM16: tripartite motif containing 16; Ub: ubiquitin; UBE2QL1: ubiquitin conjugating enzyme E2 QL1; UBXN6/UBXD1: UBX domain protein 6; VCP/p97: valosin containing protein; WIPI2: WD repeat domain, phosphoinositide interacting 2; YOD1: YOD1 deubiquitinase.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2021-2022

The use of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry for the analysis of carbohydrates and glycoconjugates is a well-established technique and this review is the 12th update of the original article published in 1999 and brings coverage of the literature to the end of 2022. As with previous review, this review also includes a few papers that describe methods appropriate to analysis by MALDI, such as sample preparation, even though the ionization method is not MALDI. The review follows the same format as previous reviews. It is divided into three sections: (1) general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, quantification and the use of computer software for structural identification. (2) Applications to various structural types such as oligo- and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals, and (3) other general areas such as medicine, industrial processes, natural products and glycan synthesis where MALDI is extensively used. Much of the material relating to applications is presented in tabular form. MALDI is still an ideal technique for carbohydrate analysis, particularly in its ability to produce single ions from each analyte and advancements in the technique and range of applications show little sign of diminishing.

The Molecular Toolbox for Linkage Type-Specific Analysis of Ubiquitin Signaling

Modification of proteins and other biomolecules with ubiquitin regulates virtually all aspects of eukaryotic cell biology. Ubiquitin can be attached to substrates as a monomer or as an array of polyubiquitin chains with defined linkages between the ubiquitin moieties. Each ubiquitin linkage type adopts a distinct structure, enabling the individual linkage types to mediate specific functions or outcomes in the cell. The dynamics, heterogeneity, and in some cases low abundance, make analysis of linkage type-specific ubiquitin signaling a challenging and complex task. Herein, the strategies and molecular tools available for enrichment, detection, and characterization of linkage type-specific ubiquitin signaling, are reviewed. The molecular "toolbox" consists of a range of molecularly different affinity reagents, including antibodies and antibody-like molecules, affimers, engineered ubiquitin-binding domains, catalytically inactive deubiquitinases, and macrocyclic peptides, each with their unique characteristics and binding modes. The molecular engineering of these ubiquitin-binding molecules makes them useful tools and reagents that can be coupled to a range of analytical methods, such as immunoblotting, fluorescence microscopy, mass spectrometry-based proteomics, or enzymatic analyses to aid in deciphering the ever-expanding complexity of ubiquitin modifications.

Insights into non-proteinaceous ubiquitination

Ubiquitination plays a key role in the regulation of numerous diverse cellular functions. This process involves the covalent attachment of ubiquitin to protein substrates by a cascade of enzymes. In recent years, various non-proteinaceous substrates of ubiquitination have been discovered, expanding the potential for the functions of ubiquitination beyond its conventional role as a post-translational modification. Here, we profile the non-proteinaceous substrates of ubiquitination reported to date, the enzymes that regulate these activities, and the mechanistic details of substrate modification. The biological functions linked to these modifications are discussed, and finally, we highlight the challenges hindering further progress in the identification and functional characterization of non-proteinaceous substrates of ubiquitination within cellular contexts.

Genetic Regulation of Cell Death: Insights from Autoinflammatory Diseases

Metazoans have evolved innate antimicrobial defenses that promote cellular survival and proliferation. Countering the inevitable molecular mechanisms by which microbes sabotage these pathways, multicellular organisms rely on an alternative, perhaps more ancient, strategy that is the immune equivalent of suicide bombing: Infection triggers cell death programs that summon localized or even systemic inflammation. The study of human genetics has now unveiled a level of complexity that refutes the naive view that cell death is merely a blunt instrument or an evolutionary afterthought. To the contrary, findings from patients with rare diseases teach us that cell death-induced inflammation is a sophisticated, tightly choreographed process. We herein review the emerging body of evidence describing a group of illnesses-inborn errors of cell death, which define many of the molecular building blocks and regulatory elements controlling cell death-induced inflammation in humans-and provide a possible road map to countering this process across the spectrum of rare and common illnesses.

Ubiquitin is directly linked via an ester to protein-conjugated mono-ADP-ribose

The prevailing view on post-translational modifications (PTMs) is that a single amino acid is modified with a single PTM at any given time. However, recent work has demonstrated crosstalk between different PTMs, some occurring on the same residue. Such interplay is seen with ADP-ribosylation and ubiquitylation. For example, DELTEX E3 ligases were reported to ubiquitylate a hydroxyl group on free NAD+ and ADP-ribose in vitro, generating a noncanonical ubiquitin ester-linked species. In this report, we show, for the first time, that this dual PTM occurs in cells on mono-ADP-ribosylated (MARylated) PARP10 on Glu/Asp sites to form a MAR ubiquitin ester. We call this process mono-ADP-ribosyl ubiquitylation or MARUbylation. Using chemical and enzymatic treatments, including a newly characterized bacterial deubiquitinase with esterase-specific activity, we discovered that multiple PARPs are MARUbylated and extended with K11-linked polyubiquitin chains when exogenously expressed. Finally, we show that in response to type I interferon stimulation, MARUbylation can occur endogenously on PARP targets. Thus, MARUbylation represents a new dual PTM that broadens our understanding of the function of PARP-mediated ADP-ribosylation in cells.

Shigella effector IpaH1.4 subverts host E3 ligase RNF213 to evade antibacterial immunity

Ubiquitination plays vital roles in modulating pathogen-host cell interactions. RNF213, a E3 ligase, can catalyze the ubiquitination of lipopolysaccharide (LPS) and is crucial for antibacterial immunity in mammals. Shigella flexneri, an LPS-containing pathogenic bacterium, has developed mechanisms to evade host antibacterial defenses during infection. However, the precise strategies by which S. flexneri circumvents RNF213-mediated antibacterial immunity remain poorly understood. Here, through comprehensive biochemical, structural and cellular analyses, we reveal that the E3 effector IpaH1.4 of S. flexneri can directly target human RNF213 via a specific interaction between the IpaH1.4 LRR domain and the RING domain of RNF213, and mediate the ubiquitination and proteasomal degradation of RNF213 in cells. Furthermore, we determine the cryo-EM structure of human RNF213 and the crystal structure of the IpaH1.4 LRR/RNF213 RING complex, elucidating the molecular mechanism underlying the specific recognition of RNF213 by IpaH1.4. Finally, our cell based functional assays demonstrate that the targeting of host RNF213 by IpaH1.4 promotes S. flexneri proliferation within infected cells. In summary, our work uncovers an unprecedented strategy employed by S. flexneri to subvert the key host immune factor RNF213, thereby facilitating bacterial proliferation during invasion.

Ubiquitin-A structural perspective

The modification of proteins and other biomolecules with the small protein ubiquitin has enthralled scientists from many disciplines for decades, creating a broad research field. Ubiquitin research is particularly rich in molecular and mechanistic understanding due to a plethora of (poly)ubiquitin structures alone and in complex with ubiquitin machineries. Furthermore, due to its favorable properties, ubiquitin serves as a model system for many biophysical and computational techniques. Here, we review the current knowledge of ubiquitin signals through a ubiquitin-centric, structural biology lens. We amalgamate the information from 240 structures in the Protein Data Bank (PDB), combined with single-molecule, molecular dynamics, and nuclear magnetic resonance (NMR) studies, to provide a comprehensive picture of ubiquitin and polyubiquitin structures and dynamics. We close with a discussion of the latest frontiers in ubiquitin research, namely the modification of ubiquitin by other post-translational modifications (PTMs) and the notion that ubiquitin is attached to biomolecules beyond proteins.

Insights into mechanisms of ubiquitin ADP-ribosylation reversal

Ubiquitination and ADP-ribosylation are two types of post-translational modification (PTM) involved in regulating various cellular activities. In a striking example of direct interplay between ubiquitination and ADP-ribosylation, the bacterial pathogen Legionella pneumophila uses its SidE family of secreted effectors to catalyze an NAD+-dependent phosphoribosyl ubiquitination of host substrates in a process involving the intermediary formation of ADP-ribosylated ubiquitin (ADPR-Ub). This noncanonical ubiquitination pathway is finely regulated by multiple Legionella effectors to ensure a balanced host subjugation. Among the various regulatory effectors, the macrodomain effector MavL has been recently shown to reverse the Ub ADP-ribosylation and regenerate intact Ub. Here, we briefly outline emerging knowledge on ubiquitination and ADP-ribosylation and tap into cases of direct cross-talk between these two PTMs. The chemistry of ADP-ribose in the context of the PTM and the reversal mechanisms of ADP-ribosylation are then highlighted. Lastly, focusing on recent structural studies on the MavL-mediated reversal of Ub ADP-ribosylation, we strive to deduce distinct mechanisms regarding the catalysis and product release of this reaction.

A closer look at the role of deubiquitinating enzymes in the Hypoxia Inducible Factor pathway

Hypoxia Inducible transcription Factors (HIFs) are central to the metazoan oxygen-sensing response. Under low oxygen conditions (hypoxia), HIFs are stabilised and govern an adaptive transcriptional programme to cope with prolonged oxygen starvation. However, when oxygen is present, HIFs are continuously degraded by the proteasome in a process involving prolyl hydroxylation and subsequent ubiquitination by the Von Hippel Lindau (VHL) E3 ligase. The essential nature of VHL in the HIF response is well established but the role of other enzymes involved in ubiquitination is less clear. Deubiquitinating enzymes (DUBs) counteract ubiquitination and provide an important regulatory aspect to many signalling pathways involving ubiquitination. In this review, we look at the complex network of ubiquitination and deubiquitination in controlling HIF signalling in normal and low oxygen tensions. We discuss the relative importance of DUBs in opposing VHL, and explore roles of DUBs more broadly in hypoxia, in both VHL and HIF independent contexts. We also consider the catalytic and non-catalytic roles of DUBs, and elaborate on the potential benefits and challenges of inhibiting these enzymes for therapeutic use.

Mass Spectral Feature Analysis of Ubiquitylated Peptides Provides Insights into Probing the Dark Ubiquitylome

Ubiquitylation is a structurally and functionally diverse post-translational modification that involves the covalent attachment of the small protein ubiquitin to other protein substrates. Trypsin-based proteomics is the most common approach for globally identifying ubiquitylation sites. However, we estimate that such methods are unable to detect ∼40% of ubiquitylation sites in the human proteome, i.e., "the dark ubiquitylome", including many important for human health and disease. In this meta-analysis of three large ubiquitylomic data sets, we performed a series of bioinformatic analyses to assess experimental features that could aid in uniquely identifying site-specific ubiquitylation events. Spectral predictions from Prosit were compared to experimental spectra of tryptic ubiquitylated peptides, revealing previously uncharacterized fragmentation of the diGly scar. Analysis of the LysC-derived ubiquitylated peptides reveals systematic, multidimensional peptide fragmentation, including diagnostic b-ions from fragmentation of the LysC ubiquitin scar. Comprehensively, these findings provide diagnostic spectral signatures of modification events that could be applied to new analysis methods for nontryptic ubiquitylomics.

Determinants of chemoselectivity in ubiquitination by the J2 family of ubiquitin-conjugating enzymes

Ubiquitin-conjugating enzymes (E2) play a crucial role in the attachment of ubiquitin to proteins. Together with ubiquitin ligases (E3), they catalyze the transfer of ubiquitin (Ub) onto lysines with high chemoselectivity. A subfamily of E2s, including yeast Ubc6 and human Ube2J2, also mediates noncanonical modification of serines, but the structural determinants for this chemical versatility remain unknown. Using a combination of X-ray crystallography, molecular dynamics (MD) simulations, and reconstitution approaches, we have uncovered a two-layered mechanism that underlies this unique reactivity. A rearrangement of the Ubc6/Ube2J2 active site enhances the reactivity of the E2-Ub thioester, facilitating attack by weaker nucleophiles. Moreover, a conserved histidine in Ubc6/Ube2J2 activates a substrate serine by general base catalysis. Binding of RING-type E3 ligases further increases the serine selectivity inherent to Ubc6/Ube2J2, via an allosteric mechanism that requires specific positioning of the ubiquitin tail at the E2 active site. Our results elucidate how subtle structural modifications to the highly conserved E2 fold yield distinct enzymatic activity.

An atypical E3 ligase safeguards the ribosome during nutrient stress

Metabolic stress must be effectively mitigated for the survival of cells and organisms. Ribosomes have emerged as signaling hubs that sense metabolic perturbations and coordinate responses that either restore homeostasis or trigger cell death. As yet, the mechanisms governing these cell fate decisions are not well understood. Here, we report an unexpected role for the atypical E3 ligase HOIL-1 in safeguarding the ribosome. We find HOIL-1 mutations associated with cardiomyopathy broadly sensitize cells to nutrient and translational stress. These signals converge on the ribotoxic stress sentinel ZAKα. Mechanistically, mutant HOIL-1 excludes a ribosome quality control E3 ligase from its functional complex and remodels the ribosome ubiquitin landscape. This quality control failure renders glucose starvation ribotoxic, precipitating a ZAKα-ATF4-xCT-driven noncanonical cell death. We further show HOIL-1 loss exacerbates cardiac dysfunction under pressure overload. These data reveal an unrecognized ribosome signaling axis and a molecular circuit controlling cell fate during nutrient stress.

DTX3L ubiquitin ligase ubiquitinates single-stranded nucleic acids

Ubiquitination typically involves covalent linking of ubiquitin (Ub) to a lysine residue on a protein substrate. Recently, new facets of this process have emerged, including Ub modification of non-proteinaceous substrates like ADP-ribose by the DELTEX E3 ligase family. Here, we show that the DELTEX family member DTX3L expands this non-proteinaceous substrate repertoire to include single-stranded DNA and RNA. Although the N-terminal region of DTX3L contains single-stranded nucleic acid binding domains and motifs, the minimal catalytically competent fragment comprises the C-terminal RING and DTC domains (RD). DTX3L-RD catalyses ubiquitination of the 3'-end of single-stranded DNA and RNA, as well as double-stranded DNA with a 3' overhang of two or more nucleotides. This modification is reversibly cleaved by deubiquitinases. NMR and biochemical analyses reveal that the DTC domain binds single-stranded DNA and facilitates the catalysis of Ub transfer from RING-bound E2-conjugated Ub. Our study unveils the direct ubiquitination of nucleic acids by DTX3L, laying the groundwork for understanding its functional implications.

The role of ubiquitination in health and disease

Ubiquitination is an enzymatic process characterized by the covalent attachment of ubiquitin to target proteins, thereby modulating their degradation, transportation, and signal transduction. By precisely regulating protein quality and quantity, ubiquitination is essential for maintaining protein homeostasis, DNA repair, cell cycle regulation, and immune responses. Nevertheless, the diversity of ubiquitin enzymes and their extensive involvement in numerous biological processes contribute to the complexity and variety of diseases resulting from their dysregulation. The ubiquitination process relies on a sophisticated enzymatic system, ubiquitin domains, and ubiquitin receptors, which collectively impart versatility to the ubiquitination pathway. The widespread presence of ubiquitin highlights its potential to induce pathological conditions. Ubiquitinated proteins are predominantly degraded through the proteasomal system, which also plays a key role in regulating protein localization and transport, as well as involvement in inflammatory pathways. This review systematically delineates the roles of ubiquitination in maintaining protein homeostasis, DNA repair, genomic stability, cell cycle regulation, cellular proliferation, and immune and inflammatory responses. Furthermore, the mechanisms by which ubiquitination is implicated in various pathologies, alongside current modulators of ubiquitination are discussed. Enhancing our comprehension of ubiquitination aims to provide novel insights into diseases involving ubiquitination and to propose innovative therapeutic strategies for clinical conditions.

Ubiquitylation of nucleic acids by DELTEX ubiquitin E3 ligase DTX3L

The recent discovery of non-proteinaceous ubiquitylation substrates broadened our understanding of this modification beyond conventional protein targets. However, the existence of additional types of substrates remains elusive. Here, we present evidence that nucleic acids can also be directly ubiquitylated via ester bond formation. DTX3L, a member of the DELTEX family E3 ubiquitin ligases, ubiquitylates DNA and RNA in vitro and that this activity is shared with DTX3, but not with the other DELTEX family members DTX1, DTX2 and DTX4. DTX3L shows preference for the 3'-terminal adenosine over other nucleotides. In addition, we demonstrate that ubiquitylation of nucleic acids is reversible by DUBs such as USP2, JOSD1 and SARS-CoV-2 PLpro. Overall, our study proposes reversible ubiquitylation of nucleic acids in vitro and discusses its potential functional implications.

SIRT6 loss causes intervertebral disc degeneration in mice by promoting senescence and SASP status

Intervertebral disc degeneration is a major risk factor contributing to chronic low back and neck pain. While the etiological factors for disc degeneration vary, age is still one of the most important risk factors. Recent studies have shown the promising role of SIRT6 in mammalian aging and skeletal tissue health, however its role in the intervertebral disc health remains unexplored. We investigated the contribution of SIRT6 to disc health by studying the age-dependent spinal phenotype of mice with conditional deletion of Sirt6 in the disc (Acan CreERT2 ; Sirt6 fl/fl ). Histological studies showed a degenerative phenotype in knockout mice compared to Sirt6 fl/fl control mice at 12 months which became pronounced at 24 months. RNA-Seq analysis of NP and AF tissues, quantitative histone analysis, and in vitro multiomics employing RNA-seq with ATAC-seq revealed that SIRT6-loss resulted in changes in acetylation and methylation status of specific Histone 3 lysine residues, thereby affecting DNA accessibility and transcriptomic landscape. A decrease in autophagy and an increase in DNA damage were also noted in Sirt6-deficient cells. Further mechanistic insights revealed that loss of SIRT6 increased senescence and SASP burden in the disc characterized by increased p21, γH2AX, IL-6, and TGF-β abundance. Taken together our study highlights the contribution of SIRT6 in modulating DNA damage, autophagy and cell senescence, and its importance in maintaining disc health during aging thereby underscoring it as a potential therapeutic target to treat intervertebral disc degeneration.

Neurological glycogen storage diseases and emerging therapeutics

Glycogen storage diseases (GSDs) comprise a group of inherited metabolic disorders characterized by defects in glycogen metabolism, leading to abnormal glycogen accumulation in multiple tissues, most notably affecting the liver, skeletal muscle, and heart. Recent findings have uncovered the importance of glycogen metabolism in the brain, sustaining a myriad of physiological functions and linking its perturbation to central nervous system (CNS) pathology. This link resulted in classification of neurological-GSDs (n-GSDs), a group of diseases with shared deficits in neurological glycogen metabolism. The n-GSD patients exhibit a spectrum of clinical presentations with common etiology while requiring tailored therapeutic approaches from the traditional GSDs. Recent research has elucidated the genetic and biochemical mechanisms and pathophysiological basis underlying different n-GSDs. Further, the last decade has witnessed some promising developments in novel therapeutic approaches, including enzyme replacement therapy (ERT), substrate reduction therapy (SRT), small molecule drugs, and gene therapy targeting key aspects of glycogen metabolism in specific n-GSDs. This preclinical progress has generated noticeable success in potentially modifying disease course and improving clinical outcomes in patients. Herein, we provide an overview of current perspectives on n-GSDs, emphasizing recent advances in understanding their molecular basis, therapeutic developments, underscore key challenges and the need to deepen our understanding of n-GSDs pathogenesis to develop better therapeutic strategies that could offer improved treatment and sustainable benefits to the patients.

Satellite glial contact enhances differentiation and maturation of human iPSC-derived sensory neurons

Sensory neurons generated from induced pluripotent stem cells (iSNs) are used to model human peripheral neuropathies, however current differentiation protocols produce sensory neurons with an embryonic phenotype. Peripheral glial cells contact sensory neurons early in development and contribute to formation of the canonical pseudounipolar morphology, but these signals are not encompassed in current iSN differentiation protocols. Here, we show that terminal differentiation of iSNs in co-culture with rodent Dorsal Root Ganglion satellite glia (rSG) advances their differentiation and maturation. Co-cultured iSNs develop a pseudounipolar morphology through contact with rSGs. This transition depends on semaphorin-plexin guidance cues and on glial gap junction signaling. In addition to morphological changes, iSNs terminally differentiated in co-culture exhibit enhanced spontaneous action potential firing, more mature gene expression, and increased susceptibility to paclitaxel induced axonal degeneration. Thus, iSNs differentiated in coculture with rSGs provide a better model for investigating human peripheral neuropathies.

Sugar-mediated non-canonical ubiquitination impairs Nrf1/NFE2L1 activation

Proteasome is essential for cell survival, and proteasome inhibition induces proteasomal gene transcription via the activated endoplasmic-reticulum-associated transcription factor nuclear factor erythroid 2-like 1 (Nrf1/NFE2L1). Nrf1 activation requires proteolytic cleavage by DDI2 and N-glycan removal by NGLY1. We previously showed that Nrf1 ubiquitination by SKP1-CUL1-F-box (SCF)FBS2/FBXO6, an N-glycan-recognizing E3 ubiquitin ligase, impairs its activation, although the molecular mechanism remained elusive. Here, we show that SCFFBS2 cooperates with the RING-between-RING (RBR)-type E3 ligase ARIH1 to ubiquitinate Nrf1 through oxyester bonds in human cells. Endo-β-N-acetylglucosaminidase (ENGASE) generates asparagine-linked N-acetyl glucosamine (N-GlcNAc) residues from N-glycans, and N-GlcNAc residues on Nrf1 served as acceptor sites for SCFFBS2-ARIH1-mediated ubiquitination. We reconstituted the polyubiquitination of N-GlcNAc and serine/threonine residues on glycopeptides and found that the RBR-specific E2 enzyme UBE2L3 is required for the assembly of atypical ubiquitin chains on Nrf1. The atypical ubiquitin chains inhibited DDI2-mediated activation. The present results identify an unconventional ubiquitination pathway that inhibits Nrf1 activation.

Discovery of ester-linked ubiquitylation of PARP10 mono-ADP-ribosylation in cells: a dual post-translational modification on Glu/Asp side chains

The prevailing view on post-translational modifications (PTMs) is that amino acid side chains in proteins are modified with a single PTM at any given time. However, a growing body of work has demonstrated crosstalk between different PTMs, some occurring on the same residue. Such interplay is seen with ADP-ribosylation and ubiquitylation, where specialized E3 ligases ubiquitylate targets for proteasomal degradation in an ADP-ribosylation-dependent manner. More recently, the DELTEX family of E3 ligases was reported to catalyze ubiquitylation of the 3'- hydroxy group of the adenine-proximal ribose of free NAD + and ADP-ribose in vitro , generating a non-canonical ubiquitin ester-linked species. In this report, we show, for the first time, that this dual PTM occurs in cells on mono-ADP-ribosylated (MARylated) PARP10 on Glu/Asp sites to form a MAR ubiquitin ester (MARUbe). We term this process m ono- A DP-ribosyl ub iquit ylation or MARUbylation. Using chemical and enzymatic treatments, including a newly characterized bacterial deubiquitinase with esterase-specific activity, we discovered that PARP10 MARUbylation is extended with K11-linked polyubiquitin chains. Finally, mechanistic studies using proteasomal and ubiquitin-activating enzyme inhibitors demonstrated that PARP10 MARUbylation leads to its proteasomal degradation, providing a functional role for this new PTM in regulating protein turnover.

Bacterial esterases reverse lipopolysaccharide ubiquitylation to block host immunity

Aspects of how Burkholderia escape the host's intrinsic immune response to replicate in the cell cytosol remain enigmatic. Here, we show that Burkholderia has evolved two mechanisms to block the activity of Ring finger protein 213 (RNF213)-mediated non-canonical ubiquitylation of bacterial lipopolysaccharide (LPS), thereby preventing the initiation of antibacterial autophagy. First, Burkholderia's polysaccharide capsule blocks RNF213 association with bacteria and second, the Burkholderia deubiquitylase (DUB), TssM, directly reverses the activity of RNF213 through a previously unrecognized esterase activity. Structural analysis provides insight into the molecular basis of TssM esterase activity, allowing it to be uncoupled from its isopeptidase function. Furthermore, a putative TssM homolog also displays esterase activity and removes ubiquitin from LPS, establishing this as a virulence mechanism. Of note, we also find that additional immune-evasion mechanisms exist, revealing that overcoming this arm of the host's immune response is critical to the pathogen.

The emerging roles of non-canonical ubiquitination in proteostasis and beyond

Ubiquitin regulates various cellular functions by posttranslationally modifying substrates with diverse ubiquitin codes. Recent discoveries of new ubiquitin chain topologies, types of bonds, and non-protein substrates have substantially expanded the complexity of the ubiquitin code. Here, we describe the ubiquitin system covering the basic principles and recent discoveries related to mechanisms, technologies, and biological importance.

Biallelic human SHARPIN loss of function induces autoinflammation and immunodeficiency

The linear ubiquitin assembly complex (LUBAC) consists of HOIP, HOIL-1 and SHARPIN and is essential for proper immune responses. Individuals with HOIP and HOIL-1 deficiencies present with severe immunodeficiency, autoinflammation and glycogen storage disease. In mice, the loss of Sharpin leads to severe dermatitis due to excessive keratinocyte cell death. Here, we report two individuals with SHARPIN deficiency who manifest autoinflammatory symptoms but unexpectedly no dermatological problems. Fibroblasts and B cells from these individuals showed attenuated canonical NF-κB responses and a propensity for cell death mediated by TNF superfamily members. Both SHARPIN-deficient and HOIP-deficient individuals showed a substantial reduction of secondary lymphoid germinal center B cell development. Treatment of one SHARPIN-deficient individual with anti-TNF therapies led to complete clinical and transcriptomic resolution of autoinflammation. These findings underscore the critical function of the LUBAC as a gatekeeper for cell death-mediated immune dysregulation in humans.

Molecular structure and function of mysterin/RNF213

Mysterin is a large intracellular protein harboring a RING finger ubiquitin ligase domain and is also referred to as RING finger protein 213 (RNF213). The author performed the first molecular cloning of the mysterin gene as the final step in genetic exploration of cerebrovascular moyamoya disease (MMD) and initiated the next round of exploration to understand its molecular and cellular functions. Although much remains unknown, accumulating findings suggest that mysterin functions in cells by targeting massive intracellular structures, such as lipid droplets (LDs) and various invasive pathogens. In the latter case, mysterin appears to directly surround and ubiquitylate the surface of pathogens and stimulate cell-autonomous antimicrobial reactions, such as xenophagy and inflammatory response. To date, multiple mutations causing MMD have been identified within and near the RING finger domain of mysterin; however, their functional relevance remains largely unknown. Besides the RING finger, mysterin harbors a dynein-like ATPase core and an RZ finger, another ubiquitin ligase domain unique to mysterin, while functional exploration of these domains has also just commenced. In this review, the author attempts to summarize the core findings regarding the molecular structure and function of the mysterin protein, with an emphasis on the perspective of MMD research.

Systematic HOIP interactome profiling reveals critical roles of linear ubiquitination in tissue homeostasis

Linear ubiquitination catalyzed by HOIL-1-interacting protein (HOIP), the key component of the linear ubiquitination assembly complex, plays fundamental roles in tissue homeostasis by executing domain-specific regulatory functions. However, a proteome-wide analysis of the domain-specific interactome of HOIP across tissues is lacking. Here, we present a comprehensive mass spectrometry-based interactome profiling of four HOIP domains in nine mouse tissues. The interaction dataset provides a high-quality HOIP interactome resource with an average of approximately 90 interactors for each bait per tissue. HOIP tissue interactome presents a systematic understanding of linear ubiquitination functions in each tissue and also shows associations of tissue functions to genetic diseases. HOIP domain interactome characterizes a set of previously undefined linear ubiquitinated substrates and elucidates the cross-talk among HOIP domains in physiological and pathological processes. Moreover, we show that linear ubiquitination of Integrin-linked protein kinase (ILK) decreases focal adhesion formation and promotes the detachment of Shigella flexneri-infected cells. Meanwhile, Hoip deficiency decreases the linear ubiquitination of Smad ubiquitination regulatory factor 1 (SMURF1) and enhances its E3 activity, finally causing a reduced bone mass phenotype in mice. Overall, our work expands the knowledge of HOIP-interacting proteins and provides a platform for further discovery of linear ubiquitination functions in tissue homeostasis.

Protein neddylation and its role in health and diseases

NEDD8 (Neural precursor cell expressed developmentally downregulated protein 8) is an ubiquitin-like protein that is covalently attached to a lysine residue of a protein substrate through a process known as neddylation, catalyzed by the enzyme cascade, namely NEDD8 activating enzyme (E1), NEDD8 conjugating enzyme (E2), and NEDD8 ligase (E3). The substrates of neddylation are categorized into cullins and non-cullin proteins. Neddylation of cullins activates CRLs (cullin RING ligases), the largest family of E3 ligases, whereas neddylation of non-cullin substrates alters their stability and activity, as well as subcellular localization. Significantly, the neddylation pathway and/or many neddylation substrates are abnormally activated or over-expressed in various human diseases, such as metabolic disorders, liver dysfunction, neurodegenerative disorders, and cancers, among others. Thus, targeting neddylation becomes an attractive strategy for the treatment of these diseases. In this review, we first provide a general introduction on the neddylation cascade, its biochemical process and regulation, and the crystal structures of neddylation enzymes in complex with cullin substrates; then discuss how neddylation governs various key biological processes via the modification of cullins and non-cullin substrates. We further review the literature data on dysregulated neddylation in several human diseases, particularly cancer, followed by an outline of current efforts in the discovery of small molecule inhibitors of neddylation as a promising therapeutic approach. Finally, few perspectives were proposed for extensive future investigations.

HOIL1 mediates MDA5 activation through ubiquitination of LGP2

The RIG-I-like receptors (RLRs), RIG-I and MDA5, are innate sensors of RNA virus infections that are critical for mounting a robust antiviral immune response. We have shown previously that HOIL1, a component of the Linear Ubiquitin Chain Assembly Complex (LUBAC), is essential for interferon (IFN) induction in response to viruses sensed by MDA5, but not for viruses sensed by RIG-I. LUBAC contains two unusual E3 ubiquitin ligases, HOIL1 and HOIP. HOIP generates methionine-1-linked polyubiquitin chains, whereas HOIL1 has recently been shown to conjugate ubiquitin onto serine and threonine residues. Here, we examined the differential requirement for HOIL1 and HOIP E3 ligase activities in RLR-mediated IFN induction. We determined that HOIL1 E3 ligase activity was critical for MDA5-dependent IFN induction, while HOIP E3 ligase activity played only a modest role in promoting IFN induction. HOIL1 E3 ligase promoted MDA5 oligomerization, its translocation to mitochondrial-associated membranes, and the formation of MAVS aggregates. We identified that HOIL1 can interact with and facilitate the ubiquitination of LGP2, a positive regulator of MDA5 oligomerization. In summary, our work identifies LGP2 ubiquitination by HOIL1 in facilitating the activation of MDA5 and the induction of a robust IFN response.

Discovery and characterization of noncanonical E2-conjugating enzymes

E2-conjugating enzymes (E2s) play a central role in the enzymatic cascade that leads to the attachment of ubiquitin to a substrate. This process, termed ubiquitylation, is required to maintain cellular homeostasis and affects almost all cellular process. By interacting with multiple E3 ligases, E2s dictate the ubiquitylation landscape within the cell. Since its discovery, ubiquitylation has been regarded as a posttranslational modification that specifically targets lysine side chains (canonical ubiquitylation). We used Matrix-Assisted Laser Desorption/Ionization-Time Of Flight Mass Spectrometry to identify and characterize a family of E2s that are instead able to conjugate ubiquitin to serine and/or threonine. We used structural modeling and prediction tools to identify the key activity determinants that these E2s use to interact with ubiquitin as well as their substrates. Our results unveil the missing E2s necessary for noncanonical ubiquitylation, underscoring the adaptability and versatility of ubiquitin modifications.

SKping cell cycle regulation: role of ubiquitin ligase SKP2 in hematological malignancies

SKP2 (S-phase kinase-associated protein 2) is a member of the F-box family of substrate-recognition subunits in the SCF ubiquitin-protein ligase complexes. It is associated with ubiquitin-mediated degradation in the mammalian cell cycle components and other target proteins involved in cell cycle progression, signal transduction, and transcription. Being an oncogene in solid tumors and hematological malignancies, it is frequently associated with drug resistance and poor disease outcomes. In the current review, we discussed the novel role of SKP2 in different hematological malignancies. Further, we performed a limited in-silico analysis to establish the involvement of SKP2 in a few publicly available cancer datasets. Interestingly, our study identified Skp2 expression to be altered in a cancer-specific manner. While it was found to be overexpressed in several cancer types, few cancer showed a down-regulation in SKP2. Our review provides evidence for developing novel SKP2 inhibitors in hematological malignancies. We also investigated the effect of SKP2 status on survival and disease progression. In addition, the role of miRNA and its associated families in regulating Skp2 expression was explored. Subsequently, we predicted common miRNAs against Skp2 genes by using miRNA-predication tools. Finally, we discussed current approaches and future prospective approaches to target the Skp2 gene by using different drugs and miRNA-based therapeutics applications in translational research.

Expanding the phenotype of RBCK1-associated polyglucosan body myopathy type 1

Polyglucosan body myopathy-1 (PGBM1) is an extremely rare glycogen storage diseases that leads to muscle weakness and cardiomyopathy due to the accumulation of polyglucosan bodies. The clinical presentation appears to be partially dependent on the genetic mutation, but no clear genotype/phenotype correlation is currently possible. We describe a 7 year old patient, who initially presented with recurrent vomiting and respiratory infections until her first year of life. Diagnostic workup revealed an achalasia and the whole exome sequencing revealed an homozygous RBCK1 (RANBP2-type and C3HC4-type zinc finger containing 1) variant (c.896_899delAGTG) located in exon 7 (mid-domain), which has also been described in 4 patients with PGBM1. The unusual presentation with gastrointestinal and respiratory symptoms before the development of progressive muscle weakness expands the phenotype of this disease.

Myofiber-type-dependent 'boulder' or 'multitudinous pebble' formations across distinct amylopectinoses

At least five enzymes including three E3 ubiquitin ligases are dedicated to glycogen's spherical structure. Absence of any reverts glycogen to a structure resembling amylopectin of the plant kingdom. This amylopectinosis (polyglucosan body formation) causes fatal neurological diseases including adult polyglucosan body disease (APBD) due to glycogen branching enzyme deficiency, Lafora disease (LD) due to deficiencies of the laforin glycogen phosphatase or the malin E3 ubiquitin ligase and type 1 polyglucosan body myopathy (PGBM1) due to RBCK1 E3 ubiquitin ligase deficiency. Little is known about these enzymes' functions in glycogen structuring. Toward understanding these functions, we undertake a comparative murine study of the amylopectinoses of APBD, LD and PGBM1. We discover that in skeletal muscle, polyglucosan bodies form as two main types, small and multitudinous ('pebbles') or giant and single ('boulders'), and that this is primarily determined by the myofiber types in which they form, 'pebbles' in glycolytic and 'boulders' in oxidative fibers. This pattern recapitulates what is known in the brain in LD, innumerable dust-like in astrocytes and single giant sized in neurons. We also show that oxidative myofibers are relatively protected against amylopectinosis, in part through highly increased glycogen branching enzyme expression. We present evidence of polyglucosan body size-dependent cell necrosis. We show that sex influences amylopectinosis in genotype, brain region and myofiber-type-specific fashion. RBCK1 is a component of the linear ubiquitin chain assembly complex (LUBAC), the only known cellular machinery for head-to-tail linear ubiquitination critical to numerous cellular pathways. We show that the amylopectinosis of RBCK1 deficiency is not due to loss of linear ubiquitination, and that another function of RBCK1 or LUBAC must exist and operate in the shaping of glycogen. This work opens multiple new avenues toward understanding the structural determinants of the mammalian carbohydrate reservoir critical to neurologic and neuromuscular function and disease.

Design and high-throughput implementation of MALDI-TOF/MS-based assays for Parkin E3 ligase activity

Parkinson's disease (PD) is a progressive neurological disorder that manifests clinically as alterations in movement as well as multiple non-motor symptoms including but not limited to cognitive and autonomic abnormalities. Loss-of-function mutations in the gene encoding the ubiquitin E3 ligase Parkin are causal for familial and juvenile PD. Among several therapeutic approaches being explored to treat or improve the prognosis of patients with PD, the use of small molecules able to reinstate or boost Parkin activity represents a potential pharmacological treatment strategy. A major barrier is the lack of high-throughput platforms for the robust and accurate quantification of Parkin activity in vitro. Here, we present two different and complementary Matrix-Assisted Laser Desorption/Ionization Time-Of-Flight Mass Spectrometry (MALDI-TOF/MS)-based approaches for the quantification of Parkin E3 ligase activity in vitro. Both approaches are scalable for high-throughput primary screening to facilitate the identification of Parkin modulators.

Structure of the extracellular region of the adhesion GPCR CELSR1 reveals a compact module which regulates G protein-coupling

Cadherin EGF Laminin G seven-pass G-type receptors (CELSRs or ADGRCs) are conserved adhesion G protein-coupled receptors which are essential for animal development. CELSRs have extracellular regions (ECRs) containing 23 adhesion domains which couple adhesion to intracellular signaling. However, molecular-level insight into CELSR function is sparsely available. We report the 4.3 Å cryo-EM reconstruction of the mCELSR1 ECR with 13 domains resolved in the structure. These domains form a compact module mediated by interdomain interactions with contact between the N- and C-terminal domains. We show the mCELSR1 ECR forms an extended species in the presence of Ca 2+ , which we propose represents the antiparallel cadherin repeat dimer. Using assays for adhesion and G protein-coupling, we assign the N-terminal CADH1-8 module as necessary for cell adhesion and we show the C-terminal CAHD9-GAIN module regulates signaling. Our work provides important molecular context to the literature on CELSR function and opens the door towards further mechanistic studies.

DELTEX E3 ligases ubiquitylate ADP-ribosyl modification on nucleic acids

Although ubiquitylation had traditionally been considered limited to proteins, the discovery of non-proteinaceous substrates (e.g. lipopolysaccharides and adenosine diphosphate ribose (ADPr)) challenged this perspective. Our recent study showed that DTX2 E3 ligase efficiently ubiquitylates ADPr. Here, we show that the ADPr ubiquitylation activity is also present in another DELTEX family member, DTX3L, analysed both as an isolated catalytic fragment and the full-length PARP9:DTX3L complex, suggesting that it is a general feature of the DELTEX family. Since structural predictions show that DTX3L possesses single-stranded nucleic acids binding ability and given the fact that nucleic acids have recently emerged as substrates for ADP-ribosylation, we asked whether DELTEX E3s might catalyse ubiquitylation of an ADPr moiety linked to nucleic acids. Indeed, we show that DTX3L and DTX2 are capable of ubiquitylating ADP-ribosylated DNA and RNA synthesized by PARPs, including PARP14. Furthermore, we demonstrate that the Ub-ADPr-nucleic acids conjugate can be reversed by two groups of hydrolases, which remove either the whole adduct (e.g. SARS-CoV-2 Mac1 or PARP14 macrodomain 1) or just the Ub (e.g. SARS-CoV-2 PLpro). Overall, this study reveals ADPr ubiquitylation as a general function of the DELTEX family E3s and presents the evidence of reversible ubiquitylation of ADP-ribosylated nucleic acids.

Mechanistic insights into the homo-dimerization of HOIL-1L and SHARPIN

HOIL-1L and SHARPIN are two essential regulatory subunits of the linear ubiquitin chain assembly complex (LUBAC), which is the only known E3 ligase complex generating linear ubiquitin chains. In addition to their LUBAC-dependent functions, HOIL-1L and SHARPIN alone play crucial roles in many LUBAC-independent cellular processes. Importantly, deficiency of HOIL-1L or SHARPIN leads to severe disorders in humans or mice. However, the mechanistic bases underlying the multi-functions of HOIL-1L and SHARPIN are still largely unknown. Here, we uncover that HOIL-1L and SHARPIN alone can form homo-dimers through their LTM motifs. We solve two crystal structures of the dimeric LTM motifs of HOIL-1L and SHARPIN, which not only elucidate the detailed molecular mechanism underpinning the dimer formations of HOIL-1L and SHARPIN, but also reveal a general mode shared by the LTM motifs of HOIL-1L and SHARPIN for forming homo-dimer or hetero-dimer. Furthermore, we elucidate that the polyglucosan body myopathy-associated HOIL-1L A18P mutation disturbs the structural folding of HOIL-1L LTM, and disrupts the dimer formation of HOIL-1L. In summary, our study provides mechanistic insights into the homo-dimerization of HOIL-1L and SHARPIN mediated by their LTM motifs, and expands our understandings of the multi-functions of HOIL-1L and SHARPIN as well as the etiology of relevant human disease caused by defective HOIL-1L.

Targeting the Ubiquitin-Proteasome System and Recent Advances in Cancer Therapy

Ubiquitination is a reversible post-translational modification based on the chemical addition of ubiquitin to proteins with regulatory effects on various signaling pathways. Ubiquitination can alter the molecular functions of tagged substrates with respect to protein turnover, biological activity, subcellular localization or protein-protein interaction. As a result, a wide variety of cellular processes are under ubiquitination-mediated control, contributing to the maintenance of cellular homeostasis. It follows that the dysregulation of ubiquitination reactions plays a relevant role in the pathogenic states of human diseases such as neurodegenerative diseases, immune-related pathologies and cancer. In recent decades, the enzymes of the ubiquitin-proteasome system (UPS), including E3 ubiquitin ligases and deubiquitinases (DUBs), have attracted attention as novel druggable targets for the development of new anticancer therapeutic approaches. This perspective article summarizes the peculiarities shared by the enzymes involved in the ubiquitination reaction which, when deregulated, can lead to tumorigenesis. Accordingly, an overview of the main pharmacological interventions based on targeting the UPS that are in clinical use or still in clinical trials is provided, also highlighting the limitations of the therapeutic efficacy of these approaches. Therefore, various attempts to circumvent drug resistance and side effects as well as UPS-related emerging technologies in anticancer therapeutics are discussed.

HOIL1 Regulates Group 3 Innate Lymphoid Cells in the Colon and Protects against Systemic Dissemination, Colonic Ulceration, and Lethality from Citrobacter rodentium Infection

Heme-oxidized IRP2 ubiquitin ligase-1 (HOIL1)-deficient patients experience chronic intestinal inflammation and diarrhea as well as increased susceptibility to bacterial infections. HOIL1 is a component of the linear ubiquitin chain assembly complex that regulates immune signaling pathways, including NF-κB-activating pathways. We have shown previously that HOIL1 is essential for survival following Citrobacter rodentium gastrointestinal infection of mice, but the mechanism of protection by HOIL1 was not examined. C. rodentium is an important murine model for human attaching and effacing pathogens, enteropathogenic and enterohemorrhagic Escherichia coli that cause diarrhea and foodborne illnesses and lead to severe disease in children and immunocompromised individuals. In this study, we found that C. rodentium infection resulted in severe colitis and dissemination of C. rodentium to systemic organs in HOIL1-deficient mice. HOIL1 was important in the innate immune response to limit early replication and dissemination of C. rodentium. Using bone marrow chimeras and cell type-specific knockout mice, we found that HOIL1 functioned in radiation-resistant cells and partly in radiation-sensitive cells and in myeloid cells to limit disease, but it was dispensable in intestinal epithelial cells. HOIL1 deficiency significantly impaired the expansion of group 3 innate lymphoid cells and their production of IL-22 during C. rodentium infection. Understanding the role HOIL1 plays in type 3 inflammation and in limiting the pathogenesis of attaching and effacing lesion-forming bacteria will provide further insight into the innate immune response to gastrointestinal pathogens and inflammatory disorders.

Catalysis of non-canonical protein ubiquitylation by the ARIH1 ubiquitin ligase

Protein ubiquitylation typically involves isopeptide bond formation between the C-terminus of ubiquitin to the side-chain amino group on Lys residues. However, several ubiquitin ligases (E3s) have recently been identified that ubiquitylate proteins on non-Lys residues. For instance, HOIL-1 belongs to the RING-in-between RING (RBR) class of E3s and has an established role in Ser ubiquitylation. Given the homology between HOIL-1 and ARIH1, an RBR E3 that functions with the large superfamily of cullin-RING E3 ligases (CRLs), a biochemical investigation was undertaken, showing ARIH1 catalyzes Ser ubiquitylation to CRL-bound substrates. However, the efficiency of ubiquitylation was exquisitely dependent on the location and chemical environment of the Ser residue within the primary structure of the substrate. Comprehensive mutagenesis of the ARIH1 Rcat domain identified residues whose mutation severely impacted both oxyester and isopeptide bond formation at the preferred site for Ser ubiquitylation while only modestly affecting Lys ubiquitylation at the physiological site. The results reveal dual isopeptide and oxyester protein ubiquitylation activities of ARIH1 and set the stage for physiological investigations into this function of emerging importance.

Ubiquitination of non-protein substrates

The covalent attachment of ubiquitin is a common regulatory mechanism in various proteins. Although it has long been thought that the substrates of ubiquitination are limited to proteins, recent studies have changed this view: ubiquitin can be conjugated to lipids, sugars, and nucleotides. Ubiquitin is linked to these substrates by the action of different classes of ubiquitin ligases that have distinct catalytic mechanisms. Ubiquitination of non-protein substrates likely serves as a signal for the recruitment of other proteins to bring about specific effects. These discoveries have expanded the concept of ubiquitination and have advanced our insight into the biology and chemistry of this well-established modification process. In this review we describe the molecular mechanisms and roles of non-protein ubiquitination and discuss the current limitations.

Mechanistic insights into the enzymatic activity of E3 ligase HOIL-1L and its regulation by the linear ubiquitin chain binding

Heme-oxidized IRP2 ubiquitin ligase 1 (HOIL-1L) serves as a unique E3 ligase to catalyze the mono-ubiquitination of relevant protein or sugar substrates and plays vital roles in numerous cellular processes in mammals. However, the molecular mechanism underpinning the E3 activity of HOIL-1L and the related regulatory mechanism remain elusive. Here, we report the crystal structure of the catalytic core region of HOIL-1L and unveil the key catalytic triad residues of HOIL-1L. Moreover, we discover that HOIL-1L contains two distinct linear di-ubiquitin binding sites that can synergistically bind to linear tetra-ubiquitin, and the binding of HOIL-1L with linear tetra-ubiquitin can promote its E3 activity. The determined HOIL-1L/linear tetra-ubiquitin complex structure not only elucidates the detailed binding mechanism of HOIL-1L with linear tetra-ubiquitin but also uncovers a unique allosteric ubiquitin-binding site for the activation of HOIL-1L. In all, our findings provide mechanistic insights into the E3 activity of HOIL-1L and its regulation by the linear ubiquitin chain binding.

Ubiquitin-targeted bacterial effectors: rule breakers of the ubiquitin system

Regulation through post-translational ubiquitin signaling underlies a large portion of eukaryotic biology. This has not gone unnoticed by invading pathogens, many of which have evolved mechanisms to manipulate or subvert the host ubiquitin system. Bacteria are particularly adept at this and rely heavily upon ubiquitin-targeted virulence factors for invasion and replication. Despite lacking a conventional ubiquitin system of their own, many bacterial ubiquitin regulators loosely follow the structural and mechanistic rules established by eukaryotic ubiquitin machinery. Others completely break these rules and have evolved novel structural folds, exhibit distinct mechanisms of regulation, or catalyze foreign ubiquitin modifications. Studying these interactions can not only reveal important aspects of bacterial pathogenesis but also shed light on unexplored areas of ubiquitin signaling and regulation. In this review, we discuss the methods by which bacteria manipulate host ubiquitin and highlight aspects that follow or break the rules of ubiquitination.

MALDI-TOF Mass Spectrometry for interrogating ubiquitin enzymes

The attachment of ubiquitin to a substrate (ubiquitination or ubiquitylation) impacts its lifetime and regulates its function within the cell. Several classes of enzymes oversee the attachment of ubiquitin to the substrate: an E1 activating enzyme that makes ubiquitin chemically susceptible prior to the following stages of conjugation and ligation, respectively mediated by E2 conjugating enzymes (E2s) and E3 ligases (E3s). Around 40 E2s and more than 600 E3s are encoded in the human genome, and their combinatorial and cooperative behaviour dictate the tight specificity necessary for the regulation of thousands of substrates. The removal of ubiquitin is orchestrated by a network of about 100 deubiquitylating enzymes (DUBs). Many cellular processes are tightly controlled by ubiquitylation, which is essential in maintaining cellular homeostasis. Because of the fundamental role(s) of ubiquitylation, there is an interest in better understanding the function and specificity of the ubiquitin machinery. Since 2014, an expanding array of Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) Mass Spectrometry (MS) assays have been developed to systematically characterise the activity of a variety of ubiquitin enzymes in vitro. Here we recapitulate how MALDI-TOF MS aided the in vitro characterization of ubiquitin enzymes and the discovery of new and unexpected of E2s and DUBs functions. Given the versatility of the MALDI-TOF MS approach, we foreseen the use of this technology to further expand our understanding of ubiquitin and ubiquitin-like enzymes.

An expanded lexicon for the ubiquitin code

Our understanding of the ubiquitin code has greatly evolved from conventional E1, E2 and E3 enzymes that modify Lys residues on specific substrates with a single type of ubiquitin chain to more complex processes that regulate and mediate ubiquitylation. In this Review, we discuss recently discovered endogenous mechanisms and unprecedented pathways by which pathogens rewrite the ubiquitin code to promote infection. These processes include unconventional ubiquitin modifications involving ester linkages with proteins, lipids and sugars, or ubiquitylation through a phosphoribosyl bridge involving Arg42 of ubiquitin. We also introduce the enzymatic pathways that write and reverse these modifications, such as the papain-like proteases of severe acute respiratory syndrome coronavirus (SARS-CoV) and SARS-CoV-2. Furthermore, structural studies have revealed that the ultimate functions of ubiquitin are mediated not simply by straightforward recognition by ubiquitin-binding domains. Instead, elaborate multivalent interactions between ubiquitylated targets or ubiquitin chains and their readers (for example, the proteasome, the MLL1 complex or DOT1L) can elicit conformational changes that regulate protein degradation or transcription. The newly discovered mechanisms provide opportunities for innovative therapeutic interventions for diseases such as cancer and infectious diseases.

Role of Astrocytes in the Pathophysiology of Lafora Disease and Other Glycogen Storage Disorders

Lafora disease is a rare disorder caused by loss of function mutations in either the EPM2A or NHLRC1 gene. The initial symptoms of this condition are most commonly epileptic seizures, but the disease progresses rapidly with dementia, neuropsychiatric symptoms, and cognitive deterioration and has a fatal outcome within 5-10 years after onset. The hallmark of the disease is the accumulation of poorly branched glycogen in the form of aggregates known as Lafora bodies in the brain and other tissues. Several reports have demonstrated that the accumulation of this abnormal glycogen underlies all the pathologic traits of the disease. For decades, Lafora bodies were thought to accumulate exclusively in neurons. However, it was recently identified that most of these glycogen aggregates are present in astrocytes. Importantly, astrocytic Lafora bodies have been shown to contribute to pathology in Lafora disease. These results identify a primary role of astrocytes in the pathophysiology of Lafora disease and have important implications for other conditions in which glycogen abnormally accumulates in astrocytes, such as Adult Polyglucosan Body disease and the buildup of Corpora amylacea in aged brains.

Regional and age-dependent changes in ubiquitination in cellular and mouse models of Spinocerebellar ataxia type 3

Spinocerebellar ataxia type 3 (SCA3), also known as Machadoâ€"Joseph disease, is the most common dominantly inherited ataxia. SCA3 is caused by a CAG repeat expansion in the ATXN3 gene that encodes an expanded tract of polyglutamine (polyQ) in the disease protein ataxin-3 (ATXN3). As a deubiquitinating enzyme, ATXN3 regulates numerous cellular processes including proteasome- and autophagy-mediated protein degradation. In SCA3 disease brain, polyQ-expanded ATXN3 accumulates with other cellular constituents, including ubiquitin (Ub)-modified proteins, in select areas like the cerebellum and the brainstem, but whether pathogenic ATXN3 affects the abundance of ubiquitinated species is unknown. Here, in mouse and cellular models of SCA3, we investigated whether elimination of murine Atxn3 or expression of wild-type or polyQ-expanded human ATXN3 alters soluble levels of overall ubiquitination, as well as K48-linked (K48-Ub) and K63-linked (K63-Ub) chains. Levels of ubiquitination were assessed in the cerebellum and brainstem of 7- and 47-week-old Atxn3 knockout and SCA3 transgenic mice, and also in relevant mouse and human cell lines. In older mice, we observed that wild-type ATXN3 impacts the cerebellar levels of K48-Ub proteins. In contrast, pathogenic ATXN3 leads to decreased brainstem abundance of K48-Ub species in younger mice and changes in both cerebellar and brainstem K63-Ub levels in an age-dependent manner: younger SCA3 mice have higher levels of K63-Ub while older mice have lower levels of K63-Ub compared to controls. Human SCA3 neuronal progenitor cells also show a relative increase in K63-Ub proteins upon autophagy inhibition. We conclude that wild-type and mutant ATXN3 differentially impact K48-Ub- and K63-Ub-modified proteins in the brain in a region- and age-dependent manner.