Mixed-linkage ubiquitin chains send mixed messages

Emerging tools and methods to study cell signalling mediated by branched ubiquitin chains

Branched ubiquitin chains are complex molecular structures in which two or more ubiquitin moieties are attached to distinct lysine residues of a single ubiquitin molecule within a polyubiquitin chain. These bifurcated architectures significantly expand the signalling capacity of the ubiquitin system. Although branched chains constitute a substantial fraction of cellular polyubiquitin, their biological functions largely remain enigmatic due to their complex nature and the associated technical challenges of studying them. Recent technological innovations have enabled the identification of key molecular players and revealed essential roles for branched chains in diverse cellular processes. In this review, we discuss the bespoke strategies that have driven these discoveries, as well as the technologies needed to advance this rapidly evolving field.

VCP/p97-associated proteins are binders and debranching enzymes of K48-K63-branched ubiquitin chains

Branched ubiquitin (Ub) chains constitute a sizable fraction of Ub polymers in human cells. Despite their abundance, our understanding of branched Ub function in cell signaling has been stunted by the absence of accessible methods and tools. Here we identify cellular branched-chain-specific binding proteins and devise approaches to probe K48-K63-branched Ub function. We establish a method to monitor cleavage of linkages within complex Ub chains and unveil ATXN3 and MINDY as debranching enzymes. We engineer a K48-K63 branch-specific nanobody and reveal the molecular basis of its specificity in crystal structures of nanobody-branched Ub chain complexes. Using this nanobody, we detect increased K48-K63-Ub branching following valosin-containing protein (VCP)/p97 inhibition and after DNA damage. Together with our discovery that multiple VCP/p97-associated proteins bind to or debranch K48-K63-linked Ub, these results suggest a function for K48-K63-branched chains in VCP/p97-related processes.

Mechanism of degrader-targeted protein ubiquitinability

Small-molecule degraders of disease-driving proteins offer a clinically proven modality with enhanced therapeutic efficacy and potential to tackle previously undrugged targets. Stable and long-lived degrader-mediated ternary complexes drive fast and profound target degradation; however, the mechanisms by which they affect target ubiquitination remain elusive. Here, we show cryo-EM structures of the VHL Cullin 2 RING E3 ligase with the degrader MZ1 directing target protein Brd4BD2 toward UBE2R1-ubiquitin, and Lys456 at optimal positioning for nucleophilic attack. In vitro ubiquitination and mass spectrometry illuminate a patch of favorably ubiquitinable lysines on one face of Brd4BD2, with cellular degradation and ubiquitinomics confirming the importance of Lys456 and nearby Lys368/Lys445, identifying the "ubiquitination zone." Our results demonstrate the proficiency of MZ1 in positioning the substrate for catalysis, the favorability of Brd4BD2 for ubiquitination by UBE2R1, and the flexibility of CRL2 for capturing suboptimal lysines. We propose a model for ubiquitinability of degrader-recruited targets, providing a mechanistic blueprint for further rational drug design.

Chemical Diversification of Enzymatically Assembled Polyubiquitin Chains to Decipher the Ubiquitin Codes Programmed on the Branch Structure

The multimerization of ubiquitins at different positions of lysine residues to form heterotypic polyubiquitin chains is a post-translational modification that is essential for the precise regulation of protein functions and degradative fates in living cells. The understanding of structure-activity relationships underlying their diverse properties has been seriously impeded by difficulties in the preparation of a series of folded heterotypic chains appropriately functionalized with different chemical tags for the systematic evaluation of their multifaceted functions. Here, we report a chemical diversification of enzymatically assembled polyubiquitin chains that enables the facile preparation of folded heterotypic chains with different functionalities. By introducing an acyl hydrazide at the C terminus of the proximal ubiquitin, polyubiquitin chains were readily diversified from the same starting materials with a variety of molecules, ranging from small molecules to biopolymers, under nondenaturing conditions. This chemical diversification allowed the systematic study of the functional differences of K63/K48 heterotypic chains based on the position of the branch point during enzymatic deubiquitination and proteasomal proteolysis, thus demonstrating critical roles of the branch position in both the positive and negative control of ubiquitin-mediated reactions. The chemical diversification of the heterotypic chains provides a robust chemical platform to reframe the understanding of how the ubiquitin codes are regulated from the viewpoint of the branch structure for the precise control of cell functions, which has not been deciphered solely on the basis of the linkage types.

SCFFBXW11 Complex Targets Interleukin-17 Receptor A for Ubiquitin-Proteasome-Mediated Degradation

Interleukin-17 (IL-17) is a pro-inflammatory cytokine that participates in innate and adaptive immune responses and plays an important role in host defense, autoimmune diseases, tissue regeneration, metabolic regulation, and tumor progression. Post-translational modifications (PTMs) are crucial for protein function, stability, cellular localization, cellular transduction, and cell death. However, PTMs of IL-17 receptor A (IL-17RA) have not been investigated. Here, we show that human IL-17RA was targeted by F-box and WD repeat domain-containing 11 (FBXW11) for ubiquitination, followed by proteasome-mediated degradation. We used bioinformatics tools and biochemical techniques to determine that FBXW11 ubiquitinated IL-17RA through a lysine 27-linked polyubiquitin chain, targeting IL-17RA for proteasomal degradation. Domain 665-804 of IL-17RA was critical for interaction with FBXW11 and subsequent ubiquitination. Our study demonstrates that FBXW11 regulates IL-17 signaling pathways at the IL-17RA level.

Dynamic interactions between SPX proteins, the ubiquitination machinery, and signalling molecules for stress adaptation at a whole-plant level

The plant macronutrient phosphorus is a scarce resource and plant-available phosphate is limiting in most soil types. Generally, a gene regulatory module called the phosphate starvation response (PSR) enables efficient phosphate acquisition by roots and translocation to other organs. Plants growing on moderate to nutrient-rich soils need to co-ordinate availability of different nutrients and repress the highly efficient PSR to adjust phosphate acquisition to the availability of other macro- and micronutrients, and in particular nitrogen. PSR repression is mediated by a small family of single SYG1/Pho81/XPR1 (SPX) domain proteins. The SPX domain binds higher order inositol pyrophosphates that signal cellular phosphorus status and modulate SPX protein interaction with PHOSPHATE STARVATION RESPONSE1 (PHR1), the central transcriptional regulator of PSR. Sequestration by SPX repressors restricts PHR1 access to PSR gene promoters. Here we focus on SPX4 that primarily acts in shoots and sequesters many transcription factors other than PHR1 in the cytosol to control processes beyond the classical PSR, such as nitrate, auxin, and jasmonic acid signalling. Unlike SPX1 and SPX2, SPX4 is subject to proteasomal degradation not only by singular E3 ligases, but also by SCF-CRL complexes. Emerging models for these different layers of control and their consequences for plant acclimation to the environment will be discussed.

Simultaneous substrate and ubiquitin modification recognition by bispecific antibodies enables detection of ubiquitinated RIP1 and RIP2

Ubiquitination is a posttranslational modification that is crucial for the dynamic regulation of diverse signaling pathways. To enhance our understanding of ubiquitination-mediated signaling, we generated a new class of bispecific antibodies that combine recognition of ubiquitination substrates and specific polyubiquitin linkages. RIP1-K63 and RIP1-linear (Lin) linkage polyubiquitin bispecific antibodies detected linkage-specific ubiquitination of the proinflammatory kinase RIP1 in cells and in tissues and revealed RIP1 ubiquitination by immunofluorescence. Similarly, ubiquitination of the RIP1-related kinase RIP2 with K63 or linear linkages was specifically detected with the RIP2-K63 and RIP2-Lin bispecific antibodies, respectively. Furthermore, using the RIP2-K63 and RIP2-Lin bispecific antibodies, we found prominent K63-linked and linear RIP2 ubiquitination in samples from patients with ulcerative colitis and Crohn's disease. We also developed a bispecific antibody (K63-Lin) that simultaneously recognizes K63-linked and linear ubiquitination of components of various signaling pathways. Together, these bispecific antibodies represent a new class of reagents with the potential to be developed for the detection of inflammatory biomarkers.

Mechanism of selective recognition of Lys48-linked polyubiquitin by macrocyclic peptide inhibitors of proteasomal degradation

Post-translational modification of proteins with polyubiquitin chains is a critical cellular signaling mechanism in eukaryotes with implications in various cellular states and processes. Unregulated ubiquitin-mediated protein degradation can be detrimental to cellular homeostasis, causing numerous diseases including cancers. Recently, macrocyclic peptides were developed that selectively target long Lysine-48-linked polyubiquitin chains (tetra-ubiquitin) to inhibit ubiquitin-proteasome system, leading to attenuation of tumor growth in vivo. However, structural determinants of the chain length and linkage selectivity by these cyclic peptides remained unclear. Here, we uncover the mechanism underlying cyclic peptide's affinity and binding selectivity by combining X-ray crystallography, solution NMR, and biochemical studies. We found that the peptide engages three consecutive ubiquitins that form a ring around the peptide and determined requirements for preferential selection of a specific trimer moiety in longer polyubiquitin chains. The structural insights gained from this work will guide the development of next-generation cyclic peptides with enhanced anti-cancer activity.

Specifying conformational heterogeneity of multi-domain proteins at atomic resolution

The conformational landscape of multi-domain proteins is inherently linked to their specific functions. This also holds for polyubiquitin chains that are assembled by two or more ubiquitin domains connected by a flexible linker thus showing a large interdomain mobility. However, molecular recognition and signal transduction are associated with particular conformational substates that are populated in solution. Here, we apply high-resolution NMR spectroscopy in combination with dual-scale MD simulations to explore the conformational space of K6-, K29-, and K33-linked diubiquitin molecules. The conformational ensembles are evaluated utilizing a paramagnetic cosolute reporting on solvent exposure plus a set of complementary NMR parameters. This approach unravels a conformational heterogeneity of diubiquitins and explains the diversity of structural models that have been determined for K6-, K29-, and K33-linked diubiquitins in free and ligand-bound states so far. We propose a general application of the approach developed here to demystify multi-domain proteins occurring in nature.

One-Pot, Photocontrolled Enzymatic Assembly of the Structure-Defined Heterotypic Polyubiquitin Chain

Heterotypic polyubiquitins are an emerging class of polyubiquitins that have attracted interest because of their potential diversity of structures and physiological functions. There is an increasing demand for structure-defined synthesis of heterotypic chains to investigate the topological factors underlying the intracellular signals that are characteristically mediated by the heterotypic chain. However, the applicability of chemical and enzymatic polyubiquitin synthesis developed to date has been limited by laborious rounds of ligation and purification or by a lack of modularity of the chain structure with respect to the length and the branch position. Here, we established a one-pot, photocontrolled synthesis of structurally defined heterotypic polyubiquitin chains. We designed ubiquitin derivatives with a photolabile protecting group at a lysine residue used for polymerization. Repetitive cycles of linkage-specific enzymatic elongation and photoinduced deprotection of the protected ubiquitin units enabled stepwise addition of ubiquitins with appropriate functionalities to control the length and branching positions. The positional control of branching was achieved without isolation of intermediates, allowing one-pot synthesis of K63 triubiqutin chains and a K63/K48 heterotypic tetraubiquitin chain with defined branching positions. The present study provides a chemical platform for the efficient construction of long polyubiquitin chains with defined branch structures that will facilitate the understanding of the essential relationships between functions and structures of the heterotypic chain that have hitherto been overlooked.

Getting to the Root of Branched Ubiquitin Chains: A Review of Current Methods and Functions

Nearly 20 years since the first branched ubiquitin (Ub) chains were identified by mass spectrometry, our understanding of these chains and their function is still evolving. This is due to the limitations of classical Ub research techniques in identifying these chains and the vast complexity of potential branched chains. Considering only lysine or N-terminal methionine attachment sites, there are already 28 different possible branch points. Taking into account recently discovered ester-linked ubiquitination, branch points of more than two linkage types, and the higher-order chain structures within which branch points exist, the diversity of branched chains is nearly infinite. This review breaks down the complexity of these chains into their general functions, what we know so far about the different linkage combinations, branched chain-optimized methodologies, and the future perspectives of branched chain research.

Progress in Anticancer Drug Development Targeting Ubiquitination-Related Factors

Ubiquitination is extensively involved in critical signaling pathways through monitoring protein stability, subcellular localization, and activity. Dysregulation of this process results in severe diseases including malignant cancers. To develop drugs targeting ubiquitination-related factors is a hotspot in research to realize better therapy of human diseases. Ubiquitination comprises three successive reactions mediated by Ub-activating enzyme E1, Ub-conjugating enzyme E2, and Ub ligase E3. As expected, multiple ubiquitination enzymes have been highlighted as targets for anticancer drug development due to their dominant effect on tumorigenesis and cancer progression. In this review, we discuss recent progresses in anticancer drug development targeting enzymatic machinery components.

Current methodologies in protein ubiquitination characterization: from ubiquitinated protein to ubiquitin chain architecture

Ubiquitination is a versatile post-translational modification (PTM), which regulates diverse fundamental features of protein substrates, including stability, activity, and localization. Unsurprisingly, dysregulation of the complex interaction between ubiquitination and deubiquitination leads to many pathologies, such as cancer and neurodegenerative diseases. The versatility of ubiquitination is a result of the complexity of ubiquitin (Ub) conjugates, ranging from a single Ub monomer to Ub polymers with different length and linkage types. To further understand the molecular mechanism of ubiquitination signaling, innovative strategies are needed to characterize the ubiquitination sites, the linkage type, and the length of Ub chain. With advances in chemical biology tools, computational methodologies, and mass spectrometry, protein ubiquitination sites and their Ub chain architecture have been extensively revealed. The obtained information on protein ubiquitination helps to crack the molecular mechanism of ubiquitination in numerous pathologies. In this review, we summarize the recent advances in protein ubiquitination analysis to gain updated knowledge in this field. In addition, the current and future challenges and barriers are also reviewed and discussed.

The role of K63-linked polyubiquitin in several types of autophagy

Lysosomal-dependent self-degradative (autophagic) mechanisms are essential for the maintenance of normal homeostasis in all eukaryotic cells. Several types of such self-degradative and recycling pathways have been identified, based on how the cellular self material can incorporate into the lysosomal lumen. Ubiquitination, a well-known and frequently occurred posttranslational modification has essential role in all cell biological processes, thus in autophagy too. The second most common type of polyubiquitin chain is the K63-linked polyubiquitin, which strongly connects to some self-degradative mechanisms in the cells. In this review, we discuss the role of this type of polyubiquitin pattern in numerous autophagic processes.

Structure of UBE2K-Ub/E3/polyUb reveals mechanisms of K48-linked Ub chain extension

Ubiquitin (Ub) chain types govern distinct biological processes. K48-linked polyUb chains target substrates for proteasomal degradation, but the mechanism of Ub chain synthesis remains elusive due to the transient nature of Ub handover. Here, we present the structure of a chemically trapped complex of the E2 UBE2K covalently linked to donor Ub and acceptor K48-linked di-Ub, primed for K48-linked Ub chain synthesis by a RING E3. The structure reveals the basis for acceptor Ub recognition by UBE2K active site residues and the C-terminal Ub-associated (UBA) domain, to impart K48-linked Ub specificity and catalysis. Furthermore, the structure unveils multiple Ub-binding surfaces on the UBA domain that allow distinct binding modes for K48- and K63-linked Ub chains. This multivalent Ub-binding feature serves to recruit UBE2K to ubiquitinated substrates to overcome weak acceptor Ub affinity and thereby promote chain elongation. These findings elucidate the mechanism of processive K48-linked polyUb chain formation by UBE2K.

Branched ubiquitin code: from basic biology to targeted protein degradation

Protein ubiquitylation regulates numerous pathways, and the diverse information encoded by various forms of ubiquitylation is known as the ubiquitin code. Recent studies revealed that branched ubiquitin chains are abundant in mammalian cells and regulate important pathways. They include proteasomal degradation of misfolded and disease-causing proteins, regulation of NF-B signaling, and apoptotic cell fate decisions. Targeted protein degradation through chemical degraders emerged as a transformative therapeutic paradigm aimed at inducing the disappearance of unwanted cellular proteins. To further improve the efficacy of target degradation and expand its applications, understanding the molecular mechanism of degraders' action from the view of ubiquitin code biology is required. In this review, I discuss the roles of the ubiquitin code in biological pathways and in chemically induced targeted protein degradation by focusing on the branched ubiquitin codes that we have characterized.

Branched ubiquitin chain binding and deubiquitination by UCH37 facilitate proteasome clearance of stress-induced inclusions

UCH37, also known as UCHL5, is a highly conserved deubiquitinating enzyme (DUB) that associates with the 26S proteasome. Recently, it was reported that UCH37 activity is stimulated by branched ubiquitin (Ub) chain architectures. To understand how UCH37 achieves its unique debranching specificity, we performed biochemical and Nuclear Magnetic Resonance (NMR) structural analyses and found that UCH37 is activated by contacts with the hydrophobic patches of both distal Ubs that emanate from a branched Ub. In addition, RPN13, which recruits UCH37 to the proteasome, further enhances branched-chain specificity by restricting linear Ub chains from having access to the UCH37 active site. In cultured human cells under conditions of proteolytic stress, we show that substrate clearance by the proteasome is promoted by both binding and deubiquitination of branched polyubiquitin by UCH37. Proteasomes containing UCH37(C88A), which is catalytically inactive, aberrantly retain polyubiquitinated species as well as the RAD23B substrate shuttle factor, suggesting a defect in recycling of the proteasome for the next round of substrate processing. These findings provide a foundation to understand how proteasome degradation of substrates modified by a unique Ub chain architecture is aided by a DUB.

The 20S as a stand-alone proteasome in cells can degrade the ubiquitin tag

The proteasome, the primary protease for ubiquitin-dependent proteolysis in eukaryotes, is usually found as a mixture of 30S, 26S, and 20S complexes. These complexes have common catalytic sites, which makes it challenging to determine their distinctive roles in intracellular proteolysis. Here, we chemically synthesize a panel of homogenous ubiquitinated proteins, and use them to compare 20S and 26S proteasomes with respect to substrate selection and peptide-product generation. We show that 20S proteasomes can degrade the ubiquitin tag along with the conjugated substrate. Ubiquitin remnants on branched peptide products identified by LC-MS/MS, and flexibility in the 20S gate observed by cryo-EM, reflect the ability of the 20S proteasome to proteolyze an isopeptide-linked ubiquitin-conjugate. Peptidomics identifies proteasome-trapped ubiquitin-derived peptides and peptides of potential 20S substrates in Hi20S cells, hypoxic cells, and human failing-heart. Moreover, elevated levels of 20S proteasomes appear to contribute to cell survival under stress associated with damaged proteins.

[Metabolic pathways controlled by E3 ligases: an opportunity for therapeutic targeting]

Since its discovery, the Ubiquitin Proteasome System (UPS) has been recognized for its major role in controlling most of the cell's metabolic pathways. In addition to its essential role in the degradation of proteins, it is also involved in the addressing, signaling or repair of DNA, which makes it a key player in cellular homeostasis. Although other control systems exist in the cell, the UPS is often referred to as the conductor. In view of its importance, any dysregulation of the UPS leads to more or less severe disorders for the cell and therefore the body, which accounts for UPS implication in many pathologies (cancer, Alzheimer's disease, Huntington's disease, etc.). UPS is made up of more than 1000 different proteins, the combinations of which allow the fine targeting of virtually all proteins in the body. UPS uses an enzymatic cascade (E1, 2 members; E2 > 35; E3 > 800) which allows the transfer of ubiquitin, a small protein of 8.5 kDa onto the protein to be targeted either for its degradation or to modify its activity. This ubiquitinylation signal is reversible and many deubiquitinylases (DUB, ∼ 80 isoforms) also have an important role. E3 enzymes are the most numerous and their function is to recognize the target protein, which makes them important players in the specific action of UPS. The very nature of E3 and the complexity of their interactions with different partners offer a very broad field of investigation and therefore significant potential for the development of therapeutic approaches. Without being exhaustive, this review illustrates the different strategies that have already been implemented to fight against different pathologies (excluding bacterial or viral infections).

Proteasome in action: substrate degradation by the 26S proteasome

Ubiquitination is the major criteria for the recognition of a substrate-protein by the 26S proteasome. Additionally, a disordered segment on the substrate — either intrinsic or induced — is critical for proteasome engagement. The proteasome is geared to interact with both of these substrate features and prepare it for degradation. To facilitate substrate accessibility, resting proteasomes are characterised by a peripheral distribution of ubiquitin receptors on the 19S regulatory particle (RP) and a wide-open lateral surface on the ATPase ring. In this substrate accepting state, the internal channel through the ATPase ring is discontinuous, thereby obstructing translocation of potential substrates. The binding of the conjugated ubiquitin to the ubiquitin receptors leads to contraction of the 19S RP. Next, the ATPases engage the substrate at a disordered segment, energetically unravel the polypeptide and translocate it towards the 20S catalytic core (CP). In this substrate engaged state, Rpn11 is repositioned at the pore of the ATPase channel to remove remaining ubiquitin modifications and accelerate translocation. C-termini of five of the six ATPases insert into corresponding lysine-pockets on the 20S α-ring to complete 20S CP gate opening. In the resulting substrate processing state, the ATPase channel is fully contiguous with the translocation channel into the 20S CP, where the substrate is proteolyzed. Complete degradation of a typical ubiquitin-conjugate takes place over a few tens of seconds while hydrolysing tens of ATP molecules in the process (50 kDa/∼50 s/∼80ATP). This article reviews recent insight into biochemical and structural features that underlie substrate recognition and processing by the 26S proteasome.

Remodeling without destruction: non-proteolytic ubiquitin chains in neural function and brain disorders

Ubiquitination is a fundamental posttranslational protein modification that regulates diverse biological processes, including those in the CNS. Several topologically and functionally distinct polyubiquitin chains can be assembled on protein substrates, modifying their fates. The classical and most prevalent polyubiquitin chains are those that tag a substrate to the proteasome for degradation, which has been established as a major mechanism driving neural circuit deconstruction and remodeling. In contrast, proteasome-independent non-proteolytic polyubiquitin chains regulate protein scaffolding, signaling complex formation, and kinase activation, and play essential roles in an array of signal transduction processes. Despite being a cornerstone in immune signaling and abundant in the mammalian brain, these non-proteolytic chains are underappreciated in neurons and synapses in the brain. Emerging studies have begun to generate exciting insights about some fundamental roles played by these non-degradative chains in neuronal function and plasticity. In addition, their roles in a number of brain diseases are being recognized. In this article, we discuss recent advances on these nonconventional ubiquitin chains in neural development, function, plasticity, and related pathologies.

Proteasome-Bound UCH37/UCHL5 Debranches Ubiquitin Chains to Promote Degradation

The linkage, length, and architecture of ubiquitin (Ub) chains are all important variables in providing tight control over many biological paradigms. There are clear roles for branched architectures in regulating proteasome-mediated degradation, but the proteins that selectively recognize and process these atypical chains are unknown. Here, using synthetic and enzyme-derived ubiquitin chains along with intact mass spectrometry, we report that UCH37/UCHL5, a proteasome-associated deubiquitinase, cleaves K48 branched chains. The activity and selectivity toward branched chains is markedly enhanced by the proteasomal Ub receptor RPN13/ADRM1. Using reconstituted proteasome complexes, we find that chain debranching promotes degradation of substrates modified with branched chains under multi-turnover conditions. These results are further supported by proteome-wide pulse-chase experiments, which show that the loss of UCH37 activity impairs global protein turnover. Our work therefore defines UCH37 as a debranching deubiquitinase important for promoting proteasomal degradation.

Branched Ubiquitination: Detection Methods, Biological Functions and Chemical Synthesis

Ubiquitination is a versatile posttranslational modification that elicits signaling roles to impact on various cellular processes and disease states. The versatility is a result of the complexity of ubiquitin conjugates, ranging from a single ubiquitin monomer to polymers with different length and linkage types. Recent studies have revealed the abundant existence of branched ubiquitin chains in which one ubiquitin molecule is connected to two or more ubiquitin moieties in the same ubiquitin polymer. Compared to the homotypic ubiquitin chain, the branched chain is recognized or processed differently by readers and erasers of the ubiquitin system, respectively, resulting in a qualitative or quantitative alteration of the functional output. Furthermore, certain types of branched ubiquitination are induced by cellular stresses, implicating their important physiological role in stress adaption. In addition, the current chemical methodologies of solid phase peptide synthesis and expanding genetic code approach have been developed to synthesize different architectures of branched ubiquitin chains. The synthesized branched ubiquitin chains have shown their significance in understanding the topologies and binding partners of the branched chains. Here, we discuss the recent progresses on the detection, functional characterization and synthesis of branched ubiquitin chains as well as the future perspectives of this emerging field.

Structural insights into ADP-ribosylation of ubiquitin by Deltex family E3 ubiquitin ligases

Cellular cross-talk between ubiquitination and other posttranslational modifications contributes to the regulation of numerous processes. One example is ADP-ribosylation of the carboxyl terminus of ubiquitin by the E3 DTX3L/ADP-ribosyltransferase PARP9 heterodimer, but the mechanism remains elusive. Here, we show that independently of PARP9, the conserved carboxyl-terminal RING and DTC (Deltex carboxyl-terminal) domains of DTX3L and other human Deltex proteins (DTX1 to DTX4) catalyze ADP-ribosylation of ubiquitin's Gly76 Structural studies reveal a hitherto unknown function of the DTC domain in binding NAD+ Deltex RING domain recruits E2 thioesterified with ubiquitin and juxtaposes it with NAD+ bound to the DTC domain to facilitate ADP-ribosylation of ubiquitin. This ubiquitin modification prevents its activation but is reversed by the linkage nonspecific deubiquitinases. Our study provides mechanistic insights into ADP-ribosylation of ubiquitin by Deltex E3s and will enable future studies directed at understanding the increasingly complex network of ubiquitin cross-talk.

The Ubiquitin Enigma: Progress in the Detection and Chemical Synthesis of Branched Ubiquitin Chains

Ubiquitin chains with distinct topologies play essential roles in eukaryotic cells. Recently, it was discovered that multiple ubiquitin units can be ligated to more than one lysine residue in the same ubiquitin to form diverse branched ubiquitin chains. Although there is increasing evidence implicating these branched chains in a plethora of biological functions, few mechanistic details have been elucidated. This concept article introduces the function, detection and chemical synthesis of branched ubiquitin chains; and offers some future perspective for this exciting new field.

Hybrid Chains: A Collaboration of Ubiquitin and Ubiquitin-Like Modifiers Introducing Cross-Functionality to the Ubiquitin Code

The Ubiquitin CODE constitutes a unique post-translational modification language relying on the covalent attachment of Ubiquitin (Ub) to substrates, with Ub serving as the minimum entity to generate a message that is translated into different cellular pathways. The creation of this message is brought about by the dedicated action of writers, erasers, and readers of the Ubiquitin CODE. This CODE is greatly expanded through the generation of polyUb chains of different architectures on substrates thus regulating their fate. Through additional post-translational modification by Ub-like proteins (UbL), hybrid Ub/UbL chains, which either alter the originally encrypted message or encode a completely new one, are formed. Hybrid Ub/UbL chains are generated under both stress or physiological conditions and seem to confer improved specificity and affinity toward their cognate receptors. In such a manner, their formation must play a specific, yet still undefined role in cellular signaling and thus understanding the UbCODE message is crucial. Here, we discuss the evidence for the existence of hybrid Ub/UbL chains in addition to the current understanding of its biology. The modification of Ub by another UbL complicates the deciphering of the spatial and temporal order of events warranting the development of a hybrid chain toolbox. We discuss this unmet need and expand upon the creation of tailored tools adapted from our previously established toolkit for the Ubiquitin Proteasome System to specifically target these hybrid Ub/UbL chains.

Branching via K11 and K48 Bestows Ubiquitin Chains with a Unique Interdomain Interface and Enhanced Affinity for Proteasomal Subunit Rpn1

Post-translational substrate modification with ubiquitin is essential for eukaryotic cellular signaling. Polymeric ubiquitin chains are assembled with specific architectures, which convey distinct signaling outcomes depending on the linkages involved. Recently, branched K11/K48-linked polyubiquitins were shown to enhance proteasomal degradation during mitosis. To better understand the underlying structural mechanisms, we determined the crystal and NMR structures of branched K11/K48-linked tri-ubiquitin and discovered a previously unobserved interdomain interface between the distal ubiquitins. Small-angle neutron scattering and site-directed mutagenesis corroborated the presence of this interface, which we hypothesized to be influential in the physiological role of branched K11/K48-linked chains. Yet, experiments probing polyubiquitin interactions-deubiquitination assays, binding to proteasomal shuttle hHR23A-showed negligible differences between branched K11/K48-linked tri-ubiquitin and related di-ubiquitins. However, significantly stronger binding affinity for branched K11/K48-linked tri-ubiquitin was observed with proteasomal subunit Rpn1, thereby suggesting a functional impact of this interdomain interface and pinpointing the mechanistic site of enhanced degradation.

Epigenetic Mechanisms and Posttranslational Modifications in Systemic Lupus Erythematosus

The complex physiology of eukaryotic cells is regulated through numerous mechanisms, including epigenetic changes and posttranslational modifications. The wide-ranging diversity of these mechanisms constitutes a way of dynamic regulation of the functionality of proteins, their activity, and their subcellular localization as well as modulation of the differential expression of genes in response to external and internal stimuli that allow an organism to respond or adapt to accordingly. However, alterations in these mechanisms have been evidenced in several autoimmune diseases, including systemic lupus erythematosus (SLE). The present review aims to provide an approach to the current knowledge of the implications of these mechanisms in SLE pathophysiology.

Crosstalk between Lys63- and Lys11-polyubiquitin signaling at DNA damage sites is driven by Cezanne

The establishment of polyubiquitin conjugates with distinct linkages play important roles in the DNA damage response. Much remains unknown about the regulation of linkage-specific ubiquitin signaling at sites of DNA damage. Here we reveal that Cezanne (also known as Otud7B) deubiquitinating enzyme promotes the recruitment of Rap80/BRCA1-A complex by binding to Lys63-polyubiquitin and targeting Lys11-polyubiquitin. Using a ubiquitin binding domain protein array screen, we identify that the UBA domains of Cezanne and Cezanne2 (also known as Otud7A) selectively bind to Lys63-linked polyubiquitin. Increased Lys11-linkage ubiquitination due to lack of Cezanne DUB activity compromises the recruitment of Rap80/BRCA1-A. Cezanne2 interacts with Cezanne, facilitating Cezanne in the recruitment of Rap80/BRCA1-A, Rad18, and 53BP1, in cellular resistance to ionizing radiation and DNA repair. Our work presents a model that Cezanne serves as a "reader" of the Lys63-linkage polyubiquitin at DNA damage sites and an "eraser" of the Lys11-linkage ubiquitination, indicating a crosstalk between linkage-specific ubiquitination at DNA damage sites.

Molecular recognition of ubiquitin and Lys63-linked diubiquitin by STAM2 UIM-SH3 dual domain: the effect of its linker length and flexibility

Multidomain proteins represent a broad spectrum of the protein landscape and are involved in various interactions. They could be considered as modular building blocks assembled in distinct fashion and connected by linkers of varying lengths and sequences. Due to their intrinsic flexibility, these linkers provide proteins a subtle way to modulate interactions and explore a wide range of conformational space. In the present study, we are seeking to understand the effect of the flexibility and dynamics of the linker involved in the STAM2 UIM-SH3 dual domain protein with respect to molecular recognition. We have engineered several constructs of UIM-SH3 with different length linkers or domain deletion. By means of SAXS and NMR experiments, we have shown that the modification of the linker modifies the flexibility and the dynamics of UIM-SH3. Indeed, the global tumbling of both the UIM and SH3 domain is different but not independent from each other while the length of the linker has an impact on the ps-ns time scale dynamics of the respective domains. Finally, the modification of the flexibility and dynamics of the linker has a drastic effect on the interaction of UIM-SH3 with Lys63-linked diubiquitin with a roughly eight-time weaker dissociation constant.

Artificially Linked Ubiquitin Dimers Characterised Structurally and Dynamically by NMR Spectroscopy

As one of the most prevalent post-translational modifications in eukaryotic cells, ubiquitylation plays vital roles in many cellular processes, such as protein degradation, DNA metabolism, and cell differentiation. Substrate proteins can be tagged by distinct types of polymeric ubiquitin (Ub) chains, which determine the eventual fate of the modified protein. A facile, click chemistry based approach for the efficient generation of linkage-defined Ub chains, including Ub dimers, was recently established. Within these chains, individual Ub moieties are connected through a triazole linkage, rather than the natural isopeptide bond. Herein, it is reported that the conformation of an artificially K48-linked Ub dimer resembles that of the natively linked dimer, with respect to structural and dynamic characteristics, as demonstrated by means of high-resolution NMR spectroscopy. Thus, it is proposed that artificially linked Ub dimers, as generated by this approach, represent potent tools for studying the inherently different properties and functions of distinct Ub chains.

Analysis of the topology of ubiquitin chains

The small protein ubiquitin and its multiple polymers are encountered free in cells and as post-translational modifications on all proteins. Different polyubiquitin three dimensional structures are shown to correlate uniquely with different cellular functions as part of the diverse ubiquitin signaling. At the same time, this multiplicity of structures provides serious challenges to the analytical biochemist. Globally applicable strategies are presented here for the analyses of polyubiquitins and of ubiquitinated proteins, which take advantage of the speed, specificity and sensitivity of top-down tandem mass spectrometry. Particular attention is given to the supervised interpretation of fragmentation as revealed in the MS/MS spectra of these branched proteins. The strategy is compatible with any MS activation technology, is applicable to all polyubiquitin linkage and chain types, can be extended to ubiquitin-like proteins, and will be compatible with and enhanced by continuing advances in LC-MS/MS instrumentation and interpretation software.

Branching Out: Improved Signaling by Heterotypic Ubiquitin Chains

Ubiquitin chains of distinct topologies control the stability, interactions, or localization of many proteins in eukaryotic cells, and thus play an essential role in cellular information transfer. It has recently been found that ubiquitin chains can be combined to produce branched conjugates that are characterized by the presence of at least two linkages within the same polymer. Akin to their homotypic counterparts, branched chains elicit a wide array of biological outputs, further expanding the versatility, specificity, and efficiency of ubiquitin-dependent signaling. This review discusses emerging understanding of the synthesis and function of branched ubiquitin chains.

A20: A multifunctional tool for regulating immunity and preventing disease

A20, also known as TNFAIP3, is a potent regulator of ubiquitin (Ub) dependent signals. A20 prevents multiple human diseases, indicating that the critical functions of this protein are clinically as well as biologically impactful. As revealed by mouse models, cell specific functions of A20 are linked to its ability to regulate diverse signaling pathways. Aberrant expression or functions of A20 in specific cell types underlie divergent disease outcomes. Discernment of A20's biochemical functions and their phenotypic outcomes will contribute to our understanding of how ubiquitination is regulated, how Ub mediated functions can prevent disease, and will pave the way for future therapeutic interventions.

Using Ubiquitin Binders to Decipher the Ubiquitin Code

Post-translational modifications (PTMs) by ubiquitin (Ub) are versatile, highly dynamic, and involved in nearly all aspects of eukaryote biological function. The reversibility and heterogeneity of Ub chains attached to protein substrates have complicated their isolation, quantification, and characterization. Strategies have emerged to isolate endogenous ubiquitylated targets, including technologies based on the use of Ub-binding peptides, such as tandem-repeated Ub-binding entities (TUBEs). TUBEs allow the identification and characterization of Ub chains, and novel substrates for deubiquitylases (DUBs) and Ub ligases (E3s). Here we review their impact on purification, analysis of pan or chain-selective polyubiquitylated proteins and underline the biological relevance of this information. Together with peptide aptamers and other Ub affinity-based approaches, TUBEs will contribute to unraveling the secrets of the Ub code.

Structural Snapshots of 26S Proteasome Reveal Tetraubiquitin-Induced Conformations

The 26S proteasome is the ATP-dependent protease responsible for regulating the proteome of eukaryotic cells through degradation of mainly ubiquitin-tagged substrates. In order to understand how proteasome responds to ubiquitin signal, we resolved an ensemble of cryo-EM structures of proteasome in the presence of K48-Ub₄, with three of them resolved at near-atomic resolution. We identified a conformation with stabilized ubiquitin receptors and a previously unreported orientation of the lid, assigned as a Ub-accepted state C1-b. We determined another structure C3-b with localized K48-Ub₄ to the toroid region of Rpn1, assigned as a substrate-processing state. Our structures indicate that tetraUb induced conformational changes in proteasome could initiate substrate degradation. We also propose a CP gate-opening mechanism involving the propagation of the motion of the lid to the gate through the Rpn6-α2 interaction. Our results enabled us to put forward a model of a functional cycle for proteasomes induced by tetraUb and nucleotide.

Assembly of branched ubiquitin oligomers by click chemistry

Ubiquitin monomers functionalized with an azide or multiple alkynes were utilized for the assembly of branched ubiquitin oligomers that exhibit stability in eukaryotic cell lysates.

Trivalent RING Assembly on Retroviral Capsids Activates TRIM5 Ubiquitination and Innate Immune Signaling

TRIM5 is a RING domain E3 ubiquitin ligase with potent antiretroviral function. TRIM5 assembles into a hexagonal lattice on retroviral capsids, causing envelopment of the infectious core. Concomitantly, TRIM5 initiates innate immune signaling and orchestrates disassembly of the viral particle, yet how these antiviral responses are regulated by capsid recognition is unclear. We show that hexagonal assembly triggers N-terminal polyubiquitination of TRIM5 that collectively drives antiviral responses. In uninfected cells, N-terminal monoubiquitination triggers non-productive TRIM5 turnover. Upon TRIM5 assembly on virus, a trivalent RING arrangement allows elongation of N-terminally anchored K63-linked ubiquitin chains (N-K63-Ub). N-K63-Ub drives TRIM5 innate immune stimulation and proteasomal degradation. Inducing ubiquitination before TRIM5 assembly triggers premature degradation and ablates antiviral restriction. Conversely, driving N-K63 ubiquitination after TRIM5 assembly enhances innate immune signaling. Thus, the hexagonal geometry of TRIM5's antiviral lattice converts a capsid-binding protein into a multifunctional antiviral platform.

Genome-wide analysis of Glycine soja ubiquitin (UBQ) genes and functional analysis of GsUBQ10 in response to alkaline stress

Ubiquitin is a highly conserved protein with multiple essential regulatory functions through the ubiquitin-proteasome system. Even though its functions in the ubiquitin-mediated protein degradation pathway are very well characterized, the function of ubiquitin genes in the regulation of the alkaline stress response is not fully established. In this study, we identified 12 potential UBQ genes in the Glycine soja genome, and analyzed their evolutionary relationship, conserved domains and promoter cis-elements. We also explored the expression profiles of G. soja UBQ genes under alkaline stress, based on the transcriptome sequencing. We found that the expression of GsUBQ10 was significantly induced by alkaline stress, and the function of GsUBQ10 was characterized by overexpression in transgenic alfalfa (Medicago sativa). Our results suggested that GsUBQ10 transgenic lines significantly improved the alkaline tolerance in alfalfa. The GsUBQ10 transgenic lines showed lower relative membrane permeability, lower malon dialdehyde content and higher catalase activity than in the wild-type plants. This indicates that GsUBQ10 is involved in regulating the reactive oxygen species accumulation under alkaline stress. Taken together, we identified an ubiquitin gene GsUBQ10 from G. soja, which plays a positive role in responses to alkaline stress in alfalfa.

Top-Down Analysis of Branched Proteins Using Mass Spectrometry

Post-translational modifications by the covalent attachment of Rub1 (NEDD8), ubiquitin, SUMO, and other small signaling proteins have profound impacts on the functions and fates of cellular proteins. Investigations of the relationship of these bioactive structures and their functions are limited by analytical methods that are scarce and tedious. A novel strategy is reported here for the analysis of branched proteins by top-down mass spectrometry and illustrated by application to four recombinant proteins and one synthetic peptide modified by covalent bonds with ubiquitin or Rub1. The approach allows an analyte to be recognized as a branched protein; the participating proteins to be identified; the site of conjugation to be defined; and other chemical, native, and recombinant modifications to be characterized. In addition to the high resolution and high accuracy provided by the mass spectrometer, success is based on sample fragmentation by electron-transfer dissociation assisted by collisional activation and on software designed for graphic interpretation and adapted for branched proteins. The strategy allows for structures of unknown, two-component branched proteins to be elucidated directly the first time and can potentially be extended to more complex systems.

Impact of different ionization states of phosphorylated Serine-65 on ubiquitin structure and interactions

The covalent attachment of ubiquitin (Ub) or Ub chains to cellular proteins is a versatile post-translational modification involved in a variety of eukaryotic cellular events. Recently, the post-translational modification of Ub itself by phosphorylation has emerged as an important component of the Ub-signaling system. Specifically, Ub phosphorylation at serine-65 was shown to activate parkin-mediated mitochondrial quality control. However, the impact of phosphorylation on Ub structure and interactions is poorly understood. Here we investigate the recently reported structural changes in Ub upon serine-65 phosphorylation, namely, the equilibrium between a native-like and a novel, alternate conformer of phosphorylated Ub (pUb). We show that this equilibrium is pH-dependent, and the two pUb conformers are linked to the different charge states of the phosphate group. We examined pUb binding to a known Ub-receptor and found that the alternate conformer is binding incompetent. Furthermore, serine-65 phosphorylation affects the conformational equilibrium of K48-linked Ub dimers. Lastly, our crystal structure of S65D Ub and NMR data indicate that phosphomimetic mutations do not adequately reproduce the salient features of pUb. Our results suggest that the pH-dependence of the conformations and binding properties of phosphorylated Ub and polyUb could provide an additional level of modulation in Ub-mediated signaling.

K63 ubiquitylation triggers proteasomal degradation by seeding branched ubiquitin chains

Different polyubiquitin chain linkages direct substrates toward distinct cellular pathways. K63-linked ubiquitylation is known to regulate proteasome-independent events such as signal transduction, but its function in the context of heterogeneous ubiquitin chains remains unclear. Here, we report that K63 ubiquitylation plays a critical role in proteasome-mediated substrate degradation by serving as a "seed" for K48/K63 branched ubiquitin chains. Quantitative analysis revealed that K48/K63 branched linkages preferentially associate with proteasomes in cells. We found that ITCH-dependent K63 ubiquitylation of the proapoptotic regulator TXNIP triggered subsequent assembly of K48/K63 branched chains by recruiting ubiquitin-interacting ligases such as UBR5, leading to TXNIP degradation. These results reveal a role for K63 chains as a substrate-specific mark for proteasomal degradation involved in regulating cell fate. Our findings provide insight into how cellular interpretation of the ubiquitin code is altered by combinations of ubiquitin linkages.

Post-translational add-ons mark the path in exosomal protein sorting

Extracellular vesicles (EVs) are released by cells to the extracellular environment to mediate inter-cellular communication. Proteins, lipids, nucleic acids and metabolites shuttled in these vesicles modulate specific functions in recipient cells. The enrichment of selected sets of proteins in EVs compared with global cellular levels suggests the existence of specific sorting mechanisms to specify EV loading. Diverse post-translational modifications (PTMs) of proteins participate in the loading of specific elements into EVs. In this review, we offer a perspective on PTMs found in EVs and discuss the specific role of some PTMs, specifically Ubiquitin and Ubiquitin-like modifiers, in exosomal sorting of protein components. The understanding of these mechanisms will provide new strategies for biomedical applications. Examples include the presence of defined PTM marks on EVs as novel biomarkers for the diagnosis and prognosis of certain diseases, or the specific import of immunogenic components into EVs for vaccine generation.

The Crystal Structure and Conformations of an Unbranched Mixed Tri-Ubiquitin Chain Containing K48 and K63 Linkages

The ability of ubiquitin to function in a wide range of cellular processes is ascribed to its capacity to cause a diverse spectrum of modifications. While a target protein can be modified with monoubiquitin, it can also be modified with ubiquitin chains. The latter include seven types of homotypic chains as well as mixed ubiquitin chains. In a mixed chain, not all the isopeptide bonds are restricted to a specific lysine of ubiquitin, resulting in a chain possessing more than one type of linkage. While structural characterization of homotypic chains has been well elucidated, less is known about mixed chains. Here we present the crystal structure of a mixed tri-ubiquitin chain at 3.1-Å resolution. In the structure, the proximal ubiquitin is connected to the middle ubiquitin via K48 and these two ubiquitins adopt a compact structure as observed in K48 di-ubiquitin. The middle ubiquitin links to the distal ubiquitin via its K63 and these ubiquitins adopt two conformations, suggesting a flexible structure. Using small-angle X-ray scattering, we unexpectedly found differences between the conformational ensembles of the above tri-ubiquitin chains and chains possessing the same linkages but in the reverse order. In addition, cleavage of the K48 linkage by DUB is faster if this linkage is at the distal end. Taken together, our results suggest that in mixed chains, not only the type of the linkages but also their sequence determine the structural and functional properties of the chain.

RNF8- and Ube2S-Dependent Ubiquitin Lysine 11-Linkage Modification in Response to DNA Damage

Ubiquitin modification of proteins plays pivotal roles in the cellular response to DNA damage. Given the complexity of ubiquitin conjugation due to the formation of poly-conjugates of different linkages, functional roles of linkage-specific ubiquitin modification at DNA damage sites are largely unclear. We identify that Lys11-linkage ubiquitin modification occurs at DNA damage sites in an ATM-dependent manner, and ubiquitin-modifying enzymes, including Ube2S E2-conjugating enzyme and RNF8 E3 ligase, are responsible for the assembly of Lys11-linkage conjugates on damaged chromatin, including histone H2A/H2AX. We show that RNF8- and Ube2S-dependent Lys11-linkage ubiquitin conjugation plays an important role in regulating DNA damage-induced transcriptional silencing, distinct from the role of Lys63-linkage ubiquitin in the recruitment of DNA damage repair proteins 53BP1 and BRCA1. Thus, our study highlights the importance of linkage-specific ubiquitination at DNA damage sites, and it reveals that Lys11-linkage ubiquitin modification plays a crucial role in the DNA damage response.