NEDD8 nucleates a multivalent cullin-RING-UBE2D ubiquitin ligation assembly

Structural and biochemical characterization establishes a detailed understanding of KEAP1-CUL3 complex assembly

KEAP1 promotes the ubiquitin-dependent degradation of NRF2 by assembling into a CUL3-dependent ubiquitin ligase complex. Oxidative and electrophilic stress inhibit KEAP1 allowing NRF2 to accumulate for the transactivation of stress response genes. To date there are no structures of the KEAP1-CUL3 interaction nor binding data to show the contributions of different domains to their binding affinity. We determined a crystal structure of the BTB and 3-box domains of human KEAP1 in complex with the CUL3 N-terminal domain that showed a heterotetrameric assembly with 2:2 stoichiometry. To support the structural data, we developed a versatile TR-FRET-based assay system to profile the binding of BTB-domain-containing proteins to CUL3 and determine the contribution of distinct protein features, revealing the importance of the CUL3 N-terminal extension for high affinity binding. We further provide direct evidence that the investigational drug CDDO does not disrupt the KEAP1-CUL3 interaction, even at high concentrations, but reduces the affinity of KEAP1-CUL3 binding. The TR-FRET-based assay system offers a generalizable platform for profiling this protein class and may form a suitable screening platform for ligands that disrupt these interactions by targeting the BTB or 3-box domains to block E3 ligase function.

Targeting NEDD8-activating enzyme for cancer therapy: developments, clinical trials, challenges and future research directions

NEDDylation, a post-translational modification through three-step enzymatic cascades, plays crucial roles in the regulation of diverse biological processes. NEDD8-activating enzyme (NAE) as the only activation enzyme in the NEDDylation modification has become an attractive target to develop anticancer drugs. To date, numerous inhibitors or agonists targeting NAE have been developed. Among them, covalent NAE inhibitors such as MLN4924 and TAS4464 currently entered into clinical trials for cancer therapy, particularly for hematological tumors. This review explains the relationships between NEDDylation and cancers, structural characteristics of NAE and multistep mechanisms of NEDD8 activation by NAE. In addition, the potential approaches to discover NAE inhibitors and detailed pharmacological mechanisms of NAE inhibitors in the clinical stage are explored in depth. Importantly, we reasonably investigate the challenges of NAE inhibitors for cancer therapy and possible development directions of NAE-targeting drugs in the future.

A ubiquitin-based effector-to-inhibitor switch coordinates early brain, craniofacial, and skin development

The molecular mechanisms that coordinate patterning of the embryonic ectoderm into spatially distinct lineages to form the nervous system, epidermis, and neural crest-derived craniofacial structures are unclear. Here, biochemical disease-variant profiling reveals a posttranslational pathway that drives early ectodermal differentiation in the vertebrate head. The anteriorly expressed ubiquitin ligase CRL3-KLHL4 restricts signaling of the ubiquitous cytoskeletal regulator CDC42. This regulation relies on the CDC42-activating complex GIT1-βPIX, which CRL3-KLHL4 exploits as a substrate-specific co-adaptor to recognize and monoubiquitylate PAK1. Surprisingly, we find that ubiquitylation converts the canonical CDC42 effector PAK1 into a CDC42 inhibitor. Loss of CRL3-KLHL4 or a disease-associated KLHL4 variant reduce PAK1 ubiquitylation causing overactivation of CDC42 signaling and defective ectodermal patterning and neurulation. Thus, tissue-specific restriction of CDC42 signaling by a ubiquitin-based effector-to-inhibitor is essential for early face, brain, and skin formation, revealing how cell-fate and morphometric changes are coordinated to ensure faithful organ development.

Structural basis for the ubiquitination of G protein βγ subunits by KCTD5/Cullin3 E3 ligase

G protein-coupled receptor (GPCR) signaling is precisely controlled to avoid overstimulation that results in detrimental consequences. Gβγ signaling is negatively regulated by a Cullin3 (Cul3)-dependent E3 ligase, KCTD5, which triggers ubiquitination and degradation of free Gβγ. Here, we report the cryo-electron microscopy structures of the KCTD5-Gβγ fusion complex and the KCTD7-Cul3 complex. KCTD5 in pentameric form engages symmetrically with five copies of Gβγ through its C-terminal domain. The unique pentameric assembly of the KCTD5/Cul3 E3 ligase places the ubiquitin-conjugating enzyme (E2) and the modification sites of Gβγ in close proximity and allows simultaneous transfer of ubiquitin from E2 to five Gβγ subunits. Moreover, we show that ubiquitination of Gβγ by KCTD5 is important for fine-tuning cyclic adenosine 3´,5´-monophosphate signaling of GPCRs. Our studies provide unprecedented insights into mechanisms of substrate recognition by unusual pentameric E3 ligases and highlight the KCTD family as emerging regulators of GPCR signaling.

The semisynthesis of site-specifically modified histones and histone-based probes of chromatin-modifying enzymes

Histone post-translational modifications (PTMs) on lysine residues, including methylation, ubiquitylation, and sumoylation, have been studied using semisynthetic histones reconstituted into nucleosomes. These studies have revealed the in vitro effects of histone PTMs on chromatin structure, gene transcription, and biochemical crosstalk. However, the dynamic and transient nature of most enzyme-chromatin interactions poses a challenge toward identifying specific enzyme-substrate interactions. To address this, we report methodology for the synthesis of two ubiquitylated activity-based probe histones, H2BK120ub(G76C) and H2BK120ub(G76Dha), that may be used to trap enzyme active-site cysteines as disulfides or in the form of thioether linkages, respectively. The general synthetic method we report for converting ubiquitylated nucleosomes into activity-based probes may also be applied to other histone sites of ubiquitylation in order to facilitate the identification of enzyme-chromatin interactions.

Glucose-induced CRL4COP1-p53 axis amplifies glycometabolism to drive tumorigenesis

The diabetes-cancer association remains underexplained. Here, we describe a glucose-signaling axis that reinforces glucose uptake and glycolysis to consolidate the Warburg effect and overcome tumor suppression. Specifically, glucose-dependent CK2 O-GlcNAcylation impedes its phosphorylation of CSN2, a modification required for the deneddylase CSN to sequester Cullin RING ligase 4 (CRL4). Glucose, therefore, elicits CSN-CRL4 dissociation to assemble the CRL4^COP1 E3 ligase, which targets p53 to derepress glycolytic enzymes. A genetic or pharmacologic disruption of the O-GlcNAc-CK2-CSN2-CRL4^COP1 axis abrogates glucose-induced p53 degradation and cancer cell proliferation. Diet-induced overnutrition upregulates the CRL4^COP1-p53 axis to promote PyMT-induced mammary tumorigenesis in wild type but not in mammary-gland-specific p53 knockout mice. These effects of overnutrition are reversed by P28, an investigational peptide inhibitor of COP1-p53 interaction. Thus, glycometabolism self-amplifies via a glucose-induced post-translational modification cascade culminating in CRL4^COP1-mediated p53 degradation. Such mutation-independent p53 checkpoint bypass may represent the carcinogenic origin and targetable vulnerability of hyperglycemia-driven cancer.

Regulatory mechanisms and therapeutic potential of JAB1 in neurological development and disorders

c-Jun activation domain binding protein-1 (JAB1) is a multifunctional regulator that plays vital roles in diverse cellular processes. It regulates AP-1 transcriptional activity and also acts as the fifth component of the COP9 signalosome complex. While JAB1 is considered an oncoprotein that triggers tumor development, recent studies have shown that it also functions in neurological development and disorders. In this review, we summarize the general features of the JAB1 gene and protein, and present recent updates on the regulation of JAB1 expression. Moreover, we also highlight the functional roles and regulatory mechanisms of JAB1 in neurodevelopmental processes such as neuronal differentiation, synaptic morphogenesis, myelination, and hair cell development and in the pathogenesis of some neurological disorders such as Alzheimer's disease, multiple sclerosis, neuropathic pain, and peripheral nerve injury. Furthermore, current challenges and prospects are discussed, including updates on drug development targeting JAB1.

Determining the Potential Roles of Branched-Chain Amino Acids in the Regulation of Muscle Growth in Common Carp (Cyprinus carpio) Based on Transcriptome and MicroRNA Sequencing

Branched-chain amino acids (BCAAs) can be critically involved in skeletal muscle growth and body energy homeostasis. Skeletal muscle growth is a complex process; some muscle-specific microRNAs (miRNAs) are involved in the regulation of muscle thickening and muscle mass. Additionally, the regulatory network between miRNA and messenger RNA (mRNA) in the modulation of the role of BCAAs on skeletal muscle growth in fish has not been studied. In this study, common carp was starved for 14 days, followed by a 14-day gavage therapy with BCAAs, to investigate some of the miRNAs and genes that contribute to the regulation of normal growth and maintenance of skeletal muscle in response to short-term BCAA starvation stress. Subsequently, the transcriptome and small RNAome sequencing of carp skeletal muscle were performed. A total of 43,414 known and 1,112 novel genes were identified, in addition to 142 known and 654 novel miRNAs targeting 22,008 and 33,824 targets, respectively. Based on their expression profiles, 2,146 differentially expressed genes (DEGs) and 84 differentially expressed miRNA (DEMs) were evaluated. Kyoto Encyclopedia of Genes and Genome pathways, including the proteasome, phagosome, autophagy in animals, proteasome activator complex, and ubiquitin-dependent protein catabolic process, were enriched for these DEGs and DEMs. Our findings revealed the role of atg5, map1lc3c, ctsl, cdc53, psma6, psme2, myl9, and mylk in skeletal muscle growth, protein synthesis, and catabolic metabolism. Furthermore, miR-135c, miR-192, miR-194, and miR-203a may play key roles in maintaining the normal activities of the organism by regulating genes related to muscle growth, protein synthesis, and catabolism. This study on transcriptome and miRNA reveals the potential molecular mechanisms underlying the regulation of muscle protein deposition and provides new insights into genetic engineering techniques to improve common carp muscle development.

Genome-scale CRISPR screen reveals neddylation to contribute to cisplatin resistance of testicular germ cell tumours

BACKGROUND

Type II testicular germ cell tumours (TGCT) are the most prevalent tumours in young men. Patients suffering from cisplatin-resistant TGCTs are facing very poor prognosis demanding novel therapeutic options. Neddylation is a known posttranslational modification mediating many important biological processes, including tumorigenesis. Overactivation of the neddylation pathway promotes carcinogenesis and tumour progression in various entities by inducing proteasomal degradation of tumour suppressors (e.g., p21, p27).

METHODS

We used a genome-scale CRISPR/Cas9 activation screen to identify cisplatin resistance factors. TGCT cell lines were treated with the neddylation inhibitor (MLN4924)/cisplatin/combination and investigated for changes in viability (XTT assay), apoptosis/cell cycle (flow cytometry) as well as in the transcriptome (3'mRNA sequencing).

RESULTS

NAE1 overexpression was detected in cisplatin-resistant colonies from the CRISPR screen. Inhibition of neddylation using MLN4924 increased cisplatin cytotoxicity in TGCT cell lines and sensitised cisplatin-resistant cells towards cisplatin. Apoptosis, G2/M-phase cell cycle arrest, γH2A.X/P27 accumulation and mesoderm/endoderm differentiation were observed in TGCT cells, while fibroblast cells were unaffected.

CONCLUSIONS

We identified overactivation of neddylation as a factor for cisplatin resistance in TGCTs and highlighted the additive effect of NAE1 inhibition by MLN4924 in combination with cisplatin as a novel treatment option for TGCTs.

A chemical strategy to study transient states in substrate ubiquitylation by cryo-EM

The characterization of short-lived intermediate states along the path of a ubiquitylation reaction remains a major challenge. In this issue of Chem, Ai et al. present a chemical trapping method for probing transient intermediates in substrate ubiquitylation. The usefulness of this approach is demonstrated by the determination of single-particle cryo-EM structures related to nucleosome ubiquitylation.

Review: A silent concert in developing plants: Dynamic assembly of cullin-RING ubiquitin ligases

Plants appear quiet: quietly, they break the ground, expand leaves, search for resources, alert each other to invaders, and heal their own wounds. In contrast to the stationary appearance, the inside world of a plant is full of movements: cells divide to increase the body mass and form new organs; signaling molecules migrate among cells and tissues to drive transcriptional cascades and developmental programs; macromolecules, such as RNAs and proteins, collaborate with different partners to maintain optimal organismal function under changing cellular and environmental conditions. All these activities require a dynamic yet appropriately controlled molecular network in plant cells. In this short review, we used the regulation of cullin-RING ubiquitin ligases (CRLs) as an example to discuss how dynamic biochemical processes contribute to plant development. CRLs comprise a large family of modular multi-unit enzymes that determine the activity and stability of diverse regulatory proteins playing crucial roles in plant growth and development. The mechanism governing the dynamic assembly of CRLs is essential for CRL activity and biological function, and it may provide insights and implications for the regulation of other dynamic multi-unit complexes involved in fundamental processes such as transcription, translation, and protein sorting in plants.

Systemwide disassembly and assembly of SCF ubiquitin ligase complexes

Cells respond to environmental cues by remodeling their inventories of multiprotein complexes. Cellular repertoires of SCF (SKP1-CUL1-F box protein) ubiquitin ligase complexes, which mediate much protein degradation, require CAND1 to distribute the limiting CUL1 subunit across the family of ∼70 different F box proteins. Yet, how a single factor coordinately assembles numerous distinct multiprotein complexes remains unknown. We obtained cryo-EM structures of CAND1-bound SCF complexes in multiple states and correlated mutational effects on structures, biochemistry, and cellular assays. The data suggest that CAND1 clasps idling catalytic domains of an inactive SCF, rolls around, and allosterically rocks and destabilizes the SCF. New SCF production proceeds in reverse, through SKP1-F box allosterically destabilizing CAND1. The CAND1-SCF conformational ensemble recycles CUL1 from inactive complexes, fueling mixing and matching of SCF parts for E3 activation in response to substrate availability. Our data reveal biogenesis of a predominant family of E3 ligases, and the molecular basis for systemwide multiprotein complex assembly.

CAND1 orchestrates CRLs through rock and roll

Proper regulation of protein degradation is essential for cell physiology. In the current issue of Cell, Baek et al. elucidated how a large class of ubiquitin ligase, known as CRL, is assembled and disassembled through a key regulator, CAND1.

Chemoproteomics-enabled discovery of a covalent molecular glue degrader targeting NF-κB

Targeted protein degradation has arisen as a powerful therapeutic modality for degrading disease targets. While proteolysis-targeting chimera (PROTAC) design is more modular, the discovery of molecular glue degraders has been more challenging. Here, we have coupled the phenotypic screening of a covalent ligand library with chemoproteomic approaches to rapidly discover a covalent molecular glue degrader and associated mechanisms. We have identified a cysteine-reactive covalent ligand EN450 that impairs leukemia cell viability in a NEDDylation and proteasome-dependent manner. Chemoproteomic profiling revealed covalent interaction of EN450 with an allosteric C111 in the E2 ubiquitin-conjugating enzyme UBE2D. Quantitative proteomic profiling revealed the degradation of the oncogenic transcription factor NFKB1 as a putative degradation target. Our study thus puts forth the discovery of a covalent molecular glue degrader that uniquely induced the proximity of an E2 with a transcription factor to induce its degradation in cancer cells.

Ubiquitinome profiling reveals in vivo UBE2D3 targets and implicates UBE2D3 in protein quality control

Ubiquitination has crucial roles in many cellular processes and dysregulation of ubiquitin machinery enzymes can result in various forms of pathogenesis. Cells only have a limited set of ubiquitin-conjugating (E2) enzymes to support the ubiquitination of many cellular targets. As individual E2 enzymes have many different substrates and interactions between E2 enzymes and their substrates can be transient, it is challenging to define all in vivo substrates of an individual E2 and the cellular processes it affects. Particularly challenging in this respect is UBE2D3, an E2 enzyme with promiscuous activity in vitro but less defined roles in vivo. Here, we set out to identify in vivo targets of UBE2D3 by using SILAC-based and label-free quantitative ubiquitin diGly proteomics to study global proteome and ubiquitinome changes associated with UBE2D3 depletion. UBE2D3 depletion changed the global proteome, with the levels of proteins from metabolic pathways, in particular retinol metabolism, being the most affected. However, the impact of UBE2D3 depletion on the ubiquitinome was much more prominent. Interestingly, molecular pathways related to mRNA translation were the most affected. Indeed, we find that ubiquitination of the ribosomal proteins RPS10 and RPS20, critical for ribosome-associated protein quality control (RQC), is dependent on UBE2D3. We show by TULIP2 methodology that RPS10 and RPS20 are direct targets of UBE2D3 and demonstrate that UBE2D3's catalytic activity is required to ubiquitinate RPS10 in vivo. In addition, our data suggest that UBE2D3 acts at multiple levels in autophagic protein quality control (PQC). Collectively, our findings show that depletion of an E2 enzyme in combination with quantitative diGly-based ubiquitinome profiling is a powerful tool to identify new in vivo E2 substrates, as we have done here for UBE2D3. Our work provides an important resource for further studies on the in vivo functions of UBE2D3.

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.

Targeted Protein Degradation through E2 Recruitment

Targeted protein degradation (TPD) with proteolysis targeting chimeras (PROTACs), heterobifunctional compounds consisting of protein targeting ligands linked to recruiters of E3 ubiquitin ligases, has arisen as a powerful therapeutic modality to induce the proximity of target proteins with E3 ligases to ubiquitinate and degrade specific proteins in cells. Thus far, PROTACs have primarily exploited the recruitment of E3 ubiquitin ligases or their substrate adapter proteins but have not exploited the recruitment of more core components of the ubiquitin-proteasome system (UPS). In this study, we used covalent chemoproteomic approaches to discover a covalent recruiter against the E2 ubiquitin conjugating enzyme UBE2D─EN67─that targets an allosteric cysteine, C111, without affecting the enzymatic activity of the protein. We demonstrated that this UBE2D recruiter could be used in heterobifunctional degraders to degrade neo-substrate targets in a UBE2D-dependent manner, including BRD4 and the androgen receptor. Overall, our data highlight the potential for the recruitment of core components of the UPS machinery, such as E2 ubiquitin conjugating enzymes, for TPD, and underscore the utility of covalent chemoproteomic strategies for identifying novel recruiters for additional components of the UPS.

Advances in the potential roles of Cullin-RING ligases in regulating autoimmune diseases

Cullin-RING ligases (CRLs) are the largest class of E3 ubiquitin ligases regulating the stability and subsequent activity of a large number of important proteins responsible for the development and progression of various diseases, including autoimmune diseases (AIDs). However, the detailed mechanisms of the pathogenesis of AIDs are complicated and involve multiple signaling pathways. An in-depth understanding of the underlying regulatory mechanisms of the initiation and progression of AIDs will aid in the development of effective therapeutic strategies. CRLs play critical roles in regulating AIDs, partially by affecting the key inflammation-associated pathways such as NF-κB, JAK/STAT, and TGF-β. In this review, we summarize and discuss the potential roles of CRLs in the inflammatory signaling pathways and pathogenesis of AIDs. Furthermore, advances in the development of novel therapeutic strategies for AIDs through targeting CRLs are also highlighted.

E3 ligase autoinhibition by C-degron mimicry maintains C-degron substrate fidelity

E3 ligase recruitment of proteins containing terminal destabilizing motifs (degrons) is emerging as a major form of regulation. How those E3s discriminate bona fide substrates from other proteins with terminal degron-like sequences remains unclear. Here, we report that human KLHDC2, a CRL2 substrate receptor targeting C-terminal Gly-Gly degrons, is regulated through interconversion between two assemblies. In the self-inactivated homotetramer, KLHDC2's C-terminal Gly-Ser motif mimics a degron and engages the substrate-binding domain of another protomer. True substrates capture the monomeric CRL2KLHDC2, driving E3 activation by neddylation and subsequent substrate ubiquitylation. Non-substrates such as NEDD8 bind KLHDC2 with high affinity, but its slow on rate prevents productive association with CRL2KLHDC2. Without substrate, neddylated CRL2KLHDC2 assemblies are deactivated via distinct mechanisms: the monomer by deneddylation and the tetramer by auto-ubiquitylation. Thus, substrate specificity is amplified by KLHDC2 self-assembly acting like a molecular timer, where only bona fide substrates may bind before E3 ligase inactivation.

Structural mechanism of CRL4-instructed STAT2 degradation via a novel cytomegaloviral DCAF receptor

Human cytomegalovirus (CMV) is a ubiquitously distributed pathogen whose rodent counterparts such as mouse and rat CMV serve as common infection models. Here, we conducted global proteome profiling of rat CMV-infected cells and uncovered a pronounced loss of the transcription factor STAT2, which is crucial for antiviral interferon signalling. Via deletion mutagenesis, we found that the viral protein E27 is required for CMV-induced STAT2 depletion. Cellular and in vitro analyses showed that E27 exploits host-cell Cullin4-RING ubiquitin ligase (CRL4) complexes to induce poly-ubiquitylation and proteasomal degradation of STAT2. Cryo-electron microscopy revealed how E27 mimics molecular surface properties of cellular CRL4 substrate receptors called DCAFs (DDB1- and Cullin4-associated factors), thereby displacing them from the catalytic core of CRL4. Moreover, structural analyses showed that E27 recruits STAT2 through a bipartite binding interface, which partially overlaps with the IRF9 binding site. Structure-based mutations in M27, the murine CMV homologue of E27, impair the interferon-suppressing capacity and virus replication in mouse models, supporting the conserved importance of DCAF mimicry for CMV immune evasion.

Recognition of the CCT5 di-Glu degron by CRL4DCAF12 is dependent on TRiC assembly

Assembly Quality Control (AQC) E3 ubiquitin ligases target incomplete or incorrectly assembled protein complexes for degradation. The CUL4-RBX1-DDB1-DCAF12 (CRL4^DCAF12 ) E3 ligase preferentially ubiquitinates proteins that carry a C-terminal double glutamate (di-Glu) motif. Reported CRL4^DCAF12 di-Glu-containing substrates include CCT5, a subunit of the TRiC chaperonin. How DCAF12 engages its substrates and the functional relationship between CRL4^DCAF12 and CCT5/TRiC is currently unknown. Here, we present the cryo-EM structure of the DDB1-DCAF12-CCT5 complex at 2.8 Å resolution. DCAF12 serves as a canonical WD40 DCAF substrate receptor and uses a positively charged pocket at the center of the β-propeller to bind the C-terminus of CCT5. DCAF12 specifically reads out the CCT5 di-Glu side chains, and contacts other visible degron amino acids through Van der Waals interactions. The CCT5 C-terminus is inaccessible in an assembled TRiC complex, and functional assays demonstrate that DCAF12 binds and ubiquitinates monomeric CCT5, but not CCT5 assembled into TRiC. Our biochemical and structural results suggest a previously unknown role for the CRL4^DCAF12 E3 ligase in overseeing the assembly of a key cellular complex.

A bifunctional molecule-assisted synthesis of mimics for use in probing the ubiquitination system

Ubiquitination regulates almost every life process of eukaryotes. The study of the ubiquitin (Ub) coupling or decoupling process and the interaction study of Ub-Ub binding protein have always been the central focus. However, such studies are challenging, owing to the transient nature of Ub-coupling enzymes and deubiquitinases in the reactions, as well as the difficulty in preparing large quantities of polyubiquitinated samples. Here we describe a recently developed strategy for the efficient preparation of analogs of Ub chains and analogs for Ub coupling and uncoupling intermediates, which facilitate the study of the ubiquitination process. The strategy includes mainly the following steps: (i) the bifunctional molecule conjugation on the only cysteine (Cys) residue of a target protein (usually a Ub or Ub-conjugating enzyme), exposing an orthogonal reactive site for native chemical ligation; (ii) covalent ligation with a Ub-derived thioester, exposing a free sulfhydryl; and (iii) (optional) a disulfide bond formation with a substrate protein (mainly with only one Cys protein) through nonactivity-based cross-linking or with a deubiquitinase (mainly with several Cys residues) through activity-based cross-linking. When the bifunctional molecule and target proteins are obtained, the final products can be prepared in milligram quantities within 2 weeks.

NEDD8-conjugating enzyme E2s: critical targets for cancer therapy

NEDD8-conjugating enzymes, E2s, include the well-studied ubiquitin-conjugating enzyme E2 M (UBE2M) and the poorly characterized ubiquitin-conjugating enzyme E2 F (UBE2F). UBE2M and UBE2F have distinct and prominent roles in catalyzing the neddylation of Cullin or non-Cullin substrates. These enzymes are overexpressed in various malignancies, conferring a worse overall survival. Targeting UBE2M to influence tumor growth by either modulating several biological responses of tumor cells (such as DNA-damage response, apoptosis, or senescence) or regulating the anti-tumor immunity holds strong therapeutic potential. Multiple inhibitors that target the interaction between UBE2M and defective cullin neddylation protein 1 (DCN1), a co-E3 for neddylation, exhibit promising anti-tumor effects. By contrast, the potential benefits of targeting UBE2F are still to be explored. It is currently reported to inhibit apoptosis and then induce cell growth; hence, targeting UBE2F serves as an effective chemo-/radiosensitizing strategy by triggering apoptosis. This review highlights the most recent advances in the roles of UBE2M and UBE2F in tumor progression, indicating these E2s as two promising anti-tumor targets.

E3-Specific Degrader Discovery by Dynamic Tracing of Substrate Receptor Abundance

Targeted protein degradation (TPD) is a new pharmacology based on small-molecule degraders that induce proximity between a protein of interest (POI) and an E3 ubiquitin ligase. Of the approximately 600 E3s encoded in the human genome, only around 2% can be co-opted with degraders. This underrepresentation is caused by a paucity of discovery approaches to identify degraders for defined E3s. This hampers a rational expansion of the druggable proteome and stymies critical advancements in the field, such as tissue- and cell-specific degradation. Here, we focus on dynamic NEDD8 conjugation, a post-translational, regulatory circuit that controls the activity of 250 cullin RING E3 ligases (CRLs). Leveraging this regulatory layer enabled us to develop a scalable assay to identify compounds that alter the interactome of an E3 of interest by tracing their abundance after pharmacologically induced auto-degradation. Initial validation studies are performed for CRBN and VHL, but proteomics studies indicate broad applicability for many CRLs. Among amenable ligases, we select CRLDCAF15 for a proof-of-concept screen, leading to the identification of a novel DCAF15-dependent molecular glue degrader inducing the degradation of RBM23 and RBM39. Together, this strategy empowers the scalable identification of degraders specific to a ligase of interest.

Activity-based profiling of cullin-RING ligase networks by conformation-specific probes

The cullin-RING E3 ligase (CRL) network comprises over 300 unique complexes that switch from inactive to activated conformations upon site-specific cullin modification by the ubiquitin-like protein NEDD8. Assessing cellular repertoires of activated CRL complexes is critical for understanding eukaryotic regulation. However, probes surveying networks controlled by site-specific ubiquitin-like protein modifications are lacking. We report development of a synthetic antibody recognizing the active conformation of a NEDD8-linked cullin. We established a pipeline probing cellular networks of activated CUL1-, CUL2-, CUL3- and CUL4-containing CRLs, revealing the CRL complexes responding to stimuli. Profiling several cell types showed their baseline neddylated CRL repertoires vary, prime efficiency of targeted protein degradation, and are differentially rewired across distinct primary cell activation pathways. Thus, conformation-specific probes can permit nonenzymatic activity-based profiling across a system of numerous multiprotein complexes, which in the case of neddylated CRLs reveals widespread regulation and could facilitate development of degrader drugs.

Structural basis for HIV-1 antagonism of host APOBEC3G via Cullin E3 ligase

Human APOBEC3G (A3G) is a virus restriction factor that inhibits HIV-1 replication and triggers lethal hypermutation on viral reverse transcripts. HIV-1 viral infectivity factor (Vif) breaches this host A3G immunity by hijacking a cellular E3 ubiquitin ligase complex to target A3G for ubiquitination and degradation. The molecular mechanism of A3G targeting by Vif-E3 ligase is unknown, limiting the antiviral efforts targeting this host-pathogen interaction crucial for HIV-1 infection. Here, we report the cryo-electron microscopy structures of A3G bound to HIV-1 Vif in complex with T cell transcription cofactor CBF-β and multiple components of the Cullin-5 RING E3 ubiquitin ligase. The structures reveal unexpected RNA-mediated interactions of Vif with A3G primarily through A3G's noncatalytic domain, while A3G's catalytic domain is poised for ubiquitin transfer. These structures elucidate the molecular mechanism by which HIV-1 Vif hijacks the host ubiquitin ligase to specifically target A3G to establish infection and offer structural information for the rational development of antiretroviral therapeutics.

An In vitro Assay to Recapitulate Hormone-Triggered and SCF-Mediated Protein Ubiquitylation

Signaling proteins trigger a sequence of molecular switches in the cell, which permit development, growth, and rapid adaptation to changing environmental conditions. SCF-type E3 ubiquitin ligases recognize signaling proteins prompting changes in their fate, one of these being ubiquitylation followed by degradation by the proteasome. SCFs together with their ubiquitylation targets (substrates) often serve as phytohormone receptors, responding and/or assembling in response to fluctuating intracellular hormone concentrations. Tracing and understanding phytohormone perception and SCF-mediated ubiquitylation of proteins could provide powerful clues on the molecular mechanisms utilized for plant adaptation. Here, we describe an adaptable in vitro system that uses recombinant proteins and enables the study of hormone-triggered SCF-substrate interaction and the dynamics of protein ubiquitylation. This system can serve to predict the requirements for protein recognition and to understand how phytohormone levels have the power to control protein fate.

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.

Mechanical stress shapes the cancer cell response to neddylation inhibition

Background

The inhibition of neddylation by the preclinical drug MLN4924 represents a new strategy to combat cancer. However, despite being effective against hematologic malignancies, its success in solid tumors, where cell–cell and cell-ECM interactions play essential roles, remains elusive.

Methods

Here, we studied the effects of MLN4924 on cell growth, migration and invasion in cultured prostate cancer cells and in disease-relevant prostate tumoroids. Using focused protein profiling, drug and RNAi screening, we analyzed cellular pathways activated by neddylation inhibition.

Results

We show that mechanical stress induced by MLN4924 in prostate cancer cells significantly affects the therapeutic outcome. The latter depends on the cell type and involves distinct Rho isoforms. In LNCaP and VCaP cells, the stimulation of RhoA and RhoB by MLN4924 markedly upregulates the level of tight junction proteins at cell–cell contacts, which augments the mechanical strain induced by Rho signaling. This “tight junction stress response” (TJSR) causes the collapse of cell monolayers and a characteristic rupture of cancer spheroids. Notably, TJSR is a major cause of drug-induced apoptosis in these cells. On the other hand, in PC3 cells that underwent partial epithelial-to-mesenchymal transition (EMT), the stimulation of RhoC induces an adverse effect by promoting amoeboid cell scattering and invasion. We identified complementary targets and drugs that allow for the induction of TJSR without stimulating RhoC.

Conclusions

Our finding that MLN4924 acts as a mechanotherapeutic opens new ways to improve the efficacy of neddylation inhibition as an anticancer approach.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13046-022-02328-y.

DeepPROTACs is a deep learning-based targeted degradation predictor for PROTACs

The rational design of PROTACs is difficult due to their obscure structure-activity relationship. This study introduces a deep neural network model - DeepPROTACs to help design potent PROTACs molecules. It can predict the degradation capacity of a proposed PROTAC molecule based on structures of given target protein and E3 ligase. The experimental dataset is mainly collected from PROTAC-DB and appropriately labeled according to the DC50 and Dmax values. In the model of DeepPROTACs, the ligands as well as the ligand binding pockets are generated and represented with graphs and fed into Graph Convolutional Networks for feature extraction. While SMILES representations of linkers are fed into a Bidirectional Long Short-Term Memory layer to generate the features. Experiments show that DeepPROTACs model achieves 77.95% average prediction accuracy and 0.8470 area under receiver operating characteristic curve on the test set. DeepPROTACs is available online at a web server ( https://bailab.siais.shanghaitech.edu.cn/services/deepprotacs/ ) and at github ( https://github.com/fenglei104/DeepPROTACs ).

The current status and future prospects for therapeutic targeting of KEAP1-NRF2 and β-TrCP-NRF2 interactions in cancer chemoresistance

Drug resistance is one of the biggest challenges in cancer treatment and limits the potential to cure patients. In many tumors, sustained activation of the protein NRF2 makes tumor cells resistant to chemo- and radiotherapy. Thus, blocking inappropriate NRF2 activity in cancers has been shown to reduce resistance in models of the disease. There is a growing scientific interest in NRF2 inhibitors. However, the compounds developed so far are not target-specific and are associated with a high degree of toxicity, hampering clinical applications. Compounds that can enhance the binding of NRF2 to its ubiquitination-facilitating regulator proteins, either KEAP1 or β-TrCP, have the potential to increase NRF2 degradation and may be of value as potential chemosensitising agents in cancer treatment. Approaches based on molecular glue-type mechanisms, in which ligands stabilise a ternary complex between a protein and its binding partner have shown to enhance β-catenin degradation by stabilising its interaction with β-TrCP. This strategy could be applied to rationally discover degradative β-TrCP-NRF2 and KEAP1-NRF2 protein-protein interaction enhancers. We are proposing a novel approach to selectively suppress NRF2 activity in tumors. It is based on recent methodology and has the potential to be a promising new addition to the arsenal of anticancer agents.

Predicting the structural basis of targeted protein degradation by integrating molecular dynamics simulations with structural mass spectrometry

Targeted protein degradation (TPD) is a promising approach in drug discovery for degrading proteins implicated in diseases. A key step in this process is the formation of a ternary complex where a heterobifunctional molecule induces proximity of an E3 ligase to a protein of interest (POI), thus facilitating ubiquitin transfer to the POI. In this work, we characterize 3 steps in the TPD process. (1) We simulate the ternary complex formation of SMARCA2 bromodomain and VHL E3 ligase by combining hydrogen-deuterium exchange mass spectrometry with weighted ensemble molecular dynamics (MD). (2) We characterize the conformational heterogeneity of the ternary complex using Hamiltonian replica exchange simulations and small-angle X-ray scattering. (3) We assess the ubiquitination of the POI in the context of the full Cullin-RING Ligase, confirming experimental ubiquitinomics results. Differences in degradation efficiency can be explained by the proximity of lysine residues on the POI relative to ubiquitin.

Discovery of small molecule ligands for the von Hippel-Lindau (VHL) E3 ligase and their use as inhibitors and PROTAC degraders

The von Hippel-Lindau (VHL) Cullin RING E3 ligase is an essential enzyme in the ubiquitin-proteasome system that recruits substrates such as the hypoxia inducible factor for ubiquitination and subsequent proteasomal degradation. The ubiquitin-proteasome pathway can be hijacked toward non-native neo-substrate proteins using proteolysis targeting chimeras (PROTACs), bifunctional molecules designed to simultaneously bind to an E3 ligase and a target protein to induce target ubiquitination and degradation. The availability of high-quality small-molecule ligands with good binding affinity for E3 ligases is fundamental for PROTAC development. Lack of good E3 ligase ligands as starting points to develop PROTAC degraders was initially a stumbling block to the development of the field. Herein, the journey towards the design of small-molecule ligands binding to VHL is presented. We cover the structure-based design of VHL ligands, their application as inhibitors in their own right, and their implementation into rationally designed, potent PROTAC degraders of various target proteins. We highlight the key findings and learnings that have provided strong foundations for the remarkable development of targeted protein degradation, and that offer a blueprint for designing new ligands for E3 ligases beyond VHL.

Adaptive exchange sustains cullin-RING ubiquitin ligase networks and proper licensing of DNA replication

Cop9 signalosome (CSN) regulates the function of cullin-RING E3 ubiquitin ligases (CRLs) by deconjugating the ubiquitin-like protein NEDD8 from the cullin subunit. To understand the physiological impact of CSN function on the CRL network and cell proliferation, we combined quantitative mass spectrometry and genome-wide CRISPR interference (CRISPRi) and CRISPR activation (CRISPRa) screens to identify factors that modulate cell viability upon inhibition of CSN by the small molecule CSN5i-3. CRL components and regulators strongly modulated the antiproliferative effects of CSN5i-3, and in addition we found two pathways involved in genome integrity, SCF^FBXO5-APC/C-GMNN and CUL4^DTL-SETD8, that contribute substantially to the toxicity of CSN inhibition. Our data highlight the importance of CSN-mediated NEDD8 deconjugation and adaptive exchange of CRL substrate receptors in sustaining CRL function and suggest approaches for leveraging CSN inhibition for the treatment of cancer.

Structure of CRL7FBXW8 reveals coupling with CUL1-RBX1/ROC1 for multi-cullin-RING E3-catalyzed ubiquitin ligation

Most cullin-RING ubiquitin ligases (CRLs) form homologous assemblies between a neddylated cullin-RING catalytic module and a variable substrate-binding receptor (for example, an F-box protein). However, the vertebrate-specific CRL7^FBXW8 is of interest because it eludes existing models, yet its constituent cullin CUL7 and F-box protein FBXW8 are essential for development, and CUL7 mutations cause 3M syndrome. In this study, cryo-EM and biochemical analyses reveal the CRL7^FBXW8 assembly. CUL7’s exclusivity for FBXW8 among all F-box proteins is explained by its unique F-box-independent binding mode. In CRL7^FBXW8, the RBX1 (also known as ROC1) RING domain is constrained in an orientation incompatible with binding E2~NEDD8 or E2~ubiquitin intermediates. Accordingly, purified recombinant CRL7^FBXW8 lacks auto-neddylation and ubiquitination activities. Instead, our data indicate that CRL7 serves as a substrate receptor linked via SKP1–FBXW8 to a neddylated CUL1–RBX1 catalytic module mediating ubiquitination. The structure reveals a distinctive CRL–CRL partnership, and provides a framework for understanding CUL7 assemblies safeguarding human health.

The cryo-EM structure of CRL7^FBXW8 shows that CUL7–RBX1 binds FBXW8–SKP1 in an F-box-independent manner. Bridged by FBXW8–SKP1, CRL7^FBXW8 forms a multi-cullin E3 ligase complex with neddylated CUL1–RBX1, which ubiquitinates a substrate recruited to CUL7.

FBXO38 Ubiquitin Ligase Controls Centromere Integrity via ZXDA/B Stability

Alterations in the gene encoding the E3 ubiquitin ligase substrate receptor FBXO38 have been associated with several diseases, including early-onset motor neuronopathy. However, the cellular processes affected by the enzymatic action of FBXO38 are not yet known. Here, we identify the zinc finger proteins ZXDA/B as its interaction partners. FBXO38 controls the stability of ZXDA/B proteins via ubiquitination and proteasome-dependent degradation. We show that ZXDA/B proteins associate with the centromeric protein CENP-B and that the interaction between ZXDA/B and FBXO38 or CENP-B is mutually exclusive. Functionally, ZXDA/B factors control the protein level of chromatin-associated CENP-B. Furthermore, their inappropriate stabilization leads to upregulation of CENP-A and CENP-B positive centromeric chromatin. Thus we demonstrate a previously unknown role of cullin-dependent protein degradation in the control of centromeric chromatin integrity.

Chasing molecular glue degraders: screening approaches

Protein-protein interactions (PPIs) govern all biological processes. Some small molecules modulate PPIs through induced protein proximity. In particular, molecular glue degraders are monovalent compounds that orchestrate interactions between a target protein and an E3 ubiquitin ligase, prompting the proteasomal degradation of the former. This and other pharmacological strategies of targeted protein degradation (e.g. proteolysis-targeting chimeras - PROTACs) overcome some limitations of traditional occupancy-based therapeutics. Here, we provide an overview of the "molecular glue" concept, with a special focus on natural and synthetic inducers of proximity to E3s. We then briefly highlight the serendipitous discoveries of some clinical and preclinical molecular glue degraders, and discuss the first examples of intentional discoveries. Specifically, we outline the different screening strategies reported in this rapidly evolving arena and our thoughts on future perspectives. By mastering the ability to influence PPIs, molecular glue degraders can induce the degradation of unligandable proteins, thus providing an exciting path forward to broaden the targetable proteome.

SDHA/B reduction promotes hepatocellular carcinoma by facilitating the deNEDDylation of cullin1 and stabilizing YAP/TAZ

BACKGROUND AND AIMS

Succinate dehydrogenase enzyme (SDH) is frequently diminished in samples from patients with hepatocellular carcinoma (HCC), and SDH reduction is associated with elevated succinate level and poor prognosis in patients with HCC. However, the underlying mechanisms of how impaired SDH activity promotes HCC remain unclear.

APPROACH AND RESULTS

In this study, we observed remarkable downregulations of SDH subunits A and B (SDHA/B) in chronic liver injury-induced murine HCC models and patient samples. Subsequent RNA sequencing, hematoxylin and eosin staining, and immunohistochemistry analyses of HCC samples revealed that Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) were significantly upregulated in HCC, with their levels inversely correlating with that of SDHA/B. YAP/TAZ stability was greatly enhanced in SDHA/B-depleted HCC cells along with accumulation of succinate. Further mechanistic analyses demonstrated that impaired activity of SDHA/B resulted in succinate accumulation, which facilitated the deNEDDylation of cullin1 and therefore disrupted the E3 ubiquitin ligase SCF^β-TrCP complex, consequently leading to YAP/TAZ stabilization and activation in HCC cells. The accelerated in vitro cell proliferation and in vivo tumor growth caused by SDHA/B reduction or succinate exposure were largely dependent on the aberrant activation of YAP/TAZ.

CONCLUSIONS

Our study demonstrated that SDHA/B reduction promotes HCC proliferation by preventing the proteasomal degradation of YAP/TAZ through modulating cullin1 NEDDylation, thus binding SDH-deficient HCC cells to YAP/TAZ pathway and rendering these cells vulnerable to YAP/TAZ inhibition. Our findings warrant further investigation on the therapeutic effects of targeting YAP/TAZ in patients with HCC displaying reduced SDHA/B or elevated succinate levels.

A conformational switch in the SCF-D3/MAX2 ubiquitin ligase facilitates strigolactone signalling

Strigolactones (SLs) are a class of plant hormones that regulate numerous processes of growth and development. SL perception and signal activation involves interaction between F-box E3 ubiquitin ligase D3/MAX2 and DWARF14 (D14) α/β-hydrolase in a SL-dependent manner and targeting of D53/SMXL6/7/8 transcriptional repressors (SMXLs) for proteasome-mediated degradation. D3/MAX2 has been shown to exist in multiple conformational states in which the C-terminal helix (CTH) undergoes a closed-to-open dynamics and regulates D14 binding and SL perception. Despite the multiple modes of D3-D14 interactions found in vitro, the residues that regulate the conformational switch of D3/MAX2 CTH in targeting D53/SMXLs and the subsequent effect on SL signalling remain unclear. Here we elucidate the functional dynamics of ASK1-D3/MAX2 in SL signalling by leveraging conformational switch mutants in vitro and in plants. We report the crystal structure of a dislodged CTH of the ASK1-D3 mutant and demonstrate that disruptions in CTH plasticity via either CRISPR-Cas9 genome editing or expression of point mutation mutants result in impairment of SL signalling. We show that the conformational switch in ASK1-D3/MAX2 CTH directly regulates ubiquitin-mediated protein degradation. A dislodged conformation involved in D53/SMXLs SL-dependent recruitment and ubiquitination and an engaged conformation are required for the release of polyubiquitinated D53/SMXLs and subsequently D14 for proteasomal degradation. Finally, we uncovered an organic acid metabolite that can directly trigger the D3/MAX2 CTH conformational switch. Our findings unravel a new regulatory function of a SKP1-CUL1-F-box ubiquitin ligase in plant signalling.

Interferon‑stimulated gene 15 promotes progression of endometrial carcinoma and weakens antitumor immune response

Endometrial carcinoma (EC) is one of the most common gynecological cancers with a poor prognosis. Therefore, clarifying the details of the molecular mechanisms is of great importance for EC diagnosis and clinical management. Interferon-stimulated gene 15 (ISG15) plays an important role in the development of various cancers. However, its role in EC remains unclear. High ISG15 expression was observed in EC, which was associated with poor clinical outcomes and pathological stage of patients with EC, thus representing a promising marker for EC progression. Further exploratory analysis revealed that the elevated ISG15 levels in EC were driven by aberrant DNA methylation, independent of copy number variation and specific transcription factor aberrations. Accordingly, knockdown of ISG15 by small interfering RNA attenuated the malignant cellular phenotype of EC cell lines, including proliferation and colony formation in vitro. Finally, investigation of the molecular mechanisms indicated that ISG15 promoted the cell cycle G1/S transition in EC. Furthermore, ISG15 promoted EC progression by activating the MYC proto-oncogene protein signaling pathway. Moreover, ECs with high levels of ISG15 harbored a more vital immune escape ability, evidenced not only by significantly less invasive CD8⁺ T cells, but also higher expression of T cell inhibitory factors, such as programmed death-ligand 1. These results suggest a tumor-promoting role of ISG15 in EC, which may be a promising marker for diagnosis, prognosis and therapeutic immunity.

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.

A review on cullin neddylation and strategies to identify its inhibitors for cancer therapy

The cullin-RING E3 ligases (CRLs) are the biggest components of the E3 ubiquitin ligase protein family, and they represent an essential role in various diseases that occur because of abnormal activation, particularly in tumors development. Regulation of CRLs needs neddylation, a post-translational modification involving an enzymatic cascade that transfers small, ubiquitin-like NEDD8 protein to CRLs. Many previous studies have confirmed neddylation as an enticing target for anticancer drug discoveries, and few recent studies have also found a significant increase in advancement in protein neddylation, including preclinical and clinical target validation to discover the neddylation inhibitor compound. In the present review, we first presented briefly the essence of CRLs' neddylation and its control, systematic analysis of CRLs, followed by the description of a few recorded chemical inhibitors of CRLs neddylation enzymes with recent examples of preclinical and clinical targets. We have also listed various structure-based pointing of protein-protein dealings in the CRLs' neddylation reaction, and last, the methods available to discover new inhibitors of neddylation are elaborated. This review will offer a concentrated, up-to-date, and detailed description of the discovery of neddylation inhibitors.

Modeling the CRL4A ligase complex to predict target protein ubiquitination induced by cereblon-recruiting PROTACs

PROteolysis TArgeting Chimeras (PROTACs) are hetero-bifunctional small molecules that can simultaneously recruit target proteins and E3 ligases to form a ternary complex, promoting target protein ubiquitination and degradation via the Ubiquitin-Proteasome System (UPS). PROTACs have gained increasing attention in recent years due to certain advantages over traditional therapeutic modalities and enabling targeting of previously "undruggable" proteins. To better understand the mechanism of PROTAC-induced Target Protein Degradation (TPD), several computational approaches have recently been developed to study and predict ternary complex formation. However, mounting evidence suggests that ubiquitination can also be a rate-limiting step in PROTAC-induced TPD. Here, we propose a structure-based computational approach to predict target protein ubiquitination induced by cereblon (CRBN)-based PROTACs by leveraging available structural information of the CRL4A ligase complex (CRBN/DDB1/CUL4A/Rbx1/NEDD8/E2/Ub). We generated ternary complex ensembles with Rosetta, modeled multiple CRL4A ligase complex conformations, and predicted ubiquitination efficiency by separating the ternary ensemble into productive and unproductive complexes based on the proximity of the ubiquitin to accessible lysines on the target protein. We validated our CRL4A ligase complex models with published ternary complex structures and additionally employed our modeling workflow to predict ubiquitination efficiencies and sites of a series of cyclin-dependent kinases (CDKs) after treatment with TL12-186, a pan-kinase PROTAC. Our predictions are consistent with CDK ubiquitination and site-directed mutagenesis of specific CDK lysine residues as measured using a NanoBRET ubiquitination assay in HEK293 cells. This work structurally links PROTAC-induced ternary formation and ubiquitination, representing an important step toward prediction of target "degradability."

Multifunctional polyphenol-based silk hydrogel alleviates oxidative stress and enhances endogenous regeneration of osteochondral defects

In osteochondral defects, oxidative stress caused by elevated levels of reactive oxygen species (ROS) can disrupt the normal endogenous repair process. In this study, a multifunctional hydrogel composed of silk fibroin (SF) and tannic acid (TA), the FDA-approved ingredients, was developed to alleviate oxidative stress and enhance osteochondral regeneration. In this proposed hydrogel, SF first interacts with TA to form a hydrogen-bonded supramolecular structure, which is subsequently enzymatically crosslinked to form a stable hydrogel. Furthermore, TA had multiple phenolic hydroxyl groups that formed interactions with the therapeutic molecule E7 peptide for controlled drug delivery. In vitro investigations showed that SF-TA and SF-TA-E7 hydrogels exhibited a multitude of biological effects including scavenging of ROS, maintaining cell viability, and promoting the proliferation of bone marrow mesenchymal stem cells (BMSCs) against oxidative stress. The proteomic analysis indicated that SF-TA and SF-TA-E7 hydrogels suppressed oxidative stress, which in turn improved cell proliferation in multiple proliferation and apoptosis-related pathways. In rabbit osteochondral defect model, SF-TA and SF-TA-E7 hydrogels promoted enhanced regeneration of both cartilage and subchondral bone as compared to hydrogel without TA incorporation. These findings indicated that the multifunctional SF-TA hydrogel provided a microenvironment suitable for the endogenous regeneration of osteochondral defects.

Clinical significance of FBXW7 loss of function in human cancers

FBXW7 (F-Box and WD Repeat Domain Containing 7) (also referred to as FBW7 or hCDC4) is a component of the Skp1-Cdc53 / Cullin-F-box-protein complex (SCF/β-TrCP). As a member of the F-box protein family, FBXW7 serves a role in phosphorylation-dependent ubiquitination and proteasome degradation of oncoproteins that play critical role(s) in oncogenesis. FBXW7 affects many regulatory functions involved in cell survival, cell proliferation, tumor invasion, DNA damage repair, genomic instability and telomere biology. This thorough review of current literature details how FBXW7 expression and functions are regulated through multiple mechanisms and how that ultimately drives tumorigenesis in a wide array of cell types. The clinical significance of FBXW7 is highlighted by the fact that FBXW7 is frequently inactivated in human lung, colon, and hematopoietic cancers. The loss of FBXW7 can serve as an independent prognostic marker and is significantly correlated with the resistance of tumor cells to chemotherapeutic agents and poorer disease outcomes. Recent evidence shows that genetic mutation of FBXW7 differentially affects the degradation of specific cellular targets resulting in a distinct and specific pattern of activation/inactivation of cell signaling pathways. The clinical significance of FBXW7 mutations in the context of tumor development, progression, and resistance to therapies as well as opportunities for targeted therapies is discussed.

Neddylation is essential for β-catenin degradation in Wnt signaling pathway

β-Catenin is a central component in the Wnt signaling pathway; its degradation has been tightly connected to ubiquitylation, but it is rarely examined by loss-of-function assays. Here we observe that endogenous β-catenin is not stabilized upon ubiquitylation depletion by a ubiquitylation inhibitor, TAK-243. We demonstrate that N-terminal phosphorylated β-catenin is quickly and strongly stabilized by a specific neddylation inhibitor, MLN4924, in all examined cell types, and that β-catenin and TCF4 interaction is strongly enhanced by inhibition of neddylation but not ubiquitylation. We also confirm that the E3 ligase β-TrCP2, but not β-TrCP1, is associated with neddylation and destruction of β-catenin. GSK3β and adenomatous polyposis coli (APC) are not required for β-catenin neddylation but essential for its subsequent degradation. Our findings not only clarify the process of β-catenin modification and degradation in the Wnt signaling pathway but also highlight the importance of reassessing previously identified ubiquitylation substrates.