Human Cdc34 employs distinct sites to coordinate attachment of ubiquitin to a substrate and assembly of polyubiquitin chains

Diversity of structure and function in Cullin E3 ligases

The cellular process by which the protein ubiquitin (Ub) is covalently attached to a protein substrate involves Ub activating (E1s) and conjugating enzymes (E2s) that work together with a large variety of E3 ligases that impart substrate specificity. The largest family of E3s is the Cullin-RING ligase (CRL) family which utilizes a wide variety of substrate receptors, adapter proteins, and cooperating ligases. Cryo-electron microscopy (cryoEM) has revealed a wide variety of structures which suggest how Ub transfer occurs. Hydrogen deuterium exchange mass spectrometry (HDXMS) has revealed the role of dynamics and expanded our knowledge of how covalent NEDD8 modification (neddylation) activates the CRLs, particularly by facilitating cooperation with additional RING-between-RING ligases to transfer Ub.

Cullin-RING ligases employ geometrically optimized catalytic partners for substrate targeting

Cullin-RING ligases (CRLs) ubiquitylate specific substrates selected from other cellular proteins. Substrate discrimination and ubiquitin transferase activity were thought to be strictly separated. Substrates are recognized by substrate receptors, such as Fbox or BCbox proteins. Meanwhile, CRLs employ assorted ubiquitin-carrying enzymes (UCEs, which are a collection of E2 and ARIH-family E3s) specialized for either initial substrate ubiquitylation (priming) or forging poly-ubiquitin chains. We discovered specific human CRL-UCE pairings governing substrate priming. The results reveal pairing of CUL2-based CRLs and UBE2R-family UCEs in cells, essential for efficient PROTAC-induced neo-substrate degradation. Despite UBE2R2's intrinsic programming to catalyze poly-ubiquitylation, CUL2 employs this UCE for geometrically precise PROTAC-dependent ubiquitylation of a neo-substrate and for rapid priming of substrates recruited to diverse receptors. Cryo-EM structures illuminate how CUL2-based CRLs engage UBE2R2 to activate substrate ubiquitylation. Thus, pairing with a specific UCE overcomes E2 catalytic limitations to drive substrate ubiquitylation and targeted protein degradation.

Mechanism of millisecond Lys48-linked poly-ubiquitin chain formation by cullin-RING ligases

E3 ubiquitin ligases, in collaboration with E2 ubiquitin-conjugating enzymes, modify proteins with poly-ubiquitin chains. Cullin-RING ligase (CRL) E3s use Cdc34/UBE2R-family E2s to build Lys48-linked poly-ubiquitin chains to control an enormous swath of eukaryotic biology. Yet the molecular mechanisms underlying this exceptional linkage specificity and millisecond kinetics of poly-ubiquitylation remain unclear. Here we obtain cryogenic-electron microscopy (cryo-EM) structures that provide pertinent insight into how such poly-ubiquitin chains are forged. The CRL RING domain not only activates the E2-bound ubiquitin but also shapes the conformation of a distinctive UBE2R2 loop, positioning both the ubiquitin to be transferred and the substrate-linked acceptor ubiquitin within the active site. The structures also reveal how the ubiquitin-like protein NEDD8 uniquely activates CRLs during chain formation. NEDD8 releases the RING domain from the CRL, but unlike previous CRL-E2 structures, does not contact UBE2R2. These findings suggest how poly-ubiquitylation may be accomplished by many E2s and E3s.

Functional conservation and divergence of the helix-turn-helix motif of E2 ubiquitin-conjugating enzymes

Polyubiquitination by E2 and E3 enzymes is crucial to cell cycle control, epigenetic regulation, and development. The hallmark of the E2 family is the ubiquitin (Ub)-conjugating (UBC) domain that forms a dynamic thioester conjugate with ubiquitin (E2~Ub). Numerous studies have focused on E2 surfaces, such as the N-terminal and crossover helices, that directly interact with an E3 or the conjugated ubiquitin to stabilize the active, "closed" state of the E2~Ub. However, it remains unclear how other E2 surfaces regulate ubiquitin transfer. Here, we demonstrate the helix-turn-helix (HTH) motif of the UBC tunes the intrinsic polyubiquitination activity through distinct functions in different E2s. Interestingly, the E2HTH motif is repurposed in UBE2S and UBE2R2 to interact with the conjugated or acceptor ubiquitin, respectively, modulating ubiquitin transfer. Furthermore, we propose that Anaphase-Promoting Complex/Cyclosome binding to the UBE2SHTH reduces the conformational space of the flexible E2~Ub, demonstrating an atypical E3-dependent activation mechanism. Altogether, we postulate the E2HTH motif evolved to provide new functionalities that can be harnessed by E3s and permits additional regulation to facilitate specific E2-E3-mediated polyubiquitination.

Cullin-RING Ubiquitin Ligase Regulatory Circuits: A Quarter Century Beyond the F-Box Hypothesis

Cullin-RING ubiquitin ligases (CRLs) are dynamic modular platforms that regulate myriad biological processes through target-specific ubiquitylation. Our knowledge of this system emerged from the F-box hypothesis, posited a quarter century ago: Numerous interchangeable F-box proteins confer specific substrate recognition for a core CUL1-based RING E3 ubiquitin ligase. This paradigm has been expanded through the evolution of a superfamily of analogous modular CRLs, with five major families and over 200 different substrate-binding receptors in humans. Regulation is achieved by numerous factors organized in circuits that dynamically control CRL activation and substrate ubiquitylation. CRLs also serve as a vast landscape for developing small molecules that reshape interactions and promote targeted ubiquitylation-dependent turnover of proteins of interest. Here, we review molecular principles underlying CRL function, the role of allosteric and conformational mechanisms in controlling substrate timing and ubiquitylation, and how the dynamics of substrate receptor interchange drives the turnover of selected target proteins to promote cellular decision-making.

Systematic identification of CDC34 that functions to stabilize EGFR and promote lung carcinogenesis

Background

How the oncoprotein epidermal growth factor receptor (EGFR) evades proteolytic degradation and accumulates in non-small cell lung cancer (NSCLC) remains unclear, and ubiquitin pathway genes (UPGs) that are critical to NSCLC needs to be systematically identified.

Methods

A total of 696 UPGs (including E1, E2, E3, and deubiquitinases) were silenced by small interfering RNA (siRNA) library in NSCLC cells, the candidates were verified, and their significance was evaluated in patients with NSCLC. The effects of a candidate gene on EGFR were investigated in vitro and in vivo.

Findings

We report 31 candidates that are required for cell proliferation, with the E2 ubiquitin conjugase CDC34 as the most significant one. CDC34 is elevated in tumor tissues in 76 of 114 (66.7%) NSCLCs and inversely associated with prognosis, is higher in smoker patients than nonsmoker patients, and is induced by tobacco carcinogens in normal human lung epithelial cells. Forced expression of CDC34 promotes, whereas knockdown of CDC34 inhibits, NSCLC cell proliferation in vitro and in vivo. CDC34 competes with c-Cbl to bind Y1045 to inhibit polyubiquitination and degradation of EGFR. In EGFR-L858R and EGFR-T790M/Del (exon 19)-driven lung tumor growth in mouse models, knockdown of CDC34 significantly inhibits tumor formation.

Interpretation

These results demonstrate that an E2 enzyme is capable of competing with E3 ligase to stabilize substrates, and CDC34 represents an attractive therapeutic target for NSCLCs.

Funding

National Key Research and Development Program of China, National Natural Science Foundation of China, and the CAMS Innovation Fund for Medical Sciences.

Enzymatic Logic of Ubiquitin Chain Assembly

Protein ubiquitination impacts virtually every biochemical pathway in eukaryotic cells. The fate of a ubiquitinated protein is largely dictated by the type of ubiquitin modification with which it is decorated, including a large variety of polymeric chains. As a result, there have been intense efforts over the last two decades to dissect the molecular details underlying the synthesis of ubiquitin chains by ubiquitin-conjugating (E2) enzymes and ubiquitin ligases (E3s). In this review, we highlight these advances. We discuss the evidence in support of the alternative models of transferring one ubiquitin at a time to a growing substrate-linked chain (sequential addition model) versus transferring a pre-assembled ubiquitin chain (en bloc model) to a substrate. Against this backdrop, we outline emerging principles of chain assembly: multisite interactions, distinct mechanisms of chain initiation and elongation, optimal positioning of ubiquitin molecules that are ultimately conjugated to each other, and substrate-assisted catalysis. Understanding the enzymatic logic of ubiquitin chain assembly has important biomedical implications, as the misregulation of many E2s and E3s and associated perturbations in ubiquitin chain formation contribute to human disease. The resurgent interest in bifunctional small molecules targeting pathogenic proteins to specific E3s for polyubiquitination and subsequent degradation provides an additional incentive to define the mechanisms responsible for efficient and specific chain synthesis and harness them for therapeutic benefit.

The ubiquitin-conjugating enzyme CDC34 is essential for cytokinesis in contrast to putative subunits of a SCF complex in Trypanosoma brucei

The ubiquitin-proteasome system is a post-translational regulatory pathway for controlling protein stability and activity that underlies many fundamental cellular processes, including cell cycle progression. Target proteins are tagged with ubiquitin molecules through the action of an enzymatic cascade composed of E1 ubiquitin activating enzymes, E2 ubiquitin conjugating enzymes, and E3 ubiquitin ligases. One of the E3 ligases known to be responsible for the ubiquitination of cell cycle regulators in eukaryotes is the SKP1-CUL1-F-box complex (SCFC). In this work, we identified and studied the function of homologue proteins of the SCFC in the life cycle of Trypanosoma brucei, the causal agent of the African sleeping sickness. Depletion of trypanosomal SCFC components TbRBX1, TbSKP1, and TbCDC34 by RNAi resulted in decreased growth rate and contrasting cell cycle abnormalities for both procyclic (PCF) and bloodstream (BSF) forms. Depletion of TbRBX1 in PCF cells interfered with kinetoplast replication, whilst depletion of TbSKP1 arrested PCF and BSF cells in the G1/S transition. Silencing of TbCDC34 in BSF cells resulted in a block in cytokinesis and caused rapid clearance of parasites from infected mice. We also show that TbCDC34 is able to conjugate ubiquitin in vitro and in vivo, and that its activity is necessary for T. brucei infection progression in mice. This study reveals that different components of a putative SCFC have contrasting phenotypes once depleted from the cells, and that TbCDC34 is essential for trypanosome replication, making it a potential target for therapeutic intervention.

African sleeping sickness is a neglected tropical disease caused by infection with the protozoan parasite Trypanosoma brucei, which is transmitted to humans by tsetse flies (Glossina genus). Treatment of the disease is complex and relies on limited pharmaceutical options. Understanding how T. brucei regulates cell cycle progression at a molecular level when alternating between the mammalian host and the insect vector could lead to better therapies. In this study, we examined different T. brucei proteins with homology to components of the SKP1-CUL1-F-box ubiquitin ligase complex (SCFC), previously characterized in other eukaryotes as a regulator of cell cycle progression. We found that depletion of the homologues of a putative SCFC cause T. brucei to develop abnormally, generating different phenotypes of the mammalian and insect stages. Interestingly, depletion of the ubiquitin conjugating enzyme TbCDC34 arrest cells in a pre-cytokinesis stage, indicating that this protein is essential for cytokinesis. In addition to improving our fundamental understanding of the molecular regulation underlying the sophisticated life cycle of T. brucei, this work pinpoints a potential target for drug development against trypanosomiasis.

S. pombe Uba1-Ubc15 Structure Reveals a Novel Regulatory Mechanism of Ubiquitin E2 Activity

Ubiquitin (Ub) E1 initiates the Ub conjugation cascade by activating and transferring Ub to tens of different E2s. How Ub E1 cooperates with E2s that differ substantially in their predicted E1-interacting residues is unknown. Here, we report the structure of S. pombe Uba1 in complex with Ubc15, a Ub E2 with intrinsically low E1-E2 Ub thioester transfer activity. The structure reveals a distinct Ubc15 binding mode that substantially alters the network of interactions at the E1-E2 interface compared to the only other available Ub E1-E2 structure. Structure-function analysis reveals that the intrinsically low activity of Ubc15 largely results from the presence of an acidic residue at its N-terminal region. Notably, Ub E2 N termini are serine/threonine rich in many other Ub E2s, leading us to hypothesize that phosphorylation of these sites may serve as a novel negative regulatory mechanism of Ub E2 activity, which we demonstrate biochemically and in cell-based assays.

Mechanism of Lysine 48 Selectivity during Polyubiquitin Chain Formation by the Ube2R1/2 Ubiquitin-Conjugating Enzyme

Lysine selectivity is of critical importance during polyubiquitin chain formation because the identity of the lysine controls the biological outcome. Ubiquitins are covalently linked in polyubiquitin chains through one of seven lysine residues on its surface and the C terminus of adjacent protomers. Lys 48-linked polyubiquitin chains signal for protein degradation; however, the structural basis for Lys 48 selectivity remains largely unknown. The ubiquitin-conjugating enzyme Ube2R1/2 has exquisite specificity for Lys 48, and computational docking of Ube2R1/2 and ubiquitin predicts that Lys 48 is guided to the active site through a key electrostatic interaction between Arg 54 on ubiquitin and Asp 143 on Ube2R1/2. The validity of this interaction was confirmed through biochemical experiments. Since structural examples involving Arg 54 in protein-ubiquitin complexes are exceedingly rare, these results provide additional insight into how ubiquitin-protein complexes can be stabilized. We discuss how these findings relate to how other ubiquitin-conjugating enzymes direct the lysine specificity of polyubiquitin chains.

The molecular basis of lysine 48 ubiquitin chain synthesis by Ube2K

The post-translational modification of proteins by ubiquitin is central to the regulation of eukaryotic cells. Substrate-bound ubiquitin chains linked by lysine 11 and 48 target proteins to the proteasome for degradation and determine protein abundance in cells, while other ubiquitin chain linkages regulate protein interactions. The specificity of chain-linkage type is usually determined by ubiquitin-conjugating enzymes (E2s). The degradative E2, Ube2K, preferentially catalyses formation of Lys48-linked chains, but like most E2s, the molecular basis for chain formation is not well understood. Here we report the crystal structure of a Ube2K~ubiquitin conjugate and demonstrate that even though it is monomeric, Ube2K can synthesize Lys48-linked ubiquitin chains. Using site-directed mutagenesis and modelling, our studies reveal a molecular understanding of the catalytic complex and identify key features required for synthesis of degradative Lys48-linked chains. The position of the acceptor ubiquitin described here is likely conserved in other E2s that catalyse Lys48-linked ubiquitin chain synthesis.

Generation of a proteolytic signal: E3/E2-mediated polyubiquitination of IκBα

A key regulatory node in NF-κB signaling is the removal of the IκBα inhibitor, whose levels are tightly controlled by the ubiquitin-proteasome system. In response to signal activation and transmission, ubiquitin E1, E2, and E3 enzymes are employed to generate a lysine 48-linked ubiquitin chain that triggers degradation of IκBα by the proteasome. In this chapter we describe an in vitro biochemical approach to reconstitute the ubiquitination system. To do so, we detail methods for the preparation of the relevant enzymes and substrate, as well as for the execution of the reaction with high efficiency. This sensitive and highly reproducible readout can be applied to the study of proteins, small molecules, and other factors that modulate IκBα ubiquitination, thereby producing outcomes that impact NF-κB signaling to advance the course of improving human health.

Pivotal role for the ubiquitin Y59-E51 loop in lysine 48 polyubiquitination

Lysine 48 (K48)-polyubiquitination is the predominant mechanism for mediating selective protein degradation, but the underlying molecular basis of selecting ubiquitin (Ub) K48 for linkage-specific chain synthesis remains elusive. Here, we present biochemical, structural, and cell-based evidence demonstrating a pivotal role for the Ub Y59-E51 loop in supporting K48-polyubiquitination. This loop is established by a hydrogen bond between Ub Y59's hydroxyl group and the backbone amide of Ub E51, as substantiated by NMR spectroscopic analysis. Loop residues Y59 and R54 are specifically required for the receptor activity enabling K48 to attack the donor Ub-E2 thiol ester in reconstituted ubiquitination catalyzed by Skp1-Cullin1-F-box (SCF)(βTrCP) E3 ligase and Cdc34 E2-conjugating enzyme. When introduced into mammalian cells, loop-disruptive mutant Ub(R54A/Y59A) diminished the production of K48-polyubiquitin chains. Importantly, conditional replacement of human endogenous Ub by Ub(R54A/Y59A) or Ub(K48R) yielded profound apoptosis at a similar extent, underscoring the global impact of the Ub Y59-E51 loop in cellular K48-polyubiquitination. Finally, disulfide cross-linking revealed interactions between the donor Ub-bound Cdc34 acidic loop and the Ub K48 site, as well as residues within the Y59-E51 loop, suggesting a mechanism in which the Ub Y59-E51 loop helps recruit the E2 acidic loop that aligns the receptor Ub K48 to the donor Ub for catalysis.

A snapshot of ubiquitin chain elongation: lysine 48-tetra-ubiquitin slows down ubiquitination

We have explored the mechanisms of polyubiquitin chain assembly with reconstituted ubiquitination of IκBα and β-catenin by the Skp1-cullin 1-βTrCP F-box protein (SCF(βTrCP)) E3 ubiquitin (Ub) ligase complex. Competition experiments revealed that SCF(βTrCP) formed a complex with IκBα and that the Nedd8 modified E3-substrate platform engaged in dynamic interactions with the Cdc34 E2 Ub conjugating enzyme for chain elongation. Using "elongation intermediates" containing β-catenin linked with Ub chains of defined length, it was observed that a Lys-48-Ub chain of a length greater than four, but not its Lys-63 linkage counterparts, slowed the rate of additional Ub conjugation. Thus, the Ub chain length and linkage impact kinetic rates of chain elongation. Given that Lys-48-tetra-Ub is packed into compact conformations due to extensive intrachain interactions between Ub subunits, this topology may limit the accessibility of SCF(βTrCP)/Cdc34 to the distal Ub Lys-48 and result in slowed elongation.

Multimodal mechanism of action for the Cdc34 acidic loop: a case study for why ubiquitin-conjugating enzymes have loops and tails

Together with ubiquitin ligases (E3), ubiquitin-conjugating enzymes (E2) are charged with the essential task of synthesizing ubiquitin chains onto protein substrates. Some 75% of the known E2s in the human proteome contain unique insertions in their primary sequences, yet it is largely unclear what effect these insertions impart on the ubiquitination reaction. Cdc34 is an important E2 with prominent roles in cell cycle regulation and signal transduction. The amino acid sequence of Cdc34 contains an insertion distal to the active site that is absent in most other E2s, yet this acidic loop (named for its four invariably conserved acidic residues) is critical for Cdc34 function both in vitro and in vivo. Here we have investigated how the acidic loop in human Cdc34 promotes ubiquitination, identifying two key molecular events during which the acidic loop exerts its influence. First, the acidic loop promotes the interaction between Cdc34 and its ubiquitin ligase partner, SCF. Second, two glutamic acid residues located on the distal side of the loop collaborate with an invariably conserved histidine on the proximal side of the loop to suppress the pKa of an ionizing species on ubiquitin or Cdc34 which greatly contributes to Cdc34 catalysis. These results demonstrate that insertions can guide E2s to their physiologically relevant ubiquitin ligases as well as provide essential modalities that promote catalysis.

Loop 7 of E2 enzymes: an ancestral conserved functional motif involved in the E2-mediated steps of the ubiquitination cascade

The ubiquitin (Ub) system controls almost every aspect of eukaryotic cell biology. Protein ubiquitination depends on the sequential action of three classes of enzymes (E1, E2 and E3). E2 Ub-conjugating enzymes have a central role in the ubiquitination pathway, interacting with both E1 and E3, and influencing the ultimate fate of the substrates. Several E2s are characterized by an extended acidic insertion in loop 7 (L7), which if mutated is known to impair the proper E2-related functions. In the present contribution, we show that acidic loop is a conserved ancestral motif in E2s, relying on the presence of alternate hydrophobic and acidic residues. Moreover, the dynamic properties of a subset of family 3 E2s, as well as their binary and ternary complexes with Ub and the cognate E3, have been investigated. Here we provide a model of L7 role in the different steps of the ubiquitination cascade of family 3 E2s. The L7 hydrophobic residues turned out to be the main determinant for the stabilization of the E2 inactive conformations by a tight network of interactions in the catalytic cleft. Moreover, phosphorylation is known from previous studies to promote E2 competent conformations for Ub charging, inducing electrostatic repulsion and acting on the L7 acidic residues. Here we show that these active conformations are stabilized by a network of hydrophobic interactions between L7 and L4, the latter being a conserved interface for E3-recruitment in several E2s. In the successive steps, L7 conserved acidic residues also provide an interaction interface for both Ub and the Rbx1 RING subdomain of the cognate E3. Our data therefore suggest a crucial role for L7 of family 3 E2s in all the E2-mediated steps of the ubiquitination cascade. Its different functions are exploited thank to its conserved hydrophobic and acidic residues in a finely orchestrate mechanism.

Selective recruitment of an E2~ubiquitin complex by an E3 ubiquitin ligase

RING E3 ligases are proteins that must selectively recruit an E2-conjugating enzyme and facilitate ubiquitin transfer to a substrate. It is not clear how a RING E3 ligase differentiates a naked E2 enzyme from the E2∼ubiquitin-conjugated form or how this is altered upon ubiquitin transfer. RING-box protein 1 (Rbx1/ROC1) is a key protein found in the Skp1/Cullin-1/F-box (SCF) E3 ubiquitin ligase complex that functions with the E2 ubiquitin conjugating enzyme CDC34. The solution structure of Rbx1/ROC1 revealed a globular RING domain (residues 40–108) stabilized by three structural zinc ions (root mean square deviation 0.30 ± 0.04 Å) along with a disordered N terminus (residues 12–39). Titration data showed that Rbx1/ROC1 preferentially recruits CDC34 in its ubiquitin-conjugated form and favors this interaction by 50-fold compared with unconjugated CDC34. Furthermore, NMR and biochemical assays identified residues in helix α2 of Rbx1/ROC1 that are essential for binding and activating CDC34∼ubiquitin for ubiquitylation. Taken together, this work provides the first direct structural and biochemical evidence showing that polyubiquitylation by the RING E3 ligase Rbx1/ROC1 requires the preferential recruitment of an E2∼ubiquitin complex and subsequent release of the unconjugated E2 protein upon ubiquitin transfer to a substrate or ubiquitin chain.

How ubiquitination regulates the TGF-β signalling pathway: new insights and new players: new isoforms of ubiquitin-activating enzymes in the E1-E3 families join the game

Ubiquitination of protein species in regulating signal transduction pathways is universally accepted as of fundamental importance for normal development, and defects in this process have been implicated in the progression of many human diseases. One pathway that has received much attention in this context is transforming growth factor-beta (TGF-β) signalling, particularly during the regulation of epithelial-mesenchymal transition (EMT) and tumour progression. While E3-ubiquitin ligases offer themselves as potential therapeutic targets, much remains to be unveiled regarding mechanisms that culminate in their regulation. With this in mind, the focus of this review highlights the regulation of the ubiquitination pathway and the significance of a recently described group of NEDD4 E3-ubiquitin ligase isoforms in the context of TGF-β pathway regulation. Moreover, we now broaden these observations to incorporate a growing number of protein isoforms within the ubiquitin ligase superfamily as a whole, and discuss their relevance in defining a new 'iso-ubiquitinome'.

An allosteric inhibitor of the human Cdc34 ubiquitin-conjugating enzyme

In the ubiquitin-proteasome system (UPS), E2 enzymes mediate the conjugation of ubiquitin to substrates and thereby control protein stability and interactions. The E2 enzyme hCdc34 catalyzes the ubiquitination of hundreds of proteins in conjunction with the cullin-RING (CRL) superfamily of E3 enzymes. We identified a small molecule termed CC0651 that selectively inhibits hCdc34. Structure determination revealed that CC0651 inserts into a cryptic binding pocket on hCdc34 distant from the catalytic site, causing subtle but wholesale displacement of E2 secondary structural elements. CC0651 analogs inhibited proliferation of human cancer cell lines and caused accumulation of the SCF(Skp2) substrate p27(Kip1). CC0651 does not affect hCdc34 interactions with E1 or E3 enzymes or the formation of the ubiquitin thioester but instead interferes with the discharge of ubiquitin to acceptor lysine residues. E2 enzymes are thus susceptible to noncatalytic site inhibition and may represent a viable class of drug target in the UPS.

An acidic loop and cognate phosphorylation sites define a molecular switch that modulates ubiquitin charging activity in Cdc34-like enzymes

E2 ubiquitin-conjugating enzymes are crucial mediators of protein ubiquitination, which strongly influence the ultimate fate of the target substrates. Recently, it has been shown that the activity of several enzymes of the ubiquitination pathway is finely tuned by phosphorylation, an ubiquitous mechanism for cellular regulation, which modulates protein conformation. In this contribution, we provide the first rationale, at the molecular level, of the regulatory mechanism mediated by casein kinase 2 (CK2) phosphorylation of E2 Cdc34-like enzymes. In particular, we identify two co-evolving signature elements in one of the larger families of E2 enzymes: an acidic insertion in β4α2 loop in the proximity of the catalytic cysteine and two conserved key serine residues within the catalytic domain, which are phosphorylated by CK2. Our investigations, using yeast Cdc34 as a model, through 2.5 µs molecular dynamics simulations and biochemical assays, define these two elements as an important phosphorylation-controlled switch that modulates opening and closing of the catalytic cleft. The mechanism relies on electrostatic repulsions between a conserved serine phosphorylated by CK2 and the acidic residues of the β4α2 loop, promoting E2 ubiquitin charging activity. Our investigation identifies a new and unexpected pivotal role for the acidic loop, providing the first evidence that this loop is crucial not only for downstream events related to ubiquitin chain assembly, but is also mandatory for the modulation of an upstream crucial step of the ubiquitin pathway: the ubiquitin charging in the E2 catalytic cleft.

E2s: structurally economical and functionally replete

Ubiquitination is a post-translational modification pathway involved in myriad cellular regulation and disease pathways. The Ub (ubiquitin) transfer cascade requires three enzyme activities: a Ub-activating (E1) enzyme, a Ub-conjugating (E2) enzyme, and a Ub ligase (E3). Because the E2 is responsible both for E3 selection and substrate modification, E2s function at the heart of the Ub transfer pathway and are responsible for much of the diversity of Ub cellular signalling. There are currently over 90 three-dimensional structures for E2s, both alone and in complex with protein binding partners, providing a wealth of information regarding how E2s are recognized by a wide variety of proteins. In the present review, we describe the prototypical E2-E3 interface and discuss limitations of current methods to identify cognate E2-E3 partners. We present non-canonical E2-protein interactions and highlight the economy of E2s in their ability to facilitate many protein-protein interactions at nearly every surface on their relatively small and compact catalytic domain. Lastly, we compare the structures of conjugated E2~Ub species, their unique protein interactions and the mechanistic insights provided by species that are poised to transfer Ub.

Association of the disordered C-terminus of CDC34 with a catalytically bound ubiquitin

Cell division cycle protein 34 (CDC34) is a key E2 ubiquitin (Ub)-conjugating enzyme responsible for the polyubiquitination of proteins controlling the G1/S stages of cell division. The acidic C-terminus of the enzyme is required for this function, although there is little structural information providing details for a mechanism. One logical time point involving the C-terminus is the CDC34-Ub thiolester complex that precedes Ub transfer to a substrate. To examine this, we used a CDC34-Ub disulfide complex that structurally mimics the thiolester intermediate. NMR spectroscopy was used to show that the CDC34 C-terminus is disordered but can intramolecularly interact with the catalytically bound Ub. Using chemical shift perturbation analysis, we mapped two interacting regions on the surface of Ub in the CDC34-Ub complex. The first site comprises a hydrophobic patch (typical of other Ub complexes) that associates with the CDC34 catalytic domain. A novel second site, dependent on the C-terminus of CDC34, comprises a lysine-rich surface (K6, K11, K29, and K33) on the opposite face of Ub. Further, NMR experiments show that this interaction is described by two slowly exchanging states-a compact conformation where the C-terminus of CDC34 interacts with bound Ub and an extended structure where the C-terminus is released. This work provides the first structural details that show how the C-terminus of CDC34 might direct a thiolester-bound Ub to control polyubiquitin chain formation.

Disruption of Sag/Rbx2/Roc2 induces radiosensitization by increasing ROS levels and blocking NF-kappaB activation in mouse embryonic stem cells

SAG (sensitive to apoptosis gene; also known as RBX2 or ROC2) is a dual-function protein with antioxidant activity when acting alone or E3 ligase activity when complexed with other components of SCF (Skp1, cullins, F-box proteins) E3 ubiquitin ligases. SAG acts as a survival protein to inhibit apoptosis induced by a variety of stresses. Our recent work showed that SAG siRNA silencing sensitized cancer cells to radiation but the mechanism responsible remains elusive. Here we report that complete elimination of Sag expression via a gene-trapping strategy significantly sensitized mouse embryonic stem (ES) cells to radiation, with a sensitizing enhancement rate of 1.5-1.6. Radiosensitization was associated with increased steady-state levels of intracellular ROS (including superoxide) 24h after irradiation as well as enhancement of radiation-induced apoptosis. Furthermore, Sag elimination abrogated IkappaBalpha degradation leading to inhibition of NF-kappaB activation. Further detailed analysis revealed that IkappaBalpha is a direct substrate of SAG-SCF(beta-TrCP) E3 ubiquitin ligase. Taken together, these results support the hypothesis that Sag elimination via gene disruption sensitizes ES cells to radiation-induced cell killing by mechanisms that involve increased steady-state levels of ROS and decreased activation of NF-kappaB.

Solution structure and dynamics of human ubiquitin conjugating enzyme Ube2g2

Ube2g2 is an E2 enzyme which functions as part of the endoplasmic reticulum-associated degradation (ERAD) pathway responsible for identification and degradation of misfolded proteins in the endoplasmic reticulum. In tandem with a cognate E3 ligase, Ube2g2 assembles K48-linked polyubiquitin chains and then transfers them to substrate, leading ultimately to proteasomal degradation of the polyubiquitin-tagged substrate. We report here the solution structure and backbone dynamics of Ube2g2 solved by nuclear magnetic resonance spectroscopy. Although the solution structure agrees well with crystallographic structures for the E2 core, catalytically important loops (encompassing residues 95-107 and 130-135) flanking the active site cysteine are poorly defined. (15)N spin relaxation and residual dipolar coupling analysis directly demonstrates that these two loops are highly dynamic in solution. These results suggest that Ube2g2 requires one or more of its protein partners, such as cognate E3, acceptor ubiquitin substrate or thiolester-linked donor ubiquitin, to assume its catalytically relevant conformation. Within the NMR structural ensemble, interactions were observed between His94 and the highly mobile loop residues Asp98 and Asp99, supporting a possible role for His94 as a general base activated by the carboxylate side-chains of Asp98 or Asp99.

The human Cdc34 carboxyl terminus contains a non-covalent ubiquitin binding activity that contributes to SCF-dependent ubiquitination

Cdc34 is an E2 ubiquitin-conjugating enzyme that functions in conjunction with SCF (Skp1.Cullin 1.F-box) E3 ubiquitin ligase to catalyze covalent attachment of polyubiquitin chains to a target protein. Here we identified direct interactions between the human Cdc34 C terminus and ubiquitin using NMR chemical shift perturbation assays. The ubiquitin binding activity was mapped to two separate Cdc34 C-terminal motifs (UBS1 and UBS2) that comprise residues 206-215 and 216-225, respectively. UBS1 and UBS2 bind to ubiquitin in the proximity of ubiquitin Lys(48) and C-terminal tail, both of which are key sites for conjugation. When bound to ubiquitin in one orientation, the Cdc34 UBS1 aromatic residues (Phe(206), Tyr(207), Tyr(210), and Tyr(211)) are probably positioned in the vicinity of ubiquitin C-terminal residue Val(70). Replacement of UBS1 aromatic residues by glycine or of ubiquitin Val(70) by alanine decreased UBS1-ubiquitin affinity interactions. UBS1 appeared to support the function of Cdc34 in vivo because human Cdc34(1-215) but not Cdc34(1-200) was able to complement the growth defect by yeast Cdc34 mutant strain. Finally, reconstituted IkappaBalpha ubiquitination analysis revealed a role for each adjacent pair of UBS1 aromatic residues (Phe(206)/Tyr(207), Tyr(210)/Tyr(211)) in conjugation, with Tyr(210) exhibiting the most pronounced catalytic function. Intriguingly, Cdc34 Tyr(210) was required for the transfer of the donor ubiquitin to a receptor lysine on either IkappaBalpha or a ubiquitin in a manner that depended on the neddylated RING sub-complex of the SCF. Taken together, our results identified a new ubiquitin binding activity within the human Cdc34 C terminus that contributes to SCF-dependent ubiquitination.

Molecular basis for lysine specificity in the yeast ubiquitin-conjugating enzyme Cdc34

Ubiquitin (Ub)-conjugating enzymes (E2s) and ubiquitin ligases (E3s) catalyze the attachment of Ub to lysine residues in substrates and Ub during monoubiquitination and polyubiquitination. Lysine selection is important for the generation of diverse substrate-Ub structures, which provides versatility to this pathway in the targeting of proteins to different fates. The mechanisms of lysine selection remain poorly understood, with previous studies suggesting that the ubiquitination site(s) is selected by the E2/E3-mediated positioning of a lysine(s) toward the E2/E3 active site. By studying the polyubiquitination of Sic1 by the E2 protein Cdc34 and the RING E3 Skp1/Cul1/F-box (SCF) protein, we now demonstrate that in addition to E2/E3-mediated positioning, proximal amino acids surrounding the lysine residues in Sic1 and Ub are critical for ubiquitination. This mechanism is linked to key residues composing the catalytic core of Cdc34 and independent of SCF. Changes to these core residues altered the lysine preference of Cdc34 and specified whether this enzyme monoubiquitinated or polyubiquitinated Sic1. These new findings indicate that compatibility between amino acids surrounding acceptor lysine residues and key amino acids in the catalytic core of ubiquitin-conjugating enzymes is an important mechanism for lysine selection during ubiquitination.

Correlation between recombinase activating gene 1 ubiquitin ligase activity and V(D)J recombination

The really interesting new gene (RING) finger ubiquitin ligase domain of the recombinase activating gene 1 (RAG1) V(D)J recombinase protein adopts a standard cross-brace architecture but co-ordinates three zinc ions as opposed to the canonical two. We demonstrated previously that disruption of the conserved zinc co-ordination sites resulted in loss of structural integrity and ubiquitin ligase (E3) activity and interfered with the ability of full-length RAG1 to support recombination. Here we present evidence that amino acids surrounding the third, non-canonical site also contribute to functional interaction with the ubiquitin conjugating (E2) enzyme CDC34, while certain residues on the RING domain's surface important for interaction between other E2-E3 pairs are less critical to the functional RAG1-CDC34 interaction in this assay. Partial reduction of ubiquitin ligase activity was significantly correlated with reduction in the ability of RAG1 to support recombination of extra-chromosomal substrates (r = 0.805, P = 0.009). While poly-ubiquitin chains could be generated, RAG1 did not promote rapid chain extension following mono-ubiquitylation of substrate, regardless of the E2 enzyme used. No single ubiquitin lysine mutant disrupted the ability of CDC34 to form ubiquitin chains on RAG1, and mass spectrometric analysis of the poly-ubiquitylated products indicated ubiquitin chain linkages through lysines 48 and 11. These data suggest that RAG1 promotes a mono-ubiquitylation reaction that is required for optimal levels of V(D)J recombination.

Building ubiquitin chains: E2 enzymes at work

The modification of proteins with ubiquitin chains can change their localization, activity and/or stability. Although ubiquitylation requires the concerted action of ubiquitin-activating enzymes (E1s), ubiquitin-conjugating enzymes (E2s) and ubiquitin ligases (E3s), it is the E2s that have recently emerged as key mediators of chain assembly. These enzymes are able to govern the switch from ubiquitin chain initiation to elongation, regulate the processivity of chain formation and establish the topology of assembled chains, thereby determining the consequences of ubiquitylation for the modified proteins.

Origin and evolution of ubiquitin-conjugating enzymes from Guillardia theta nucleomorph to hominoid

The origin of eukaryotic ubiquitin-conjugating enzymes (E2s) can be traced back to the Guillardia theta nucleomorph about 2500 million years ago (Mya). E2s are largely vertically inherited over eukaryotic evolution [Lespinet, O., Wolf, Y.I., Koonin, E.V., Aravind, L., 2002. The role of lineage-specific gene family expansion in the evolution of eukaryotes. Genome Res. 1048-1059], while mammal E2s experienced evolution of multigene families by gene duplications which have been accompanied by the increase in the species complexity. Because of alternatively splicing, primate-specific expansions of E2s happened once again at a transcriptional level. Both of them resulted in increasing genomic complexity and diversity of primate E2 proteomic function. The evolutionary processes of human E2 gene structure during expansions were accompanied by exon duplication and exonization of intronic sequences. Exonizations of Transposable Elements (TEs) in UBE2D3, UBE2L3 and UBE2V1 genes from primates indicate that exaptation of TEs also plays important roles in the structural innovation of primate-specific E2s and may create alternative splicing isoforms at a transcriptional level. Estimates for the ratio of dN/dS suggest that a strong purifying selection had acted upon protein-coding sequences of their orthologous UBE2D2, UBE2A, UBE2N, UBE2I and Rbx1 genes from animals, plants and fungi. The similar rates of synonymous substitutions are in accordance with the neutral mutation-random drift hypothesis of molecular evolution. Systematic detection of the origin and evolution of E2s, analyzing the evolution of E2 multigene families by gene duplications and the evolutionary processes of E2s during expansions, and testing its evolutionary force using E2s from distant phylogenetic lineages may advance our distinguishing of ancestral E2s from created E2s, and reveal previously unknown relationships between E2s and metazoan complexity. Analysis of these conserved proteins provides strong support for a close relationship between social amoeba and eukaryote, choanoflagellate and metazoan, and for the central roles of social amoeba and choanoflagellate in the origin and evolution of eukaryote and metazoan. Retracing the different stages of primate E2 exonization by monitoring genomic events over 63 Myr of primate evolution will advance our understanding of how TEs dynamically modified primate transcriptome and proteome in the past, and continue to do so.

Autoinhibitory regulation of SCF-mediated ubiquitination by human cullin 1's C-terminal tail

SCF (Skp1 x CUL1 x F-box protein x ROC1) E3 ubiquitin ligase and Cdc34 E2-conjugating enzyme catalyze polyubiquitination in a precisely regulated fashion. Here, we describe biochemical evidence suggesting an autoinhibitory role played by the human CUL1 ECTD (extreme C-terminal domain; spanning the C-terminal 50 amino acids), a region that is predicted to contact the ROC1 RING finger protein by structural studies. We showed that ECTD did not contribute to CUL1's stable association with ROC1. Remarkably, deletion of ECTD, or missense mutations designed to disrupt the predicted ECTD x ROC1 interaction, markedly increased the ability of SCF(betaTrCP2) to promote IkappaB alpha polyubiquitination and polyubiquitin chain assembly by Cdc34 in vitro. Thus, disruption of ECTD yields in vitro effects that parallel SCF activation by Nedd8 conjugation to CUL1. We propose that SCF may be subject to autoinhibitory regulation, in which Nedd8 conjugation acts as a molecular switch to drive the E3 into an active state by diminishing the inhibitory ECTD x ROC1 interaction.