Ubiquitin-specific Protease 11 (USP11) Deubiquitinates Hybrid Small Ubiquitin-like Modifier (SUMO)-Ubiquitin Chains to Counteract RING Finger Protein 4 (RNF4)

Decoding the ubiquitin landscape by cutting-edge ubiquitinomic approaches

Functional consequences of protein ubiquitination have gone far beyond the degradation regulation as was initially imagined during its discovery 40 years back. The state-of-the-art has revealed the plethora of signaling pathways that are largely regulated by ubiquitination process in eukaryotes. To no surprise, ubiquitination is often dysregulated in many human diseases, including cancer, neurodegeneration and infection. Hence it has become a major focus with high-gain research value for many investigators to unravel new proteoforms, that are the targets of this ubiquitination modification. Despite many biochemical or proteomic approaches available for ubiquitination detection, mass-spectrometry stood out to be the most efficient and transformative technology to read this complex modification script. Here in this review, we have discussed how different ubiquitin codes can be decoded qualitatively and quantitatively following various sequential proteomic approaches to date reported and indicated the current limitations with scope for improvements.

DoUBLing up: ubiquitin and ubiquitin-like proteases in genome stability

Maintaining stability of the genome requires dedicated DNA repair and signalling processes that are essential for the faithful duplication and propagation of chromosomes. These DNA damage response (DDR) mechanisms counteract the potentially mutagenic impact of daily genotoxic stresses from both exogenous and endogenous sources. Inherent to these DNA repair pathways is the activity of protein factors that instigate repair processes in response to DNA lesions. The regulation, coordination, and orchestration of these DDR factors is carried out, in a large part, by post-translational modifications, such as phosphorylation, ubiquitylation, and modification with ubiquitin-like proteins (UBLs). The importance of ubiquitylation and UBLylation with SUMO in DNA repair is well established, with the modified targets and downstream signalling consequences relatively well characterised. However, the role of dedicated erasers for ubiquitin and UBLs, known as deubiquitylases (DUBs) and ubiquitin-like proteases (ULPs) respectively, in genome stability is less well established, particularly for emerging UBLs such as ISG15 and UFM1. In this review, we provide an overview of the known regulatory roles and mechanisms of DUBs and ULPs involved in genome stability pathways. Expanding our understanding of the molecular agents and mechanisms underlying the removal of ubiquitin and UBL modifications will be fundamental for progressing our knowledge of the DDR and likely provide new therapeutic avenues for relevant human diseases, such as cancer.

Friend or foe? Reciprocal regulation between E3 ubiquitin ligases and deubiquitinases

Protein ubiquitination is a post-translational modification that entails the covalent attachment of the small protein ubiquitin (Ub), which acts as a signal to direct protein stability, localization, or interactions. The Ub code is written by a family of enzymes called E3 Ub ligases (∼600 members in humans), which can catalyze the transfer of either a single ubiquitin or the formation of a diverse array of polyubiquitin chains. This code can be edited or erased by a different set of enzymes termed deubiquitinases (DUBs; ∼100 members in humans). While enzymes from these distinct families have seemingly opposing activities, certain E3-DUB pairings can also synergize to regulate vital cellular processes like gene expression, autophagy, innate immunity, and cell proliferation. In this review, we highlight recent studies describing Ub ligase-DUB interactions and focus on their relationships.

Paralogue-Specific Roles of SUMO1 and SUMO2/3 in Protein Quality Control and Associated Diseases

Small ubiquitin-related modifiers (SUMOs) function as post-translational protein modifications and regulate nearly every aspect of cellular function. While a single ubiquitin protein is expressed across eukaryotic organisms, multiple SUMO paralogues with distinct biomolecular properties have been identified in plants and vertebrates. Five SUMO paralogues have been characterized in humans, with SUMO1, SUMO2 and SUMO3 being the best studied. SUMO2 and SUMO3 share 97% protein sequence homology (and are thus referred to as SUMO2/3) but only 47% homology with SUMO1. To date, thousands of putative sumoylation substrates have been identified thanks to advanced proteomic techniques, but the identification of SUMO1- and SUMO2/3-specific modifications and their unique functions in physiology and pathology are not well understood. The SUMO2/3 paralogues play an important role in proteostasis, converging with ubiquitylation to mediate protein degradation. This function is achieved primarily through SUMO-targeted ubiquitin ligases (STUbLs), which preferentially bind and ubiquitylate poly-SUMO2/3 modified proteins. Effects of the SUMO1 paralogue on protein solubility and aggregation independent of STUbLs and proteasomal degradation have also been reported. Consistent with these functions, sumoylation is implicated in multiple human diseases associated with disturbed proteostasis, and a broad range of pathogenic proteins have been identified as SUMO1 and SUMO2/3 substrates. A better understanding of paralogue-specific functions of SUMO1 and SUMO2/3 in cellular protein quality control may therefore provide novel insights into disease pathogenesis and therapeutic innovation. This review summarizes current understandings of the roles of sumoylation in protein quality control and associated diseases, with a focus on the specific effects of SUMO1 and SUMO2/3 paralogues.

The SUMOylation and ubiquitination crosstalk in cancer

BACKGROUND

The cancer occurrence and progression are largely affected by the post-translational modifications (PTMs) of proteins. Currently, it has been shown that the relationship between ubiquitination and SUMOylation is highly complex and interactive. SUMOylation affects the process of ubiquitination and degradation of substrates. Contrarily, SUMOylation-related proteins are also regulated by the ubiquitination process thus altering their protein levels or activity. Emerging evidence suggests that the abnormal regulation between this crosstalk may lead to tumorigenesis.

PURPOSE

In this review, we have discussed the study of the relationship between ubiquitination and SUMOylation, as well as the possibility of a corresponding application in tumor therapy.

METHODS

The relevant literatures from PubMed have been reviewed for this article.

CONCLUSION

The interaction between ubiquitination and SUMOylation is crucial for the occurrence and development of cancer. A greater understanding of the crosstalk of SUMOylation and ubiquitination may be more conducive to the development of more selective and effective SUMOylation inhibitors, as well as a promotion of synergy with other tumor treatment strategies.

RNF4 and USP7 cooperate in ubiquitin-regulated steps of DNA replication

DNA replication requires precise regulation achieved through post-translational modifications, including ubiquitination and SUMOylation. These modifications are linked by the SUMO-targeted E3 ubiquitin ligases (STUbLs). Ring finger protein 4 (RNF4), one of only two mammalian STUbLs, participates in double-strand break repair and resolving DNA-protein cross-links. However, its role in DNA replication has been poorly understood. Using CRISPR/Cas9 genetic screens, we discovered an unexpected dependency of RNF4 mutants on ubiquitin specific peptidase 7 (USP7) for survival in TP53-null retinal pigment epithelial cells. TP53-/-/RNF4-/-/USP7-/- triple knockout (TKO) cells displayed defects in DNA replication that cause genomic instability. These defects were exacerbated by the proteasome inhibitor bortezomib, which limited the nuclear ubiquitin pool. A shortage of free ubiquitin suppressed the ataxia telangiectasia and Rad3-related (ATR)-mediated checkpoint response, leading to increased cell death. In conclusion, RNF4 and USP7 work cooperatively to sustain a functional level of nuclear ubiquitin to maintain the integrity of the genome.

SUMO enhances unfolding of SUMO-polyubiquitin-modified substrates by the Ufd1/Npl4/Cdc48 complex

The Ufd1/Npl4/Cdc48 complex is a universal protein segregase that plays key roles in eukaryotic cellular processes. Its functions orchestrating the clearance or removal of polyubiquitylated targets are established; however, prior studies suggest that the complex also targets substrates modified by the ubiquitin-like protein SUMO. Here, we show that interactions between Ufd1 and SUMO enhance unfolding of substrates modified by SUMO-polyubiquitin hybrid chains by the budding yeast Ufd1/Npl4/Cdc48 complex compared to substrates modified by polyubiquitin chains, a difference that is accentuated when the complex has a choice between these substrates. Incubating Ufd1/Npl4/Cdc48 with a substrate modified by a SUMO-polyubiquitin hybrid chain produced a series of single-particle cryo-EM structures that reveal features of interactions between Ufd1/Npl4/Cdc48 and ubiquitin prior to and during unfolding of ubiquitin. These results are consistent with cellular functions for SUMO and ubiquitin modifications and support a physical model wherein Ufd1/Npl4/Cdc48, SUMO, and ubiquitin conjugation pathways converge to promote clearance of proteins modified with SUMO and polyubiquitin.

Ubiquitin and SUMO as timers during DNA replication

Every time a cell copies its DNA the genetic material is exposed to the acquisition of mutations and genomic alterations that corrupt the information passed on to daughter cells. A tight temporal regulation of DNA replication is necessary to ensure the full copy of the DNA while preventing the appearance of genomic instability. Protein modification by ubiquitin and SUMO constitutes a very complex and versatile system that allows the coordinated control of protein stability, activity and interactome. In chromatin, their action is complemented by the AAA+ ATPase VCP/p97 that recognizes and removes ubiquitylated and SUMOylated factors from specific cellular compartments. The concerted action of the ubiquitin/SUMO system and VCP/p97 determines every step of DNA replication enforcing the ordered activation/inactivation, loading/unloading and stabilization/destabilization of replication factors. Here we analyze the mechanisms used by ubiquitin/SUMO and VCP/p97 to establish molecular timers throughout DNA replication and their relevance in maintaining genome stability. We propose that these PTMs are the main molecular watch of DNA replication from origin recognition to replisome disassembly.

X-linked ubiquitin-specific peptidase 11 increases tauopathy vulnerability in women

Although women experience significantly higher tau burden and increased risk for Alzheimer's disease (AD) than men, the underlying mechanism for this vulnerability has not been explained. Here, we demonstrate through in vitro and in vivo models, as well as human AD brain tissue, that X-linked ubiquitin specific peptidase 11 (USP11) augments pathological tau aggregation via tau deubiquitination initiated at lysine-281. Removal of ubiquitin provides access for enzymatic tau acetylation at lysines 281 and 274. USP11 escapes complete X-inactivation, and female mice and people both exhibit higher USP11 levels than males. Genetic elimination of usp11 in a tauopathy mouse model preferentially protects females from acetylated tau accumulation, tau pathology, and cognitive impairment. USP11 levels also strongly associate positively with tau pathology in females but not males. Thus, inhibiting USP11-mediated tau deubiquitination may provide an effective therapeutic opportunity to protect women from increased vulnerability to AD and other tauopathies.

Signalling mechanisms and cellular functions of SUMO

Sumoylation is an essential post-translational modification that is catalysed by a small number of modifying enzymes but regulates thousands of target proteins in a dynamic manner. Small ubiquitin-like modifiers (SUMOs) can be attached to target proteins as one or more monomers or in the form of polymers of different types. Non-covalent readers recognize SUMO-modified proteins via SUMO interaction motifs. SUMO simultaneously modifies groups of functionally related proteins to regulate predominantly nuclear processes, including gene expression, the DNA damage response, RNA processing, cell cycle progression and proteostasis. Recent progress has increased our understanding of the cellular and pathophysiological roles of SUMO modifications, extending their functions to the regulation of immunity, pluripotency and nuclear body assembly in response to oxidative stress, which partly occurs through the recently characterized mechanism of liquid-liquid phase separation. Such progress in understanding the roles and regulation of sumoylation opens new avenues for the targeting of SUMO to treat disease, and indeed the first drug blocking sumoylation is currently under investigation in clinical trials as a possible anticancer agent.

Insights in Post-Translational Modifications: Ubiquitin and SUMO

Both ubiquitination and SUMOylation are dynamic post-translational modifications that regulate thousands of target proteins to control virtually every cellular process. Unfortunately, the detailed mechanisms of how all these cellular processes are regulated by both modifications remain unclear. Target proteins can be modified by one or several moieties, giving rise to polymers of different morphology. The conjugation cascades of both modifications comprise a few activating and conjugating enzymes but close to thousands of ligating enzymes (E3s) in the case of ubiquitination. As a result, these E3s give substrate specificity and can form polymers on a target protein. Polymers can be quickly modified forming branches or cleaving chains leading the target protein to its cellular fate. The recent development of mass spectrometry(MS) -based approaches has increased the understanding of ubiquitination and SUMOylation by finding essential modified targets in particular signaling pathways. Here, we perform a concise overview comprising from the basic mechanisms of both ubiquitination and SUMOylation to recent MS-based approaches aimed to find specific targets for particular E3 enzymes.

Coordinating DNA Replication and Mitosis through Ubiquitin/SUMO and CDK1

Post-translational modification of the DNA replication machinery by ubiquitin and SUMO plays key roles in the faithful duplication of the genetic information. Among other functions, ubiquitination and SUMOylation serve as signals for the extraction of factors from chromatin by the AAA ATPase VCP. In addition to the regulation of DNA replication initiation and elongation, we now know that ubiquitination mediates the disassembly of the replisome after DNA replication termination, a process that is essential to preserve genomic stability. Here, we review the recent evidence showing how active DNA replication restricts replisome ubiquitination to prevent the premature disassembly of the DNA replication machinery. Ubiquitination also mediates the removal of the replisome to allow DNA repair. Further, we discuss the interplay between ubiquitin-mediated replisome disassembly and the activation of CDK1 that is required to set up the transition from the S phase to mitosis. We propose the existence of a ubiquitin–CDK1 relay, where the disassembly of terminated replisomes increases CDK1 activity that, in turn, favors the ubiquitination and disassembly of more replisomes. This model has important implications for the mechanism of action of cancer therapies that induce the untimely activation of CDK1, thereby triggering premature replisome disassembly and DNA damage.

SUMO-Targeted Ubiquitin Ligases and Their Functions in Maintaining Genome Stability

Small ubiquitin-like modifier (SUMO)-targeted E3 ubiquitin ligases (STUbLs) are specialized enzymes that recognize SUMOylated proteins and attach ubiquitin to them. They therefore connect the cellular SUMOylation and ubiquitination circuits. STUbLs participate in diverse molecular processes that span cell cycle regulated events, including DNA repair, replication, mitosis, and transcription. They operate during unperturbed conditions and in response to challenges, such as genotoxic stress. These E3 ubiquitin ligases modify their target substrates by catalyzing ubiquitin chains that form different linkages, resulting in proteolytic or non-proteolytic outcomes. Often, STUbLs function in compartmentalized environments, such as the nuclear envelope or kinetochore, and actively aid in nuclear relocalization of damaged DNA and stalled replication forks to promote DNA repair or fork restart. Furthermore, STUbLs reside in the same vicinity as SUMO proteases and deubiquitinases (DUBs), providing spatiotemporal control of their targets. In this review, we focus on the molecular mechanisms by which STUbLs help to maintain genome stability across different species.

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

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

SUMO Chains Rule on Chromatin Occupancy

The dynamic and reversible post-translational modification of proteins and protein complexes with the ubiquitin-related SUMO modifier regulates a wide variety of nuclear functions, such as transcription, replication and DNA repair. SUMO can be attached as a monomer to its targets, but can also form polymeric SUMO chains. While monoSUMOylation is generally involved in the assembly of protein complexes, multi- or polySUMOylation may have very different consequences. The evolutionary conserved paradigmatic signaling process initiated by multi- or polySUMOylation is the SUMO-targeted Ubiquitin ligase (StUbL) pathway, where the presence of multiple SUMO moieties primes ubiquitylation by the mammalian E3 ubiquitin ligases RNF4 or RNF111, or the yeast Slx5/8 heterodimer. The mammalian SUMO chain-specific isopeptidases SENP6 or SENP7, or yeast Ulp2, counterbalance chain formation thereby limiting StUbL activity. Many facets of SUMO chain signaling are still incompletely understood, mainly because only a limited number of polySUMOylated substrates have been identified. Here we summarize recent work that revealed a highly interconnected network of candidate polySUMO modified proteins functioning in DNA damage response and chromatin organization. Based on these datasets and published work on distinct polySUMO-regulated processes we discuss overarching concepts in SUMO chain function. We propose an evolutionary conserved role of polySUMOylation in orchestrating chromatin dynamics and genome stability networks by balancing chromatin-residency of protein complexes. This concept will be exemplified in processes, such as centromere/kinetochore organization, sister chromatid cohesion, DNA repair and replication.

SUMO-Mediated Regulation of Nuclear Functions and Signaling Processes

Since the discovery of SUMO twenty years ago, SUMO conjugation has become a widely recognized post-translational modification that targets a myriad of proteins in many processes. Great progress has been made in understanding the SUMO pathway enzymes, substrate sumoylation, and the interplay between sumoylation and other regulatory mechanisms in a variety of contexts. As these research directions continue to generate insights into SUMO-based regulation, several mechanisms by which sumoylation and desumoylation can orchestrate large biological effects are emerging. These include the ability to target multiple proteins within the same cellular structure or process, respond dynamically to external and internal stimuli, and modulate signaling pathways involving other post-translational modifications. Focusing on nuclear function and intracellular signaling, this review highlights a broad spectrum of historical data and recent advances with the aim of providing an overview of mechanisms underlying SUMO-mediated global effects to stimulate further inquiry into intriguing roles of SUMO.

SUMO-specific proteases and isopeptidases of the SENP family at a glance

The ubiquitin-related SUMO system controls many cellular signaling networks. In mammalian cells, three SUMO forms (SUMO1, SUMO2 and SUMO3) act as covalent modifiers of up to thousands of cellular proteins. SUMO conjugation affects cell function mainly by regulating the plasticity of protein networks. Importantly, the modification is reversible and highly dynamic. Cysteine proteases of the sentrin-specific protease (SENP) family reverse SUMO conjugation in mammalian cells. In this Cell Science at a Glance article and the accompanying poster, we will summarize how the six members of the mammalian SENP family orchestrate multifaceted deconjugation events to coordinate cell processes, such as gene expression, the DNA damage response and inflammation.

The Biology of SUMO-Targeted Ubiquitin Ligases in Drosophila Development, Immunity, and Cancer

The ubiquitin and SUMO (small ubiquitin-like modifier) pathways modify proteins that in turn regulate diverse cellular processes, embryonic development, and adult tissue physiology. These pathways were originally discovered biochemically in vitro, leading to a long-standing challenge of elucidating both the molecular cross-talk between these pathways and their biological importance. Recent discoveries in Drosophila established that ubiquitin and SUMO pathways are interconnected via evolutionally conserved SUMO-targeted ubiquitin ligase (STUbL) proteins. STUbL are RING ubiquitin ligases that recognize SUMOylated substrates and catalyze their ubiquitination, and include Degringolade (Dgrn) in Drosophila and RNF4 and RNF111 in humans. STUbL are essential for early development of both the fly and mouse embryos. In the fly embryo, Dgrn regulates early cell cycle progression, sex determination, zygotic gene transcription, segmentation, and neurogenesis, among other processes. In the fly adult, Dgrn is required for systemic immune response to pathogens and intestinal stem cell regeneration upon infection. These functions of Dgrn are highly conserved in humans, where RNF4-dependent ubiquitination potentiates key oncoproteins, thereby accelerating tumorigenesis. Here, we review the lessons learned to date in Drosophila and highlight their relevance to cancer biology.

SUMO in the DNA Double-Stranded Break Response: Similarities, Differences, and Cooperation with Ubiquitin

In recent years, our knowledge of the varied role that ubiquitination plays in promoting signal amplification, novel protein interactions, and protein turnover has progressed rapidly. This is particularly remarkable in the examination of how DNA double-stranded breaks (DSBs) are repaired, with many components of the ubiquitin (Ub) conjugation, de-conjugation, and recognition machinery now identified as key factors in DSB repair. In addition, a member of the Ub-like family, small Ub-like modifier (SUMO), has also been recognised as integral for efficient repair. Here, we summarise our emerging understanding of SUMOylation both as a distinct modification and as a cooperative modification with Ub, using the cellular response to DNA DSBs as the primary setting to compare these modifications.

SUMO, a small, but powerful, regulator of double-strand break repair

The response to a DNA double-stranded break in mammalian cells is a process of sensing and signalling the lesion. It results in halting the cell cycle and local transcription and in the mediation of the DNA repair process itself. The response is launched through a series of post-translational modification signalling events coordinated by phosphorylation and ubiquitination. More recently modifications of proteins by Small Ubiquitin-like MOdifier (SUMO) isoforms have also been found to be key to coordination of the response (Morris et al. 2009 Nature462, 886-890 (doi:10.1038/nature08593); Galanty et al. 2009 Nature462, 935-939 (doi:10.1038/nature08657)). However our understanding of the role of SUMOylation is slight compared with our growing knowledge of how ubiquitin drives signal amplification and key chromatin interactions. In this review we consider our current knowledge of how SUMO isoforms, SUMO conjugation machinery, SUMO proteases and SUMO-interacting proteins contribute to directing altered chromatin states and to repair-protein kinetics at a double-stranded DNA lesion in mammalian cells. We also consider the gaps in our understanding.This article is part of the themed issue 'Chromatin modifiers and remodellers in DNA repair and signalling'.

Functions of SUMO in the Maintenance of Genome Stability

Like in most other areas of cellular metabolism, the functions of the ubiquitin-like modifier SUMO in the maintenance of genome stability are manifold and varied. Perturbations of global sumoylation causes a wide spectrum of phenotypes associated with defects in DNA maintenance, such as hypersensitivity to DNA-damaging agents, gross chromosomal rearrangements and loss of entire chromosomes. Consistent with these observations, many key factors involved in various DNA repair pathways have been identified as SUMO substrates. However, establishing a functional connection between a given SUMO target, the cognate SUMO ligase and a relevant phenotype has remained a challenge, mainly because of the difficulties involved in identifying important modification sites and downstream effectors that specifically recognize the target in its sumoylated state. This review will give an overview over the major pathways of DNA repair and genome maintenance influenced by the SUMO system and discuss selected examples of SUMO's actions in these pathways where the biological consequences of the modification have been elucidated.

TRIM5α Promotes Ubiquitination of Rta from Epstein-Barr Virus to Attenuate Lytic Progression

Replication and transcription activator (Rta), a key protein expressed by Epstein–Barr virus (EBV) during the immediate-early stage of the lytic cycle, is responsible for the activation of viral lytic genes. In this study, GST-pulldown and coimmunoprecipitation assays showed that Rta interacts in vitro and in vivo with TRIM5α, a host factor known to be involved in the restriction of retroviral infections. Confocal microscopy results revealed that Rta colocalizes with TRIM5α in the nucleus during lytic progression. The interaction involves 190 amino acids in the N-terminal of Rta and the RING domain in TRIM5α, and it was further found that TRIM5α acts as an E3 ubiquitin ligase to promote Rta ubiquitination. Overexpression of TRIM5α reduced the transactivating capabilities of Rta, while reducing TRIM5α expression enhanced EBV lytic protein expression and DNA replication. Taken together, these results point to a critical role for TRIM5α in attenuating EBV lytic progression through the targeting of Rta for ubiquitination, and suggest that the restrictive capabilities of TRIM5α may go beyond retroviral infections.

USP7/HAUSP: A SUMO deubiquitinase at the heart of DNA replication

DNA replication is both highly conserved and controlled. Problematic DNA replication can lead to genomic instability and therefore carcinogenesis. Numerous mechanisms work together to achieve this tight control and increasing evidence suggests that post-translational modifications (phosphorylation, ubiquitination, SUMOylation) of DNA replication proteins play a pivotal role in this process. Here we discuss such modifications in the light of a recent article that describes a novel role for the deubiquitinase (DUB) USP7/HAUSP in the control of DNA replication. USP7 achieves this function by an unusual and novel mechanism, namely deubiquitination of SUMOylated proteins at the replication fork, making USP7 also a SUMO DUB (SDUB). This work extends previous observations of increased levels of SUMO and low levels of ubiquitin at the on-going replication fork. Here, we discuss this novel study, its contribution to the DNA replication and genomic stability field and what questions arise from this work.

Ubiquitin-dependent and independent roles of SUMO in proteostasis

Cellular proteomes are continuously undergoing alterations as a result of new production of proteins, protein folding, and degradation of proteins. The proper equilibrium of these processes is known as proteostasis, implying that proteomes are in homeostasis. Stress conditions can affect proteostasis due to the accumulation of misfolded proteins as a result of overloading the degradation machinery. Proteostasis is affected in neurodegenerative diseases like Alzheimer's disease, Parkinson's disease, and multiple polyglutamine disorders including Huntington's disease. Owing to a lack of proteostasis, neuronal cells build up toxic protein aggregates in these diseases. Here, we review the role of the ubiquitin-like posttranslational modification SUMO in proteostasis. SUMO alone contributes to protein homeostasis by influencing protein signaling or solubility. However, the main contribution of SUMO to proteostasis is the ability to cooperate with, complement, and balance the ubiquitin-proteasome system at multiple levels. We discuss the identification of enzymes involved in the interplay between SUMO and ubiquitin, exploring the complexity of this crosstalk which regulates proteostasis. These enzymes include SUMO-targeted ubiquitin ligases and ubiquitin proteases counteracting these ligases. Additionally, we review the role of SUMO in brain-related diseases, where SUMO is primarily investigated because of its role during formation of aggregates, either independently or in cooperation with ubiquitin. Detailed understanding of the role of SUMO in these diseases could lead to novel treatment options.

USP7 is a SUMO deubiquitinase essential for DNA replication

Post-translational modification of proteins by ubiquitin (Ub) and Ub-like modifiers regulates DNA replication. We have previously shown that chromatin around replisomes is rich in SUMO and poor in Ub, whereas mature chromatin exhibits an opposite pattern. How this SUMO-rich, Ub-poor environment is maintained at sites of DNA replication in mammalian cells remains unexplored. Here we identify USP7 as a replisome-enriched SUMO deubiquitinase that is essential for DNA replication. By acting on SUMO and SUMOylated proteins, USP7 counteracts their ubiquitination. Inhibition or genetic deletion of USP7 leads to the accumulation of Ub on SUMOylated proteins, which are displaced away from replisomes. Our findings provide a model explaining the differential accumulation of SUMO and Ub at replication forks and identify an essential role of USP7 in DNA replication that should be considered in the development of USP7 inhibitors as anticancer agents.

A mechanism for the suppression of homologous recombination in G1 cells

DNA repair by homologous recombination is highly suppressed in G1 cells to ensure that mitotic recombination occurs solely between sister chromatids. Although many homologous recombination factors are cell-cycle regulated, the identity of the events that are both necessary and sufficient to suppress recombination in G1 cells is unknown. Here we report that the cell cycle controls the interaction of BRCA1 with PALB2-BRCA2 to constrain BRCA2 function to the S/G2 phases in human cells. We found that the BRCA1-interaction site on PALB2 is targeted by an E3 ubiquitin ligase composed of KEAP1, a PALB2-interacting protein, in complex with cullin-3 (CUL3)-RBX1 (ref. 6). PALB2 ubiquitylation suppresses its interaction with BRCA1 and is counteracted by the deubiquitylase USP11, which is itself under cell cycle control. Restoration of the BRCA1-PALB2 interaction combined with the activation of DNA-end resection is sufficient to induce homologous recombination in G1, as measured by RAD51 recruitment, unscheduled DNA synthesis and a CRISPR-Cas9-based gene-targeting assay. We conclude that the mechanism prohibiting homologous recombination in G1 minimally consists of the suppression of DNA-end resection coupled with a multi-step block of the recruitment of BRCA2 to DNA damage sites that involves the inhibition of BRCA1-PALB2-BRCA2 complex assembly. We speculate that the ability to induce homologous recombination in G1 cells with defined factors could spur the development of gene-targeting applications in non-dividing cells.

Role of Deubiquitinating Enzymes in DNA Repair

Both proteolytic and nonproteolytic functions of ubiquitination are essential regulatory mechanisms for promoting DNA repair and the DNA damage response in mammalian cells. Deubiquitinating enzymes (DUBs) have emerged as key players in the maintenance of genome stability. In this minireview, we discuss the recent findings on human DUBs that participate in genome maintenance, with a focus on the role of DUBs in the modulation of DNA repair and DNA damage signaling.