SUMO-1 regulates the conformational dynamics of thymine-DNA Glycosylase regulatory domain and competes with its DNA binding activity

Potential role of PIM1 inhibition in the treatment of SARS-CoV-2 infection

BACKGROUND

SARS-CoV-2 infection involves disturbing multiple molecular pathways related to immunity and cellular functions. PIM1 is a serine/threonine-protein kinase found to be involved in the pathogenesis of several viral infections. One PIM1 substrate, Myc, was reported to interact with TMPRSS2, which is crucial for SARS-CoV-2 cell entry. PIM1 inhibitors were reported to have antiviral activity through multiple mechanisms related to immunity and proliferation. This study aimed to evaluate the antiviral activity of 2-pyridone PIM1 inhibitor against SARS-CoV-2 and its potential role in hindering the progression of COVID-19. It also aimed to assess PIM1 inhibitor's effect on the expression of several genes of Notch signaling and Wnt pathways. In vitro study was conducted on Vero-E6 cells infected by SARS-CoV-2 "NRC-03-nhCoV" virus. Protein-protein interaction of the study genes was assessed to evaluate their relation to cell proliferation and immunity. The effect of 2-pyridone PIM1 inhibitor treatment on viral load and mRNA expression of target genes was assessed at three time points.

RESULTS

Treatment with 2-pyridone PIM1 inhibitor showed potential antiviral activity against SARS-CoV-2 (IC50 of 37.255 µg/ml), significantly lowering the viral load. Functional enrichments of the studied genes include negative regulation of growth rate, several biological processes involved in cell proliferation, and Interleukin-4 production, with interleukin-6 as a predicted functional partner. These results suggest an interplay between study genes with relation to cell proliferation and immunity. Following in vitro SARS-CoV-2 infection, Notch pathway genes, CTNNB1, SUMO1, and TDG, were found to be overexpressed compared to uninfected cells. Treatment with 2-pyridone PIM1 inhibitor significantly lowers the expression levels of study genes, restoring Notch1 and BCL9 to the control level while decreasing Notch2 and CTNNB1 below control levels.

CONCLUSION

2-pyridone PIM1 inhibitor could hinder cellular entry of SARS-CoV-2 and modulate several pathways implicated in immunity, suggesting a potential benefit in the development of anti-SARS-CoV-2 therapeutic approach.

Structural Insight into Binding of the ZZ Domain of HERC2 to Histone H3 and SUMO1

Human ubiquitin ligase HERC2, a component of the DNA repair machinery, has been linked to neurological diseases and cancer. Here, we show that the ZZ domain of HERC2 (HERC2_ZZ) binds to histone H3 tail and tolerates posttranslational modifications commonly present in H3. The crystal structure of the HERC2_ZZ:H3 complex provides the molecular basis for this interaction and highlights a critical role of the negatively charged site of HERC2_ZZ in capturing of A1 of H3. NMR, mutagenesis, and fluorescence data reveal that HERC2_ZZ binds to H3 and the N-terminal tail of SUMO1, a previously reported ligand of HERC2_ZZ, with comparable affinities. Like H3, the N-terminal tail of SUMO1 occupies the same negatively charged site of HERC2_ZZ in the crystal structure of the complex, although in contrast to H3 it adopts an α-helical conformation. Our data suggest that HERC2_ZZ may play a role in mediating the association of HERC2 with chromatin.

Desumoylation of RNA polymerase III lies at the core of the Sumo stress response in yeast

Post-translational modification by small ubiquitin-like modifier (Sumo) regulates many cellular processes, including the adaptive response to various types of stress, referred to as the Sumo stress response (SSR). However, it remains unclear whether the SSR involves a common set of core proteins regardless of the type of stress or whether each particular type of stress induces a stress-specific SSR that targets a unique, largely nonoverlapping set of Sumo substrates. In this study, we used MS and a Gene Ontology approach to identify differentially sumoylated proteins during heat stress, hyperosmotic stress, oxidative stress, nitrogen starvation, and DNA alkylation in Saccharomyces cerevisiae cells. Our results indicate that each stress triggers a specific SSR signature centered on proteins involved in transcription, translation, and chromatin regulation. Strikingly, whereas the various stress-specific SSRs were largely nonoverlapping, all types of stress tested here resulted in desumoylation of subunits of RNA polymerase III, which correlated with a decrease in tRNA synthesis. We conclude that desumoylation and subsequent inhibition of RNA polymerase III constitutes the core of all stress-specific SSRs in yeast.

Identification of a noncanonical RNA binding domain in the U2 snRNP protein SF3A1

During splicing of pre-mRNA, 5' and 3' splice sites are brought within proximity by interactions between the pre-mRNA bound U1 and U2 snRNPs, followed by recruitment of the tri-snRNP for assembly of the mature spliceosome. Previously, we identified an interaction between the U2 snRNP-specific protein SF3A1 and the stem-loop 4 (SL4) of the U1 snRNA that occurs during the early steps of spliceosome assembly. Although harboring many annotated domains, SF3A1 lacks a canonical RNA binding domain. To identify the U1-SL4 binding region in SF3A1, we expressed amino- and carboxy-terminal deletion constructs using a HeLa cell-based cell free expression system. UV-crosslinking of the truncated proteins with ³²P-U1-SL4 RNA identified the carboxy-terminal ubiquitin-like (UBL) domain of SF3A1 as the RNA binding region. Characterization of the interaction between SF3A1-UBL and U1-SL4 by electrophoretic mobility shift assay and surface plasmon resonance determined high binding affinity (K_D = ∼97 nM), and revealed the double-stranded G-C rich stem of U1-SL4 as an important feature for binding to the UBL domain. Further, mutations of two conserved tyrosine residues, Y772 and Y773, were found to cause a two- and fivefold decrease in the binding affinity for U1-SL4, respectively. Finally, we found that SF3A1-UBL can specifically pull down the U1 snRNP from HeLa nuclear extract, demonstrating its capacity to bind U1-SL4 in the context of the intact snRNP. Thus, the data show that the UBL domain of SF3A1 can function as an RNA binding domain and that mutations in this region may interfere with U1-SL4 binding.

Defining the impact of sumoylation on substrate binding and catalysis by thymine DNA glycosylase

Thymine DNA glycosylase (TDG) excises thymine from mutagenic G·T mispairs generated by deamination of 5-methylcytosine (mC) and it removes two mC derivatives, 5−formylcytosine (fC) and 5−carboxylcytosine (caC), in a multistep pathway for DNA demethylation. TDG is modified by small ubiquitin-like modifier (SUMO) proteins, but the impact of sumoylation on TDG activity is poorly defined and the functions of TDG sumoylation remain unclear. We determined the effect of TDG sumoylation, by SUMO-1 or SUMO-2, on substrate binding and catalytic parameters. Single turnover experiments reveal that sumoylation dramatically impairs TDG base-excision activity, such that G·T activity is reduced by ≥45-fold and fC and caC are excised slowly, with a reaction half-life of ≥9 min (37°C). Fluorescence anisotropy studies reveal that unmodified TDG binds tightly to G·fC and G·caC substrates, with dissociation constants in the low nanomolar range. While sumoylation of TDG weakens substrate binding, the residual affinity is substantial and is comparable to that of biochemically-characterized readers of fC and caC. Our findings raise the possibility that sumoylation enables TDG to function, at least transiently, as reader of fC and caC. Notably, sumoylation could potentially facilitate TDG recruitment of other proteins, including transcription factors or epigenetic regulators, to these sites in DNA.

Transient protein-protein complexes in base excision repair

Transient protein-protein complexes are of great importance for organizing multiple enzymatic reactions into productive reaction pathways. Base excision repair (BER), a process of critical importance for maintaining genome stability against a plethora of DNA-damaging factors, involves several enzymes, including DNA glycosylases, AP endonucleases, DNA polymerases, DNA ligases and accessory proteins acting sequentially on the same damaged site in DNA. Rather than being assembled into one stable multisubunit complex, these enzymes pass the repair intermediates between them in a highly coordinated manner. In this review, we discuss the nature and the role of transient complexes arising during BER as deduced from structural and kinetic data. Almost all of the transient complexes are DNA-mediated, although some may also exist in solution and strengthen under specific conditions. The best-studied example, the interactions between DNA glycosylases and AP endonucleases, is discussed in more detail to provide a framework for distinguishing between stable and transient complexes based on the kinetic data. Communicated by Ramaswamy H. Sarma.

SUMOylation coordinates BERosome assembly in active DNA demethylation during cell differentiation

During active DNA demethylation, 5-methylcytosine (5mC) is oxidized by TET proteins to 5-formyl-/5-carboxylcytosine (5fC/5caC) for replacement by unmethylated C by TDG-initiated DNA base excision repair (BER). Base excision generates fragile abasic sites (AP-sites) in DNA and has to be coordinated with subsequent repair steps to limit accumulation of genome destabilizing secondary DNA lesions. Here, we show that 5fC/5caC is generated at a high rate in genomes of differentiating mouse embryonic stem cells and that SUMOylation and the BER protein XRCC1 play critical roles in orchestrating TDG-initiated BER of these lesions. SUMOylation of XRCC1 facilitates physical interaction with TDG and promotes the assembly of a TDG-BER core complex. Within this TDG-BERosome, SUMO is transferred from XRCC1 and coupled to the SUMO acceptor lysine in TDG, promoting its dissociation while assuring the engagement of the BER machinery to complete demethylation. Although well-studied, the biological importance of TDG SUMOylation has remained obscure. Here, we demonstrate that SUMOylation of TDG suppresses DNA strand-break accumulation and toxicity to PARP inhibition in differentiating mESCs and is essential for neural lineage commitment.

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'.

Regulation of DNA demethylation by the XPC DNA repair complex in somatic and pluripotent stem cells

In this study, Ho et al. research the mechanism by which TDG-dependent DNA demethylation occurs in a rapid and site-specific manner. Their findings demonstrate two distinct but complementary mechanisms by which XPC influences gene regulation by coordinating efficient TDG-mediated DNA demethylation along with active transcription during somatic cell reprogramming.

Faithful resetting of the epigenetic memory of a somatic cell to a pluripotent state during cellular reprogramming requires DNA methylation to silence somatic gene expression and dynamic DNA demethylation to activate pluripotency gene transcription. The removal of methylated cytosines requires the base excision repair enzyme TDG, but the mechanism by which TDG-dependent DNA demethylation occurs in a rapid and site-specific manner remains unclear. Here we show that the XPC DNA repair complex is a potent accelerator of global and locus-specific DNA demethylation in somatic and pluripotent stem cells. XPC cooperates with TDG genome-wide to stimulate the turnover of essential intermediates by overcoming slow TDG–abasic product dissociation during active DNA demethylation. We further establish that DNA demethylation induced by XPC expression in somatic cells overcomes an early epigenetic barrier in cellular reprogramming and facilitates the generation of more robust induced pluripotent stem cells, characterized by enhanced pluripotency-associated gene expression and self-renewal capacity. Taken together with our previous studies establishing the XPC complex as a transcriptional coactivator, our findings underscore two distinct but complementary mechanisms by which XPC influences gene regulation by coordinating efficient TDG-mediated DNA demethylation along with active transcription during somatic cell reprogramming.

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.

Identification of SUMO modification sites in the base excision repair protein, Ntg1

DNA damaging agents are a constant threat to genomes in both the nucleus and the mitochondria. To combat this threat, a suite of DNA repair pathways cooperate to repair numerous types of DNA damage. If left unrepaired, these damages can result in the accumulation of mutations which can lead to deleterious consequences including cancer and neurodegenerative disorders. The base excision repair (BER) pathway is highly conserved from bacteria to humans and is primarily responsible for the removal and subsequent repair of toxic and mutagenic oxidative DNA lesions. Although the biochemical steps that occur in the BER pathway have been well defined, little is known about how the BER machinery is regulated. The budding yeast, Saccharomyces cerevisiae is a powerful model system to biochemically and genetically dissect BER. BER is initiated by DNA N-glycosylases, such as S. cerevisiae Ntg1. Previous work demonstrates that Ntg1 is post-translationally modified by SUMO in response to oxidative DNA damage suggesting that this modification could modulate the function of Ntg1. In this study, we mapped the specific sites of SUMO modification within Ntg1 and identified the enzymes responsible for sumoylating/desumoylating Ntg1. Using a non-sumoylatable version of Ntg1, ntg1ΔSUMO, we performed an initial assessment of the functional impact of Ntg1 SUMO modification in the cellular response to DNA damage. Finally, we demonstrate that, similar to Ntg1, the human homologue of Ntg1, NTHL1, can also be SUMO-modified in response to oxidative stress. Our results suggest that SUMO modification of BER proteins could be a conserved mechanism to coordinate cellular responses to DNA damage.

Base Excision Repair, a Pathway Regulated by Posttranslational Modifications

Base excision repair (BER) is an essential DNA repair pathway involved in the maintenance of genome stability and thus in the prevention of human diseases, such as premature aging, neurodegenerative diseases, and cancer. Protein posttranslational modifications (PTMs), including acetylation, methylation, phosphorylation, SUMOylation, and ubiquitylation, have emerged as important contributors in controlling cellular BER protein levels, enzymatic activities, protein-protein interactions, and protein cellular localization. These PTMs therefore play key roles in regulating the BER pathway and are consequently crucial for coordinating an efficient cellular DNA damage response. In this review, we summarize the presently available data on characterized PTMs of key BER proteins, the functional consequences of these modifications at the protein level, and also the impact on BER in vitro and in vivo.

Characterizing Requirements for Small Ubiquitin-like Modifier (SUMO) Modification and Binding on Base Excision Repair Activity of Thymine-DNA Glycosylase in Vivo

Thymine-DNA glycosylase (TDG) plays critical roles in DNA base excision repair and DNA demethylation. It has been proposed, based on structural studies and in vitro biochemistry, that sumoylation is required for efficient TDG enzymatic turnover following base excision. However, whether sumoylation is required for TDG activity in vivo has not previously been tested. We have developed an in vivo assay for TDG activity that takes advantage of its recently discovered role in DNA demethylation and selective recognition and repair of 5-carboxylcytosine. Using this assay, we investigated the role of sumoylation in regulating TDG activity through the use of TDG mutants defective for sumoylation and Small Ubiquitin-like Modifier (SUMO) binding and by altering TDG sumoylation through SUMO and SUMO protease overexpression experiments. Our findings indicate that sumoylation and SUMO binding are not essential for TDG-mediated excision and repair of 5-carboxylcytosine bases. Moreover, in vitro assays revealed that apurinic/apyrimidinic nuclease 1 provides nearly maximum stimulation of TDG processing of G·caC substrates. Thus, under our assay conditions, apurinic/apyrimidinic nuclease 1-mediated stimulation or other mechanisms sufficiently alleviate TDG product inhibition and promote its enzymatic turnover in vivo.

The current state of eukaryotic DNA base damage and repair

DNA damage is a natural hazard of life. The most common DNA lesions are base, sugar, and single-strand break damage resulting from oxidation, alkylation, deamination, and spontaneous hydrolysis. If left unrepaired, such lesions can become fixed in the genome as permanent mutations. Thus, evolution has led to the creation of several highly conserved, partially redundant pathways to repair or mitigate the effects of DNA base damage. The biochemical mechanisms of these pathways have been well characterized and the impact of this work was recently highlighted by the selection of Tomas Lindahl, Aziz Sancar and Paul Modrich as the recipients of the 2015 Nobel Prize in Chemistry for their seminal work in defining DNA repair pathways. However, how these repair pathways are regulated and interconnected is still being elucidated. This review focuses on the classical base excision repair and strand incision pathways in eukaryotes, considering both Saccharomyces cerevisiae and humans, and extends to some important questions and challenges facing the field of DNA base damage repair.

SUMO-regulated transcription: challenging the dogma

The small ubiquitin-like modifier SUMO regulates many aspects of cellular physiology to maintain cell homeostasis, both under normal conditions and during cell stress. Components of the transcriptional apparatus and chromatin are among the most prominent SUMO substrates. The prevailing view is that SUMO serves to repress transcription. However, as we will discuss in this review, this model needs to be refined, because recent studies have revealed that SUMO can also have profound positive effects on transcription.

SUMO-mediated regulation of DNA damage repair and responses

Sumoylation has important roles during DNA damage repair and responses. Recent broad-scope and substrate-based studies have shed light on the regulation and significance of sumoylation during these processes. An emerging paradigm is that sumoylation of many DNA metabolism proteins is controlled by DNA engagement. Such 'on-site modification' can explain low substrate modification levels and has important implications in sumoylation mechanisms and effects. New studies also suggest that sumoylation can regulate a process through an ensemble effect or via major substrates. Additionally, we describe new trends in the functional effects of sumoylation, such as bi-directional changes in biomolecule binding and multilevel coordination with other modifications. These emerging themes and models will stimulate our thinking and research in sumoylation and genome maintenance.

A germline polymorphism of thymine DNA glycosylase induces genomic instability and cellular transformation

Thymine DNA glycosylase (TDG) functions in base excision repair, a DNA repair pathway that acts in a lesion-specific manner to correct individual damaged or altered bases. TDG preferentially catalyzes the removal of thymine and uracil paired with guanine, and is also active on 5-fluorouracil (5-FU) paired with adenine or guanine. The rs4135113 single nucleotide polymorphism (SNP) of TDG is found in 10% of the global population. This coding SNP results in the alteration of Gly199 to Ser. Gly199 is part of a loop responsible for stabilizing the flipped abasic nucleotide in the active site pocket. Biochemical analyses indicate that G199S exhibits tighter binding to both its substrate and abasic product. The persistent accumulation of abasic sites in cells expressing G199S leads to the induction of double-strand breaks (DSBs). Cells expressing the G199S variant also activate a DNA damage response. When expressed in cells, G199S induces genomic instability and cellular transformation. Together, these results suggest that individuals harboring the G199S variant may have increased risk for developing cancer.

DNA repair is vital to the survival and propagation of cells. It helps protect DNA from becoming permanently damaged and prevents cells from becoming cancerous. The base excision repair (BER) pathway is responsible for the removal of up to 20,000 lesions/cell/day. Thymine DNA glycosylase (TDG) is one of the DNA glycosylases that initiates BER. There is a germline variant of TDG that is found in 10% of the global population, where amino acid residue glycine 199 is mutated to serine. Here, we provide evidence that TDG variant G199S binds significantly more tightly to its abasic product and leads to increased DNA strand breaks in cells. We go on to show that G199S induces genomic instability, in the form of chromosomal aberrations, and leads to cellular transformation, both hallmarks of tumorigenesis. Collectively, our work suggests that a germline variant of TDG can drive carcinogenesis.

MxA interacts with and is modified by the SUMOylation machinery

Mx proteins are evolutionarily conserved dynamin-like large GTPases involved in viral resistance triggered by types I and III interferons. The human MxA is a cytoplasmic protein that confers resistance to a large number of viruses. The MxA protein is also known to self-assembly into high molecular weight homo-oligomers. Using a yeast two-hybrid screen, we identified 27 MxA binding partners, some of which are related to the SUMOylation machinery. The interaction of MxA with Small-Ubiquitin MOdifier 1 (SUMO1) and Ubiquitin conjugating enzyme 9 (Ubc9) was confirmed by co-immunoprecipitation and co-localization by confocal microscopy. We identified one SUMO conjugation site at lysine 48 and two putative SUMO interacting motifs (SIMa and SIMb). We showed that MxA interacts with the EIL loop of SUMO1 in a SIM-independent manner via its CID-GED domain. The yeast two-hybrid mapping also revealed that Ubc9 binds to the MxA GTPase domain. Mutation in the putative SIMa and SIMb, which are located in the GTPase binding domain, reduced MxA antiviral activity. In addition, we showed that MxA can be conjugated to SUMO2 or SUMO3 at lysine 48 and that the SUMOylation-deficient mutant of MxA (MxAK48R) retained its capacity to oligomerize and to inhibit Vesicular Stomatitis Virus (VSV) and Influenza A Virus replication, suggesting that MxA SUMOylation is not essential for its antiviral activity.

Monitoring regulation of DNA repair activities of cultured cells in-gel using the comet assay

Base excision repair (BER) is the predominant cellular mechanism by which human cells repair DNA base damage, sites of base loss, and DNA single strand breaks of various complexity, that are generated in their thousands in every human cell per day as a consequence of cellular metabolism and exogenous agents, including ionizing radiation. Over the last three decades the comet assay has been employed in scientific research to examine the cellular response to these types of DNA damage in cultured cells, therefore revealing the efficiency and capacity of BER. We have recently pioneered new research demonstrating an important role for post-translational modifications (particularly ubiquitylation) in the regulation of cellular levels of BER proteins, and that subtle changes (∼20-50%) in protein levels following siRNA knockdown of E3 ubiquitin ligases or deubiquitylation enzymes can manifest in significant changes in DNA repair capacity monitored using the comet assay. For example, we have shown that the E3 ubiquitin ligase Mule, the tumor suppressor protein ARF, and the deubiquitylation enzyme USP47 modulate DNA repair by controlling cellular levels of DNA polymerase β, and also that polynucleotide kinase phosphatase levels are controlled by ATM-dependant phosphorylation and Cul4A-DDB1-STRAP-dependent ubiquitylation. In these studies we employed a modification of the comet assay whereby cultured cells, following DNA damage treatment, are embedded in agarose and allowed to repair in-gel prior to lysis and electrophoresis. Whilst this method does have its limitations, it avoids the extensive cell culture-based processing associated with the traditional approach using attached cells and also allows for the examination of much more precise DNA repair kinetics. In this review we will describe, using this modified comet assay, our accumulating evidence that ubiquitylation-dependant regulation of BER proteins has important consequences for overall cellular DNA repair capacity.

Thymine DNA glycosylase is a CRL4Cdt2 substrate

The E3 ubiquitin ligase CRL4(Cdt2) targets proteins for destruction in S phase and after DNA damage by coupling ubiquitylation to DNA-bound proliferating cell nuclear antigen (PCNA). Coupling to PCNA involves a PCNA-interacting peptide (PIP) degron motif in the substrate that recruits CRL4(Cdt2) while binding to PCNA. In vertebrates, CRL4(Cdt2) promotes degradation of proteins whose presence in S phase is deleterious, including Cdt1, Set8, and p21. Here, we show that CRL4(Cdt2) targets thymine DNA glycosylase (TDG), a base excision repair enzyme that is involved in DNA demethylation. TDG contains a conserved and nearly perfect match to the PIP degron consensus. TDG is ubiquitylated and destroyed in a PCNA-, Cdt2-, and PIP degron-dependent manner during DNA repair in Xenopus egg extract. The protein can also be destroyed during DNA replication in this system. During Xenopus development, TDG first accumulates during gastrulation, and its expression is down-regulated by CRL4(Cdt2). Our results expand the group of vertebrate CRL4(Cdt2) substrates to include a bona fide DNA repair enzyme.

E2-mediated small ubiquitin-like modifier (SUMO) modification of thymine DNA glycosylase is efficient but not selective for the enzyme-product complex

Thymine DNA glycosylase (TDG) initiates the repair of G·T mismatches that arise by deamination of 5-methylcytosine (mC), and it excises 5-formylcytosine and 5-carboxylcytosine, oxidized forms of mC. TDG functions in active DNA demethylation and is essential for embryonic development. TDG forms a tight enzyme-product complex with abasic DNA, which severely impedes enzymatic turnover. Modification of TDG by small ubiquitin-like modifier (SUMO) proteins weakens its binding to abasic DNA. It was proposed that sumoylation of product-bound TDG regulates product release, with SUMO conjugation and deconjugation needed for each catalytic cycle, but this model remains unsubstantiated. We examined the efficiency and specificity of TDG sumoylation using in vitro assays with purified E1 and E2 enzymes, finding that TDG is modified efficiently by SUMO-1 and SUMO-2. Remarkably, we observed similar modification rates for free TDG and TDG bound to abasic or undamaged DNA. To examine the conjugation step directly, we determined modification rates (kobs) using preformed E2∼SUMO-1 thioester. The hyperbolic dependence of kobs on TDG concentration gives kmax = 1.6 min(-1) and K1/2 = 0.55 μM, suggesting that E2∼SUMO-1 has higher affinity for TDG than for the SUMO targets RanGAP1 and p53 (peptide). Whereas sumoylation substantially weakens TDG binding to DNA, TDG∼SUMO-1 still binds relatively tightly to AP-DNA (Kd ∼50 nM). Although E2∼SUMO-1 exhibits no specificity for product-bound TDG, the relatively high conjugation efficiency raises the possibility that E2-mediated sumoylation could stimulate product release in vivo. This and other implications for the biological role and mechanism of TDG sumoylation are discussed.

SUMO-modification and elimination of the active DNA demethylation enzyme TDG in cultured human cells

Thymine DNA glycosylase (TDG) is a base excision repair enzyme that interacts with the small ubiquitin-related modifier (SUMO)-targeted ubiquitin E3 ligase RNF4 and functions in the active DNA demethylation pathway. Here we showed that both SUMOylated and non-modified forms of endogenous TDG fluctuated during the cell cycle and in response to drugs that perturbed cell cycle progression, including hydroxyurea and nocodazole. Additionally, we detected a SUMOylation-independent association between TDG and RNF4 in vitro as well as in vivo, and observed that both forms of TDG were efficiently degraded in RNF4-depleted cells when arrested at S phase. Our findings provide insights into the in vivo dynamics of TDG SUMOylation and further clarify the TDG-RNF4 interaction.

Thymine DNA glycosylase is a positive regulator of Wnt signaling in colorectal cancer

Wnt signaling plays an important role in colorectal cancer (CRC). Although the mechanisms of β-catenin degradation have been well studied, the mechanism by which β-catenin activates transcription is still not fully understood. While screening a panel of DNA demethylases, we found that thymine DNA glycosylase (TDG) up-regulated Wnt signaling. TDG interacts with the transcription factor TCF4 and coactivator CREB-binding protein/p300 in the Wnt pathway. Knocking down TDG by shRNAs inhibited the proliferation of CRC cells in vitro and in vivo. In CRC patients, TDG levels were significantly higher in tumor tissues than in the adjacent normal tissues. These results suggest that TDG warrants consideration as a potential biomarker for CRC and as a target for CRC treatment.

A TDG/CBP/RARα ternary complex mediates the retinoic acid-dependent expression of DNA methylation-sensitive genes

The thymine DNA glycosylase (TDG) is a multifunctional enzyme, which is essential for embryonic development. It mediates the base excision repair (BER) of G:T and G:U DNA mismatches arising from the deamination of 5-methyl cytosine (5-MeC) and cytosine, respectively. Recent studies have pointed at a role of TDG during the active demethylation of 5-MeC within CpG islands. TDG interacts with the histone acetylase CREB-binding protein (CBP) to activate CBP-dependent transcription. In addition, TDG also interacts with the retinoic acid receptor α (RARα), resulting in the activation of RARα target genes. Here we provide evidence for the existence of a functional ternary complex containing TDG, CBP and activated RARα. Using global transcriptome profiling, we uncover a coupling of de novo methylation-sensitive and RA-dependent transcription, which coincides with a significant subset of CBP target genes. The introduction of a point mutation in TDG, which neither affects overall protein structure nor BER activity, leads to a significant loss in ternary complex stability, resulting in the deregulation of RA targets involved in cellular networks associated with DNA replication, recombination and repair. We thus demonstrate for the first time a direct coupling of TDG’s epigenomic and transcription regulatory function through ternary complexes with CBP and RARα.

SUMO1 negatively regulates the transcriptional activity of EVI1 and significantly increases its co-localization with EVI1 after treatment with arsenic trioxide

Aberrant expression of the proto-oncogene EVI1 (ecotropic virus integration site1) has been implicated not only in myeloid or lymphoid malignancies but also in colon, ovarian and breast cancers. Despite its importance in oncogenesis, the regulatory factors and mechanisms that potentiate the function of EVI1 and its consequences are partially known. Here we demonstrated that EVI1 is post-translationally modified by SUMO1 at lysine residues 533, 698 and 874. Although both EVI1 and SUMO1 were found to co-localize in nuclear speckles, the sumoylation mutant of EVI1 failed to co-localize with SUMO1. Sumoylation abrogated the DNA binding efficiency of EVI1 and also affected EVI1 mediated transactivation. The SUMO ligase PIASy was found to play a bi-directional role on EVI1, PIASy enhanced EVI1 sumoylation and augmented sumoylated EVI1 mediated repression. PIASy was also found to interact with EVI1 and impaired EVI1 transcriptional activity independent of its ligase activity. Arsenic trioxide (ATO) known to act as an antileukemic agent for acute promyelocytic leukemia (APL) not only enhanced EVI1 sumoylation but also enhanced the co-localization of EVI1 and SUMO1 in nuclear bodies distinct from PML nuclear bodies. ATO treatment also affected the Bcl-xL protein expression in EVI1 positive cell line. Thus, the results showed that arsenic treatment enhanced EVI1 sumoylation, deregulated Bcl-xL, which eventually may induce apoptosis in EVI1 positive cancer cells. The study for the first time explores and reports sumoylation of EVI1, which plays an essential role in regulating its function.

A click-and-release pyrrolysine analogue

What's the catch? A pyrrolysine analogue bearing a terminal alkyne and an ester functionality can be incorporated into recombinant proteins and render them amenable to capture by the click reaction and subsequent release through ester hydrolysis. The utility of this pyrrolysine-inspired technology is demonstrated for the identification of SUMOylation sites.

SUMO-conjugating enzyme E2 UBC9 mediates viral immediate-early protein SUMOylation in crayfish to facilitate reproduction of white spot syndrome virus

Successful viruses have evolved superior strategies to escape host defenses or exploit host biological pathways. Most of the viral immediate-early (ie) genes are essential for viral infection and depend solely on host proteins; however, the molecular mechanisms are poorly understood. In this study, we focused on the modification of viral IE proteins by the crayfish small ubiquitin-related modifier (SUMO) and investigated the role of SUMOylation during the viral life cycle. SUMO and SUMO ubiquitin-conjugating enzyme 9 (UBC9) involved in SUMOylation were identified in red swamp crayfish (Procambarus clarkii). Both SUMO and UBC9 were upregulated in crayfish challenged with white spot syndrome virus (WSSV). Replication of WSSV genes increased in crayfish injected with recombinant SUMO or UBC9, but injection of mutant SUMO or UBC9 protein had no effect. Subsequently, we analyzed the mechanism by which crayfish SUMOylation facilitates WSSV replication. Crayfish UBC9 bound to all three WSSV IE proteins tested, and one of these IE proteins (WSV051) was covalently modified by SUMO in vitro. The expression of viral ie genes was affected and that of late genes was significantly inhibited in UBC9-silenced or SUMO-silenced crayfish, and the inhibition effect was rescued by injection of recombinant SUMO or UBC9. The results of this study demonstrate that viral IE proteins can be modified by crayfish SUMOylation, prompt the expression of viral genes, and ultimately benefit WSSV replication. Understanding of the mechanisms by which viruses exploit host components will greatly improve our knowledge of the virus-host "arms race" and contribute to the development of novel methods against virulent viruses.

Recent advances in the structural mechanisms of DNA glycosylases

DNA glycosylases safeguard the genome by locating and excising a diverse array of aberrant nucleobases created from oxidation, alkylation, and deamination of DNA. Since the discovery 28years ago that these enzymes employ a base flipping mechanism to trap their substrates, six different protein architectures have been identified to perform the same basic task. Work over the past several years has unraveled details for how the various DNA glycosylases survey DNA, detect damage within the duplex, select for the correct modification, and catalyze base excision. Here, we provide a broad overview of these latest advances in glycosylase mechanisms gleaned from structural enzymology, highlighting features common to all glycosylases as well as key differences that define their particular substrate specificities.

DNA damage-inducible SUMOylation of HERC2 promotes RNF8 binding via a novel SUMO-binding Zinc finger

SUMOylation of the ubiquitin ligase HERC2 promotes efficient chromatin licensing in the vicinity of DNA double-strand breaks.

Nonproteolytic ubiquitylation of chromatin surrounding deoxyribonucleic acid (DNA) double-strand breaks (DSBs) by the RNF8/RNF168/HERC2 ubiquitin ligases facilitates restoration of genome integrity by licensing chromatin to concentrate genome caretaker proteins near the lesions. In parallel, SUMOylation of so-far elusive upstream DSB regulators is also required for execution of this ubiquitin-dependent chromatin response. We show that HERC2 and RNF168 are novel DNA damage–dependent SUMOylation targets in human cells. In response to DSBs, both HERC2 and RNF168 were specifically modified with SUMO1 at DSB sites in a manner dependent on the SUMO E3 ligase PIAS4. SUMOylation of HERC2 was required for its DSB-induced association with RNF8 and for stabilizing the RNF8–Ubc13 complex. We also demonstrate that the ZZ Zinc finger in HERC2 defined a novel SUMO-specific binding module, which together with its concomitant SUMOylation and T4827 phosphorylation promoted binding to RNF8. Our findings provide novel insight into the regulatory complexity of how ubiquitylation and SUMOylation cooperate to orchestrate protein interactions with DSB repair foci.

The SUMO pathway promotes basic helix-loop-helix proneural factor activity via a direct effect on the Zn finger protein senseless

During development, proneural transcription factors of the basic helix-loop-helix (bHLH) family are required to commit cells to a neural fate. In Drosophila neurogenesis, a key mechanism promoting sense organ precursor (SOP) fate is the synergy between proneural factors and their coactivator Senseless in transcriptional activation of target genes. Here we present evidence that posttranslational modification by SUMO enhances this synergy via an effect on Senseless protein. We show that Senseless is a direct target for SUMO modification and that mutagenesis of a predicted SUMOylation motif in Senseless reduces Senseless/proneural synergy both in vivo and in cell culture. We propose that SUMOylation of Senseless via lysine 509 promotes its synergy with proneural proteins during transcriptional activation and hence regulates an important step in neurogenesis leading to the formation and maturation of the SOPs.

Regulation of base excision repair in eukaryotes by dynamic localization strategies

This chapter discusses base excision repair (BER) and the known mechanisms defined thus far regulating BER in eukaryotes. Unlike the situation with nucleotide excision repair and double-strand break repair, little is known about how BER is regulated to allow for efficient and accurate repair of many types of DNA base damage in both nuclear and mitochondrial genomes. Regulation of BER has been proposed to occur at multiple, different levels including transcription, posttranslational modification, protein-protein interactions, and protein localization; however, none of these regulatory mechanisms characterized thus far affect a large spectrum of BER proteins. This chapter discusses a recently discovered mode of BER regulation defined in budding yeast cells that involves mobilization of DNA repair proteins to DNA-containing organelles in response to genotoxic stress.

The ubiquitin- and SUMO-dependent signaling response to DNA double-strand breaks

DNA double-strand breaks (DSBs) represent the most destructive type of chromosomal lesion and trigger rapid chromatin restructuring accompanied by accumulation of proteins in the vicinity of the DSB. Non-proteolytic ubiquitylation of chromatin surrounding DSBs, mediated by the RNF8/RNF168 ubiquitin ligase cascade, has emerged as a key mechanism for restoration of genome integrity by licensing the DSB-modified chromatin to concentrate genome caretaker proteins such as 53BP1 and BRCA1 near the lesions. In parallel, SUMOylation of upstream DSB regulators is also required for execution of this ubiquitin-dependent chromatin response, but its molecular basis is currently unclear. Here, we discuss recent insights into how ubiquitin- and SUMO-dependent signaling processes cooperate to orchestrate protein interactions with sites of DNA damage to facilitate DSB repair.

SUMO-1 possesses DNA binding activity

BACKGROUND

Conjugation of small ubiquitin-related modifiers (SUMOs) is a frequent post-translational modification of proteins. SUMOs can also temporally associate with protein-targets via SUMO binding motifs (SBMs). Protein sumoylation has been identified as an important regulatory mechanism especially in the regulation of transcription and the maintenance of genome stability. The precise molecular mechanisms by which SUMO conjugation and association act are, however, not understood.

FINDINGS

Using NMR spectroscopy and protein-DNA cross-linking experiments, we demonstrate here that SUMO-1 can specifically interact with dsDNA in a sequence-independent fashion. We also show that SUMO-1 binding to DNA can compete with other protein-DNA interactions at the example of the regulatory domain of Thymine-DNA Glycosylase and, based on these competition studies, estimate the DNA binding constant of SUMO1 in the range 1 mM.

CONCLUSION

This finding provides an important insight into how SUMO-1 might exert its activity. SUMO-1 might play a general role in destabilizing DNA bound protein complexes thereby operating in a bottle-opener way of fashion, explaining its pivotal role in regulating the activity of many central transcription and DNA repair complexes.