Extensive DNA damage-induced sumoylation contributes to replication and repair and acts in addition to the mec1 checkpoint

Unwinding Helicase MCM Functionality for Diagnosis and Therapeutics of Replication Abnormalities Associated with Cancer: A Review

The minichromosome maintenance (MCM) protein is a component of an active helicase that is essential for the initiation of DNA replication. Dysregulation of MCM functions contribute to abnormal cell proliferation and genomic instability. The interactions of MCM with cellular factors, including Cdc45 and GINS, determine the formation of active helicase and functioning of helicase. The functioning of MCM determines the fate of DNA replication and, thus, genomic integrity. This complex is upregulated in precancerous cells and can act as an important tool for diagnostic applications. The MCM protein complex can be an important broad-spectrum therapeutic target in various cancers. Investigations have supported the potential and applications of MCM in cancer diagnosis and its therapeutics. In this article, we discuss the physiological roles of MCM and its associated factors in DNA replication and cancer pathogenesis.

Srs2 binding to PCNA and its sumoylation contribute to RPA antagonism during the DNA damage response

Activation of the DNA damage checkpoint upon genotoxin treatment induces a multitude of cellular changes, such as cell cycle arrest, to cope with genome stress. After prolonged genotoxin treatment, the checkpoint can be downregulated to allow cell cycle and growth resumption. In yeast, downregulation of the DNA damage checkpoint requires the Srs2 DNA helicase, which removes the ssDNA binding complex RPA and the associated Mec1 checkpoint kinase from DNA, thus dampening Mec1 activation. However, it is unclear whether the 'anti-checkpoint' role of Srs2 is temporally and spatially regulated to both allow timely checkpoint termination and to prevent superfluous RPA removal. Here we address this question by examining regulatory elements of Srs2, including its phosphorylation, sumoylation, and protein-interaction sites. Our genetic analyses and checkpoint level assessment suggest that the RPA countering role of Srs2 is promoted by Srs2 binding to PCNA, which is known to recruit Srs2 to subsets of ssDNA regions. RPA antagonism is further fostered by Srs2 sumoylation, which we found depends on the Srs2-PCNA interaction. Srs2 sumoylation is additionally reliant on Mec1 and peaks after Mec1 activity reaches maximal levels. Collectively, our data provide evidence for a two-step model wherein checkpoint downregulation is facilitated by PCNA-mediated Srs2 recruitment to ssDNA-RPA filaments and the subsequent Srs2 sumoylation stimulated upon Mec1 hyperactivation. We propose that this mechanism allows Mec1 hyperactivation to trigger checkpoint recovery.

Dynamics of DNA damage-induced nuclear inclusions are regulated by SUMOylation of Btn2

Spatial compartmentalization is a key facet of protein quality control that serves to store disassembled or non-native proteins until triage to the refolding or degradation machinery can occur in a regulated manner. Yeast cells sequester nuclear proteins at intranuclear quality control bodies (INQ) in response to various stresses, although the regulation of this process remains poorly understood. Here we reveal the SUMO modification of the small heat shock protein Btn2 under DNA damage and place Btn2 SUMOylation in a pathway promoting protein clearance from INQ structures. Along with other chaperones, and degradation machinery, Btn2-SUMO promotes INQ clearance from cells recovering from genotoxic stress. These data link small heat shock protein post-translational modification to the regulation of protein sequestration in the yeast nucleus.

PTEN-mediated dephosphorylation of 53BP1 confers cellular resistance to DNA damage in cancer cells

Homologous recombination (HR) repair for DNA double-strand breaks (DSBs) is critical for maintaining genome stability and conferring the resistance of tumor cells to chemotherapy. Nuclear PTEN which contains both phosphatidylinositol 3,4,5-trisphosphate 3-phosphatase and protein phosphatase plays a key role in HR repair, but the underlying mechanism remains largely elusive. We find that SUMOylated PTEN promotes HR repair but represses nonhomologous end joining (NHEJ) repair by directly dephosphorylating TP53-binding protein 1 (53BP1). During DNA damage responses (DDR), tumor suppressor ARF (p14ARF) was phosphorylated and then interacted efficiently with PTEN, thus promoting PTEN SUMOylation as an atypical SUMO E3 ligase. Interestingly, SUMOylated PTEN was subsequently recruited to the chromatin at DSB sites. This was because SUMO1 that was conjugated to PTEN was recognized and bound by the SUMO-interacting motif (SIM) of breast cancer type 1 susceptibility protein (BRCA1), which has been located to the core of 53BP1 foci on chromatin during S/G2 stage. Furthermore, these chromatin-loaded PTEN directly and specifically dephosphorylated phosphothreonine-543 (pT543) of 53BP1, resulting in the dissociation of the 53BP1 complex, which facilitated DNA end resection and ongoing HR repair. SUMOylation-site-mutated PTENK254R mice also showed decreased DNA damage repair in vivo. Blocking the PTEN SUMOylation pathway with either a SUMOylation inhibitor or a p14ARF(2-13) peptide sensitized tumor cells to chemotherapy. Our study therefore provides a new mechanistic understanding of PTEN in HR repair and clinical intervention of chemoresistant tumors.

Where and when to start: Regulating DNA replication origin activity in eukaryotic genomes

In eukaryotic genomes, hundreds to thousands of potential start sites of DNA replication named origins are dispersed across each of the linear chromosomes. During S-phase, only a subset of origins is selected in a stochastic manner to assemble bidirectional replication forks and initiate DNA synthesis. Despite substantial progress in our understanding of this complex process, a comprehensive 'identity code' that defines origins based on specific nucleotide sequences, DNA structural features, the local chromatin environment, or 3D genome architecture is still missing. In this article, we review the genetic and epigenetic features of replication origins in yeast and metazoan chromosomes and highlight recent insights into how this flexibility in origin usage contributes to nuclear organization, cell growth, differentiation, and genome stability.

SUMO in the regulation of DNA repair and transcription at nuclear pores

Two related post-translational modifications, the covalent linkage of Ubiquitin and the Small Ubiquitin-related MOdifier (SUMO) to lysine residues, play key roles in the regulation of both DNA repair pathway choice and transcription. Whereas ubiquitination is generally associated with proteasome-mediated protein degradation, the impact of sumoylation has been more mysterious. In the cell nucleus, sumoylation effects are largely mediated by the relocalization of the modified targets, particularly in response to DNA damage. This is governed in part by the concentration of SUMO protease at nuclear pores [Melchior, F et al. (2003) Trends Biochem Sci 28, 612-618; Ptak, C and Wozniak, RW (2017) Adv Exp Med Biol 963, 111-126]. We review here the roles of sumoylation in determining genomic locus positioning relative to the nuclear envelope and to nuclear pores, to facilitate repair and regulate transcription.

SUMOylation of yeast Pso2 enhances its translocation and accumulation in the mitochondria and suppresses methyl methanesulfonate-induced mitochondrial DNA damage

Saccharomyces cerevisiae Pso2/SNM1 is essential for DNA interstrand crosslink (ICL) repair; however, its mechanism of action remains incompletely understood. While recent work has revealed that Pso2/Snm1 is dual-localized in the nucleus and mitochondria, it remains unclear whether cell-intrinsic and -extrinsic factors regulate its subcellular localization and function. Herein, we show that Pso2 undergoes ubiquitination and phosphorylation, but not SUMOylation, in unstressed cells. Unexpectedly, we found that methyl methanesulfonate (MMS), rather than ICL-forming agents, induced robust SUMOylation of Pso2 on two conserved residues, K97 and K575, and that SUMOylation markedly increased its abundance in the mitochondria. Reciprocally, SUMOylation had no discernible impact on Pso2 translocation to the nucleus, despite the presence of steady-state levels of SUMOylated Pso2 across the cell cycle. Furthermore, substitution of the invariant residues K97 and K575 by arginine in the Pso2 SUMO consensus motifs severely impaired SUMOylation and abolished its translocation to the mitochondria of MMS-treated wild type cells, but not in unstressed cells. We demonstrate that whilst Siz1 and Siz2 SUMO E3 ligases catalyze Pso2 SUMOylation, the former plays a dominant role. Notably, we found that the phenotypic characteristics of the SUMOylation-defective mutant Pso2K97R/K575R closely mirrored those observed in the Pso2Δ petite mutant. Additionally, leveraging next-generation sequencing analysis, we demonstrate that Pso2 mitigates MMS-induced damage to mitochondrial DNA (mtDNA). Viewed together, our work offers previously unknown insights into the link between genotoxic stress-induced SUMOylation of Pso2 and its preferential targeting to the mitochondria, as well as its role in attenuating MMS-induced mtDNA damage.

A R-loop sensing pathway mediates the relocation of transcribed genes to nuclear pore complexes

Nuclear pore complexes (NPCs) have increasingly recognized interactions with the genome, as exemplified in yeast, where they bind transcribed or damaged chromatin. By combining genome-wide approaches with live imaging of model loci, we uncover a correlation between NPC association and the accumulation of R-loops, which are genotoxic structures formed through hybridization of nascent RNAs with their DNA templates. Manipulating hybrid formation demonstrates that R-loop accumulation per se, rather than transcription or R-loop-dependent damages, is the primary trigger for relocation to NPCs. Mechanistically, R-loop-dependent repositioning involves their recognition by the ssDNA-binding protein RPA, and SUMO-dependent interactions with NPC-associated factors. Preventing R-loop-dependent relocation leads to lethality in hybrid-accumulating conditions, while NPC tethering of a model hybrid-prone locus attenuates R-loop-dependent genetic instability. Remarkably, this relocation pathway involves molecular factors similar to those required for the association of stalled replication forks with NPCs, supporting the existence of convergent mechanisms for sensing transcriptional and genotoxic stresses.

Actionable loss of SLF2 drives B-cell lymphomagenesis and impairs the DNA damage response

The DNA damage response (DDR) acts as a barrier to malignant transformation and is often impaired during tumorigenesis. Exploiting the impaired DDR can be a promising therapeutic strategy; however, the mechanisms of inactivation and corresponding biomarkers are incompletely understood. Starting from an unbiased screening approach, we identified the SMC5-SMC6 Complex Localization Factor 2 (SLF2) as a regulator of the DDR and biomarker for a B-cell lymphoma (BCL) patient subgroup with an adverse prognosis. SLF2-deficiency leads to loss of DDR factors including Claspin (CLSPN) and consequently impairs CHK1 activation. In line with this mechanism, genetic deletion of Slf2 drives lymphomagenesis in vivo. Tumor cells lacking SLF2 are characterized by a high level of DNA damage, which leads to alterations of the post-translational SUMOylation pathway as a safeguard. The resulting co-dependency confers synthetic lethality to a clinically applicable SUMOylation inhibitor (SUMOi), and inhibitors of the DDR pathway act highly synergistic with SUMOi. Together, our results identify SLF2 as a DDR regulator and reveal co-targeting of the DDR and SUMOylation as a promising strategy for treating aggressive lymphoma.

Compartmentalization of the SUMO/RNF4 pathway by SLX4 drives DNA repair

SLX4, disabled in the Fanconi anemia group P, is a scaffolding protein that coordinates the action of structure-specific endonucleases and other proteins involved in the replication-coupled repair of DNA interstrand cross-links. Here, we show that SLX4 dimerization and SUMO-SIM interactions drive the assembly of SLX4 membraneless compartments in the nucleus called condensates. Super-resolution microscopy reveals that SLX4 forms chromatin-bound clusters of nanocondensates. We report that SLX4 compartmentalizes the SUMO-RNF4 signaling pathway. SENP6 and RNF4 regulate the assembly and disassembly of SLX4 condensates, respectively. SLX4 condensation per se triggers the selective modification of proteins by SUMO and ubiquitin. Specifically, SLX4 condensation induces ubiquitylation and chromatin extraction of topoisomerase 1 DNA-protein cross-links. SLX4 condensation also induces the nucleolytic degradation of newly replicated DNA. We propose that the compartmentalization of proteins by SLX4 through site-specific interactions ensures the spatiotemporal control of protein modifications and nucleolytic reactions during DNA repair.

SUMOylation regulates pre-mRNA splicing to overcome DNA damage in fungi

Pathogenic fungi are subject to DNA damage stress derived from host immune responses during infection. Small ubiquitin-like modifier (SUMO) modification and precursor (pre)-mRNA splicing are both involved in DNA damage response (DDR). However, the mechanisms of how SUMOylation and splicing coordinated in DDR remain largely unknown. Combining with biochemical analysis, RNA-Seq method, and biological analysis, we report that SUMO pathway participates in DDR and virulence in Fusarium graminearum, a causal agent of Fusarium head blight of cereal crops world-wide. Interestingly, a key transcription factor FgSR is SUMOylated upon DNA damage stress. SUMOylation regulates FgSR nuclear-cytoplasmic partitioning and its phosphorylation by FgMec1, and promotes its interaction with chromatin remodeling complex SWI/SNF for activating the expression of DDR-related genes. Moreover, the SWI/SNF complex was found to further recruit splicing-related NineTeen Complex, subsequently modulates pre-mRNA splicing during DDR. Our findings reveal a novel function of SUMOylation in DDR by regulating a transcription factor to orchestrate gene expression and pre-mRNA splicing to overcome DNA damage during the infection of F. graminearum, which advances the understanding of the delicate regulation of DDR by SUMOylation in pathogenic fungi, and extends the knowledge of cooperation of SUMOylation and pre-mRNA splicing in DDR in eukaryotes.

Alternative splicing of the SUMO1/2/3 transcripts affects cellular SUMOylation and produces functionally distinct SUMO protein isoforms

Substantial increases in the conjugation of the main human SUMO paralogs, SUMO1, SUMO2, and SUMO3, are observed upon exposure to different cellular stressors, and such increases are considered important to facilitate cell survival to stress. Despite their critical cellular role, little is known about how the levels of the SUMO modifiers are regulated in the cell, particularly as it relates to the changes observed upon stress. Here we characterize the contribution of alternative splicing towards regulating the expression of the main human SUMO paralogs under normalcy and three different stress conditions, heat-shock, cold-shock, and Influenza A Virus infection. Our data reveal that the normally spliced transcript variants are the predominant mature mRNAs produced from the SUMO genes and that the transcript coding for SUMO2 is by far the most abundant of all. We also provide evidence that alternatively spliced transcripts coding for protein isoforms of the prototypical SUMO proteins, which we refer to as the SUMO alphas, are also produced, and that their abundance and nuclear export are affected by stress in a stress- and cell-specific manner. Additionally, we provide evidence that the SUMO alphas are actively synthesized in the cell as their coding mRNAs are found associated with translating ribosomes. Finally, we provide evidence that the SUMO alphas are functionally different from their prototypical counterparts, with SUMO1α and SUMO2α being non-conjugatable to protein targets, SUMO3α being conjugatable but targeting a seemingly different subset of protein from those targeted by SUMO3, and all three SUMO alphas displaying different cellular distributions from those of the prototypical SUMOs. Thus, alternative splicing appears to be an important contributor to the regulation of the expression of the SUMO proteins and the cellular functions of the SUMOylation system.

Heat shock protein 27 deficiency promotes ferrous ion absorption and enhances acyl-Coenzyme A synthetase long-chain family member 4 stability to promote glioblastoma cell ferroptosis

BACKGROUND

Glioblastoma is one of the malignant tumors of the central nervous system with high lethality, high disability and low survival rate. Effective induction of its death is one of the existing challenges. In recent studies, heat shock protein 27 (HSP27) has been shown to be associated with ferroptosis; therefore, targeting HSP27 may be a potential therapeutic approach for GBM.

METHODS

Immunohistochemistry and western blot analysis were used to detect the expression of HSP27 in GBM tissues. CCK8, plate clone formation assay, EdU proliferation assay for cell proliferation ability, PI, LDH release assay for cell viability. Reactive oxygen, iron levels, and mitochondrial potential for HSP27 silencing were assayed for ferrotosis in vitro. Western blotting and IP were used to verify the relationship between HSP27 and ACSL4. The effect of knockdown of HSP27 on tumor growth capacity was assessed in an intracranial xenograft model.

RESULTS

HSP27 was significantly highly expressed in GBM. In vitro experiments, knockdown of HSP27 significantly induced ferroptosis in GBM cells. IP and western blot demonstrated a sumo-ization link between HSP27 and ACSL4. In vivo experiments, HSP27 deficiency retarded tumor growth rate by promoting ferroptosis.

CONCLUSIONS

HSP27 deficiency promotes GBM ferroptosis. Targeting HSP27 may serve as a new direction for GBM treatment.

Multi-layered chromatin proteomics identifies cell vulnerabilities in DNA repair

The DNA damage response (DDR) is essential to maintain genome stability, and its deregulation predisposes to carcinogenesis while encompassing attractive targets for cancer therapy. Chromatin governs the DDR via the concerted interplay among different layers, including DNA, histone post-translational modifications (hPTMs) and chromatin-associated proteins. Here, we employ multi-layered proteomics to characterize chromatin-mediated functional interactions of repair proteins, signatures of hPTMs and the DNA-bound proteome during DNA double-strand break (DSB) repair at high temporal resolution. Our data illuminate the dynamics of known and novel DDR-associated factors both at chromatin and at DSBs. We functionally attribute novel chromatin-associated proteins to repair by non-homologous end-joining (NHEJ), homologous recombination (HR) and DSB repair pathway choice. We reveal histone reader ATAD2, microtubule organizer TPX2 and histone methyltransferase G9A as regulators of HR and involved in poly-ADP-ribose polymerase-inhibitor sensitivity. Furthermore, we distinguish hPTMs that are globally induced by DNA damage from those specifically acquired at sites flanking DSBs (γH2AX foci-specific) and profiled their dynamics during the DDR. Integration of complementary chromatin layers implicates G9A-mediated monomethylation of H3K56 in DSBs repair via HR. Our data provide a dynamic chromatin-centered view of the DDR that can be further mined to identify novel mechanistic links and cell vulnerabilities in DSB repair.

Stress combined with loss of the Candida albicans SUMO protease Ulp2 triggers selection of aneuploidy via a two-step process

A delicate balance between genome stability and instability ensures genome integrity while generating genetic diversity, a critical step for evolution. Indeed, while excessive genome instability is harmful, moderated genome instability can drive adaptation to novel environments by maximising genetic variation. Candida albicans, a human fungal pathogen that colonises different parts of the human body, adapts rapidly and frequently to different hostile host microenvironments. In this organism, the ability to generate large-scale genomic variation is a key adaptative mechanism triggering dangerous infections even in the presence of antifungal drugs. Understanding how fitter novel karyotypes are selected is key to determining how C. albicans and other microbial pathogens establish infections. Here, we identified the SUMO protease Ulp2 as a regulator of C. albicans genome integrity through genetic screening. Deletion of ULP2 leads to increased genome instability, enhanced genome variation and reduced fitness in the absence of additional stress. The combined stress caused by the lack of ULP2 and antifungal drug treatment leads to the selection of adaptive segmental aneuploidies that partially rescue the fitness defects of ulp2Δ/Δ cells. Short and long-read genomic sequencing demonstrates that these novel genotypes are selected via a two-step process leading to the formation of novel chromosomal fragments with breakpoints at microhomology regions and DNA repeats.

Site-specific proteomic strategies to identify ubiquitin and SUMO modifications: Challenges and opportunities

Ubiquitin and SUMO modify thousands of substrates to regulate most cellular processes. System-wide identification of ubiquitin and SUMO substrates provides global understanding of their cellular functions. In this review, we discuss the biological importance of site-specific modifications by ubiquitin and SUMO regulating the DNA damage response, protein quality control and cell cycle progression. Furthermore we discuss the machinery responsible for these modifications and methods to purify and identify ubiquitin and SUMO modified sites by mass spectrometry. We provide a framework to aid in the selection of appropriate purification, digestion and acquisition strategies suited to answer different biological questions. We highlight opportunities in the field for employing innovative technologies, as well as discuss challenges and long-standing questions in the field that are difficult to address with the currently available tools, emphasizing the need for further innovation.

Role of SUMOylation in Neurodegenerative Diseases

Neurodegenerative diseases (NDDs) are irreversible, progressive diseases with no effective treatment. The hallmark of NDDs is the aggregation of misfolded, modified proteins, which impair neuronal vulnerability and cause brain damage. The loss of synaptic connection and the progressive loss of neurons result in cognitive defects. Several dysregulated proteins and overlapping molecular mechanisms contribute to the pathophysiology of NDDs. Post-translational modifications (PTMs) are essential regulators of protein function, trafficking, and maintaining neuronal hemostasis. The conjugation of a small ubiquitin-like modifier (SUMO) is a reversible, dynamic PTM required for synaptic and cognitive function. The onset and progression of neurodegenerative diseases are associated with aberrant SUMOylation. In this review, we have summarized the role of SUMOylation in regulating critical proteins involved in the onset and progression of several NDDs.

Yeast ORC sumoylation status fine-tunes origin licensing

Sumoylation is emerging as a posttranslation modification important for regulating chromosome duplication and stability. The origin recognition complex (ORC) that directs DNA replication initiation by loading the MCM replicative helicase onto origins is sumoylated in both yeast and human cells. However, the biological consequences of ORC sumoylation are unclear. Here we report the effects of hypersumoylation and hyposumoylation of yeast ORC on ORC activity and origin function using multiple approaches. ORC hypersumoylation preferentially reduced the function of a subset of early origins, while Orc2 hyposumoylation had an opposing effect. Mechanistically, ORC hypersumoylation reduced MCM loading in vitro and diminished MCM chromatin association in vivo. Either hypersumoylation or hyposumoylation of ORC resulted in genome instability and the dependence of yeast on other genome maintenance factors, providing evidence that appropriate ORC sumoylation levels are important for cell fitness. Thus, yeast ORC sumoylation status must be properly controlled to achieve optimal origin function across the genome and genome stability.

Site-specific MCM sumoylation prevents genome rearrangements by controlling origin-bound MCM

Timely completion of eukaryotic genome duplication requires coordinated DNA replication initiation at multiple origins. Replication begins with the loading of the Mini-Chromosome Maintenance (MCM) complex, proceeds by the activation of the Cdc45-MCM-GINS (CMG) helicase, and ends with CMG removal after chromosomes are fully replicated. Post-translational modifications on the MCM and associated factors ensure an orderly transit of these steps. Although the mechanisms of CMG activation and removal are partially understood, regulated MCM loading is not, leaving an incomplete understanding of how DNA replication begins. Here we describe a site-specific modification of Mcm3 by the Small Ubiquitin-like MOdifier (SUMO). Mutations that prevent this modification reduce the MCM loaded at replication origins and lower CMG levels, resulting in impaired cell growth, delayed chromosomal replication, and the accumulation of gross chromosomal rearrangements (GCRs). These findings demonstrate the existence of a SUMO-dependent regulation of origin-bound MCM and show that this pathway is needed to prevent genome rearrangements.

Faithful replication of the genome is essential for the survival and health of all living organisms. The eukaryotic genome presents a unique and difficult challenge: its enormous size demands the coordinated action of numerous DNA replication origins to ensure timely completion of genome duplication. Although the mechanisms that control the activation and removal of DNA replisome are partially understood, whether and how cells regulate the loading of the Mini-Chromosome Maintenance (MCM) complex, the precursor of the DNA replisome, at replication origins are not. Because mutations to MCM-loading factors and enzymes that catalyze reversible protein sumoylation cause substantial gross chromosomal rearrangements (GCRs) that characterize the cancer genome, understanding regulated MCM loading is one of the most pressing questions in the field. Here, we identified a site-specific SUMO modification of MCM and found that mutation disabling this modification causes severe growth defect and impaired DNA replication. These defects are attributable to reduced MCM at DNA replication origins, resulting in a lower DNA replisome level and a dramatic accumulation of GCRs. Thus, these findings identify a hitherto unknown regulatory mechanism: Site-specific MCM sumoylation regulates origin-bound MCM, and this prevents genome rearrangements.

Smc5/6 Complex Promotes Rad3ATR Checkpoint Signaling at the Perturbed Replication Fork through Sumoylation of the RecQ Helicase Rqh1

Smc5/6, like cohesin and condensin, is a structural maintenance of chromosomes complex crucial for genome stability. Unlike cohesin and condensin, Smc5/6 carries an essential Nse2 subunit with SUMO E3 ligase activity. While screening for new DNA replication checkpoint mutants in fission yeast, we have identified two previously uncharacterized mutants in Smc5/6. Characterization of the mutants and a series of previously reported Smc5/6 mutants uncovered that sumoylation of the RecQ helicase Rqh1 by Nse2 facilitates the checkpoint signaling at the replication fork. We found that mutations that eliminate the sumoylation sites or the helicase activity of Rqh1 compromised the checkpoint signaling similar to a nse2 mutant lacking the ligase activity. Surprisingly, introducing a sumoylation site mutation to a helicase-inactive rqh1 mutant promoted cell survival under stress. These findings, together with other genetic data, support a mechanism that sumoylation of Rqh1 by Smc5/6-Nse2 recruits Rqh1 or modulates its helicase activity at the fork to facilitate the checkpoint signaling. Since the Smc5/6 complex, Rqh1, and the replication checkpoint are conserved in eukaryotes, a similar checkpoint mechanism may be operating in human cells.

SUMO-mediated recruitment allows timely function of the Yen1 nuclease in mitotic cells

The post-translational modification of DNA damage response proteins with SUMO is an important mechanism to orchestrate a timely and orderly recruitment of repair factors to damage sites. After DNA replication stress and double-strand break formation, a number of repair factors are SUMOylated and interact with other SUMOylated factors, including the Yen1 nuclease. Yen1 plays a critical role in ensuring genome stability and unperturbed chromosome segregation by removing covalently linked DNA intermediates between sister chromatids that are formed by homologous recombination. Here we show how this important role of Yen1 depends on interactions mediated by non-covalent binding to SUMOylated partners. Mutations in the motifs that allow SUMO-mediated recruitment of Yen1 impair its ability to resolve DNA intermediates and result in chromosome mis-segregation and increased genome instability.

Sumoylation in Physiology, Pathology and Therapy

Sumoylation is an essential post-translational modification that has evolved to regulate intricate networks within emerging complexities of eukaryotic cells. Thousands of target substrates are modified by SUMO peptides, leading to changes in protein function, stability or localization, often by modulating interactions. At the cellular level, sumoylation functions as a key regulator of transcription, nuclear integrity, proliferation, senescence, lineage commitment and stemness. A growing number of prokaryotic and viral proteins are also emerging as prime sumoylation targets, highlighting the role of this modification during infection and in immune processes. Sumoylation also oversees epigenetic processes. Accordingly, at the physiological level, it acts as a crucial regulator of development. Yet, perhaps the most prominent function of sumoylation, from mammals to plants, is its role in orchestrating organismal responses to environmental stresses ranging from hypoxia to nutrient stress. Consequently, a growing list of pathological conditions, including cancer and neurodegeneration, have now been unambiguously associated with either aberrant sumoylation of specific proteins and/or dysregulated global cellular sumoylation. Therapeutic enforcement of sumoylation can also accomplish remarkable clinical responses in various diseases, notably acute promyelocytic leukemia (APL). In this review, we will discuss how this modification is emerging as a novel drug target, highlighting from the perspective of translational medicine, its potential and limitations.

SUMO-Based Regulation of Nuclear Positioning to Spatially Regulate Homologous Recombination Activities at Replication Stress Sites

DNA lesions have properties that allow them to escape their nuclear compartment to achieve DNA repair in another one. Recent studies uncovered that the replication fork, when its progression is impaired, exhibits increased mobility when changing nuclear positioning and anchors to nuclear pore complexes, where specific types of homologous recombination pathways take place. In yeast models, increasing evidence points out that nuclear positioning is regulated by small ubiquitin-like modifier (SUMO) metabolism, which is pivotal to maintaining genome integrity at sites of replication stress. Here, we review how SUMO-based pathways are instrumental to spatially segregate the subsequent steps of homologous recombination during replication fork restart. In particular, we discussed how routing towards nuclear pore complex anchorage allows distinct homologous recombination pathways to take place at halted replication forks.

Advances in SUMO-based regulation of homologous recombination

Homologous Recombination (HR) is a critical DNA repair mechanism for a range of genome lesions. HR is responsible for mending DNA double strand breaks (DSBs) using intact template DNA. In addition, many HR proteins help cope with DNA lesions generated from DNA replication and telomere deficiency. The functions of HR proteins are often regulated by protein modifications that can quickly and reversibly adjust substrate proteins' attributes. Sumoylation is one of the prevalent modifications that affects all steps of the HR processes and exerts diverse regulation on substrates. This review aims to summarize the most recent advances in our understanding of SUMO-based HR regulation and highlight some key questions that remain to be elucidated.

SUMO orchestrates multiple alternative DNA-protein crosslink repair pathways

Endogenous metabolites, environmental agents, and therapeutic drugs promote formation of covalent DNA-protein crosslinks (DPCs). Persistent DPCs compromise genome integrity and are eliminated by multiple repair pathways. Aberrant Top1-DNA crosslinks, or Top1ccs, are processed by Tdp1 and Wss1 functioning in parallel pathways in Saccharomyces cerevisiae. It remains obscure how cells choose between diverse mechanisms of DPC repair. Here, we show that several SUMO biogenesis factors (Ulp1, Siz2, Slx5, and Slx8) control repair of Top1cc or an analogous DPC lesion. Genetic analysis reveals that SUMO promotes Top1cc processing in the absence of Tdp1 but has an inhibitory role if cells additionally lack Wss1. In the tdp1Δ wss1Δ mutant, the E3 SUMO ligase Siz2 stimulates sumoylation in the vicinity of the DPC, but not SUMO conjugation to Top1. This Siz2-dependent sumoylation inhibits alternative DPC repair mechanisms, including Ddi1. Our findings suggest that SUMO tunes available repair pathways to facilitate faithful DPC repair.

DNA asymmetry promotes SUMO modification of the single-stranded DNA-binding protein RPA

Repair of DNA double‐stranded breaks by homologous recombination (HR) is dependent on DNA end resection and on post‐translational modification of repair factors. In budding yeast, single‐stranded DNA is coated by replication protein A (RPA) following DNA end resection, and DNA–RPA complexes are then SUMO‐modified by the E3 ligase Siz2 to promote repair. Here, we show using enzymatic assays that DNA duplexes containing 3' single‐stranded DNA overhangs increase the rate of RPA SUMO modification by Siz2. The SAP domain of Siz2 binds DNA duplexes and makes a key contribution to this process as highlighted by models and a crystal structure of Siz2 and by assays performed using protein mutants. Enzymatic assays performed using DNA that can accommodate multiple RPA proteins suggest a model in which the SUMO‐RPA signal is amplified by successive rounds of Siz2‐dependent SUMO modification of RPA and dissociation of SUMO‐RPA at the junction between single‐ and double‐stranded DNA. Our results provide insights on how DNA architecture scaffolds a substrate and E3 ligase to promote SUMO modification in the context of DNA repair.

DNA damage repair: historical perspectives, mechanistic pathways and clinical translation for targeted cancer therapy

Genomic instability is the hallmark of various cancers with the increasing accumulation of DNA damage. The application of radiotherapy and chemotherapy in cancer treatment is typically based on this property of cancers. However, the adverse effects including normal tissues injury are also accompanied by the radiotherapy and chemotherapy. Targeted cancer therapy has the potential to suppress cancer cells' DNA damage response through tailoring therapy to cancer patients lacking specific DNA damage response functions. Obviously, understanding the broader role of DNA damage repair in cancers has became a basic and attractive strategy for targeted cancer therapy, in particular, raising novel hypothesis or theory in this field on the basis of previous scientists' findings would be important for future promising druggable emerging targets. In this review, we first illustrate the timeline steps for the understanding the roles of DNA damage repair in the promotion of cancer and cancer therapy developed, then we summarize the mechanisms regarding DNA damage repair associated with targeted cancer therapy, highlighting the specific proteins behind targeting DNA damage repair that initiate functioning abnormally duo to extrinsic harm by environmental DNA damage factors, also, the DNA damage baseline drift leads to the harmful intrinsic targeted cancer therapy. In addition, clinical therapeutic drugs for DNA damage and repair including therapeutic effects, as well as the strategy and scheme of relative clinical trials were intensive discussed. Based on this background, we suggest two hypotheses, namely "environmental gear selection" to describe DNA damage repair pathway evolution, and "DNA damage baseline drift", which may play a magnified role in mediating repair during cancer treatment. This two new hypothesis would shed new light on targeted cancer therapy, provide a much better or more comprehensive holistic view and also promote the development of new research direction and new overcoming strategies for patients.

The Multifaceted Roles of Ku70/80

DNA double-strand breaks (DSBs) are accidental lesions generated by various endogenous or exogenous stresses. DSBs are also genetically programmed events during the V(D)J recombination process, meiosis, or other genome rearrangements, and they are intentionally generated to kill cancer during chemo- and radiotherapy. Most DSBs are processed in mammalian cells by the classical nonhomologous end-joining (c-NHEJ) pathway. Understanding the molecular basis of c-NHEJ has major outcomes in several fields, including radiobiology, cancer therapy, immune disease, and genome editing. The heterodimer Ku70/80 (Ku) is a central actor of the c-NHEJ as it rapidly recognizes broken DNA ends in the cell and protects them from nuclease activity. It subsequently recruits many c-NHEJ effectors, including nucleases, polymerases, and the DNA ligase 4 complex. Beyond its DNA repair function, Ku is also involved in several other DNA metabolism processes. Here, we review the structural and functional data on the DNA and RNA recognition properties of Ku implicated in DNA repair and in telomeres maintenance.

Rad52 SUMOylation functions as a molecular switch that determines a balance between the Rad51- and Rad59-dependent survivors

Functional telomeres in yeast lacking telomerase can be restored by rare Rad51- or Rad59-dependent recombination events that lead to type I and type II survivors, respectively. We previously proposed that polySUMOylation of proteins and the SUMO-targeted ubiquitin ligase Slx5-Slx8 are key factors in type II recombination. Here, we show that SUMOylation of Rad52 favors the formation of type I survivors. Conversely, preventing Rad52 SUMOylation partially bypasses the requirement of Slx5-Slx8 for type II recombination. We further report that SUMO-dependent proteasomal degradation favors type II recombination. Finally, inactivation of Rad59, but not Rad51, impairs the relocation of eroded telomeres to the Nuclear Pore complexes (NPCs). We propose that Rad59 cooperates with non-SUMOylated Rad52 to promote type II recombination at NPCs, resulting in the emergence of more robust survivors akin to ALT cancer cells. Finally, neither Rad59 nor Rad51 is required by itself for the survival of established type II survivors.

The Srs2 helicase dampens DNA damage checkpoint by recycling RPA from chromatin

The DNA damage checkpoint induces many cellular changes to cope with genotoxic stress. However, persistent checkpoint signaling can be detrimental to growth partly due to blockage of cell cycle resumption. Checkpoint dampening is essential to counter such harmful effects, but its mechanisms remain to be understood. Here, we show that the DNA helicase Srs2 removes a key checkpoint sensor complex, RPA, from chromatin to down-regulate checkpoint signaling in budding yeast. The Srs2 and RPA antagonism is supported by their numerous suppressive genetic interactions. Importantly, moderate reduction of RPA binding to single-strand DNA (ssDNA) rescues hypercheckpoint signaling caused by the loss of Srs2 or its helicase activity. This rescue correlates with a reduction in the accumulated RPA and the associated checkpoint kinase on chromatin in srs2 mutants. Moreover, our data suggest that Srs2 regulation of RPA is separable from its roles in recombinational repair and critically contributes to genotoxin resistance. We conclude that dampening checkpoint by Srs2-mediated RPA recycling from chromatin aids cellular survival of genotoxic stress and has potential implications in other types of DNA transactions.

Shared and distinct roles of Esc2 and Mms21 in suppressing genome rearrangements and regulating intracellular sumoylation

Protein sumoylation, especially when catalyzed by the Mms21 SUMO E3 ligase, plays a major role in suppressing duplication-mediated gross chromosomal rearrangements (dGCRs). How Mms21 targets its substrates in the cell is insufficiently understood. Here, we demonstrate that Esc2, a protein with SUMO-like domains (SLDs), recruits the Ubc9 SUMO conjugating enzyme to specifically facilitate Mms21-dependent sumoylation and suppress dGCRs. The D430R mutation in Esc2 impairs its binding to Ubc9 and causes a synergistic growth defect and accumulation of dGCRs with mutations that delete the Siz1 and Siz2 E3 ligases. By contrast, esc2-D430R does not appreciably affect sensitivity to DNA damage or the dGCRs caused by the catalytically inactive mms21-CH. Moreover, proteome-wide analysis of intracellular sumoylation demonstrates that esc2-D430R specifically down-regulates sumoylation levels of Mms21-preferred targets, including the nucleolar proteins, components of the SMC complexes and the MCM complex that acts as the catalytic core of the replicative DNA helicase. These effects closely resemble those caused by mms21-CH, and are relatively unaffected by deleting Siz1 and Siz2. Thus, by recruiting Ubc9, Esc2 facilitates Mms21-dependent sumoylation to suppress the accumulation of dGCRs independent of Siz1 and Siz2.

Esc2 orchestrates substrate-specific sumoylation by acting as a SUMO E2 cofactor in genome maintenance

In this study, Li et al. set out to investigate the conserved genome stability factor Esc2 in budding yeast and its roles in DNA damage-induced sumoylation. Using in vitro and in vivo approaches, the authors propose that Esc2 acts as a SUMO E2 cofactor at distinct DNA structures to promote the sumoylation of specific substrates and genome maintenance.

SUMO modification regulates diverse cellular processes by targeting hundreds of proteins. However, the limited number of sumoylation enzymes raises the question of how such a large number of substrates are efficiently modified. Specifically, how genome maintenance factors are dynamically sumoylated at DNA replication and repair sites to modulate their functions is poorly understood. Here, we demonstrate a role for the conserved yeast Esc2 protein in this process by acting as a SUMO E2 cofactor. Esc2 is required for genome stability and binds to Holliday junctions and replication fork structures. Our targeted screen found that Esc2 promotes the sumoylation of a Holliday junction dissolution complex and specific replisome proteins. Esc2 does not elicit these effects via stable interactions with substrates or their common SUMO E3. Rather, we show that a SUMO-like domain of Esc2 stimulates sumoylation by exploiting a noncovalent SUMO binding site on the E2 enzyme. This role of Esc2 in sumoylation is required for Holliday junction clearance and genome stability. Our findings thus suggest that Esc2 acts as a SUMO E2 cofactor at distinct DNA structures to promote the sumoylation of specific substrates and genome maintenance.

The nuclear pore primes recombination-dependent DNA synthesis at arrested forks by promoting SUMO removal

Nuclear Pore complexes (NPCs) act as docking sites to anchor particular DNA lesions facilitating DNA repair by elusive mechanisms. Using replication fork barriers in fission yeast, we report that relocation of arrested forks to NPCs occurred after Rad51 loading and its enzymatic activity. The E3 SUMO ligase Pli1 acts at arrested forks to safeguard integrity of nascent strands and generates poly-SUMOylation which promote relocation to NPCs but impede the resumption of DNA synthesis by homologous recombination (HR). Anchorage to NPCs allows SUMO removal by the SENP SUMO protease Ulp1 and the proteasome, promoting timely resumption of DNA synthesis. Preventing Pli1-mediated SUMO chains was sufficient to bypass the need for anchorage to NPCs and the inhibitory effect of poly-SUMOylation on HR-mediated DNA synthesis. Our work establishes a novel spatial control of Recombination-Dependent Replication (RDR) at a unique sequence that is distinct from mechanisms engaged at collapsed-forks and breaks within repeated sequences.

In yeast, collapsed forks shift to the nuclear periphery to associate with two distinct perinuclear anchorage sites such as the nuclear pore complex. Here, the authors reveal the mechanisms engaged at nuclear pore complex facilitating fork integrity and restart via SUMO regulation.

DNA Damage-Induced Nucleosome Depletion Enhances Homology Search Independently of Local Break Movement

To determine whether double-strand break (DSB) mobility enhances the physical search for an ectopic template during homology-directed repair (HDR), we tested the effects of factors that control chromatin dynamics, including cohesin loading and kinetochore anchoring. The former but not the latter is altered in response to DSBs. Loss of the nonhistone high-mobility group protein Nhp6 reduces histone occupancy and increases chromatin movement, decompaction, and ectopic HDR. The loss of nucleosome remodeler INO80-C did the opposite. To see whether enhanced HDR depends on DSB mobility or the global chromatin response, we tested the ubiquitin ligase mutant uls1Δ, which selectively impairs local but not global movement in response to a DSB. Strand invasion occurs in uls1Δ cells with wild-type kinetics, arguing that global histone depletion rather than DSB movement is rate limiting for HDR. Impaired break movement in uls1Δ correlates with elevated MRX and cohesin loading, despite normal resection and checkpoint activation.

Location, Location, Location: The Role of Nuclear Positioning in the Repair of Collapsed Forks and Protection of Genome Stability

Components of the nuclear pore complex (NPC) have been shown to play a crucial role in protecting against replication stress, and recovery from some types of stalled or collapsed replication forks requires movement of the DNA to the NPC in order to maintain genome stability. The role that nuclear positioning has on DNA repair has been investigated in several systems that inhibit normal replication. These include structure forming sequences (expanded CAG repeats), protein mediated stalls (replication fork barriers (RFBs)), stalls within the telomere sequence, and the use of drugs known to stall or collapse replication forks (HU + MMS or aphidicolin). Recently, the mechanism of relocation for collapsed replication forks to the NPC has been elucidated. Here, we will review the types of replication stress that relocate to the NPC, the current models for the mechanism of relocation, and the currently known protective effects of this movement.

Adenoviral strategies to overcome innate cellular responses to infection

Viruses alter host cell processes to optimize their replication cycle. Human adenoviruses (Ad) encode proteins that promote viral macromolecular synthesis and counteract innate and adaptive responses to infection. The focus of this review is on how Ad evades innate cellular responses to infection, including an interferon (IFN) response and a DNA damage response (DDR). Ad blocks the IFN response by inhibiting cytoplasmic signaling pathways and the activation of IFN-stimulated genes (ISGs), as well as the functions of ISG products, such as PML. Ad also inhibits DDR sensors, for instance, the Mre11-Rad50-Nbs1 complex, and DDR effectors like DNA ligase IV. These innate cellular responses impact many different viruses, and studies on Ad have provided broad insight into these areas.

Sumoylation of the DNA polymerase ε by the Smc5/6 complex contributes to DNA replication

DNA polymerase epsilon (Pol ε) is critical for genome duplication, but little is known about how post-translational modification regulates its function. Here we report that the Pol ε catalytic subunit Pol2 in yeast is sumoylated at a single lysine within a catalytic domain insertion uniquely possessed by Pol2 family members. We found that Pol2 sumoylation occurs specifically in S phase and is increased under conditions of replication fork blockade. Analyses of the genetic requirements of this modification indicate that Pol2 sumoylation is associated with replication fork progression and dependent on the Smc5/6 SUMO ligase known to promote DNA synthesis. Consistently, the pol2 sumoylation mutant phenotype suggests impaired replication progression and increased levels of gross chromosomal rearrangements. Our findings thus indicate a direct role for SUMO in Pol2-mediated DNA synthesis and a molecular basis for Smc5/6-mediated regulation of genome stability.

DNA replication factors are tightly regulated to ensure genome duplication accuracy and efficiency. Among these factors, the Pol ε replicative polymerase plays a vital role by copying half of the genome every cell cycle. However, little is known about how this critical enzyme is regulated. Here we describe SUMO-based regulation of the catalytic subunit of Pol ε, Pol2. Our data suggest that Pol2 sumoylation occurs during replication elongation, particularly when replication forks encounter template obstacles. This modification is mediated by the conserved Smc5/6 SUMO ligase complex and occurs at a single site within the Pol2 catalytic domain. Several observations suggest that Pol2 sumoylation makes positive contributions to the synthesis of DNA regions enriched with template barriers and helps to prevent large-scale genomic alterations. Our work thus provides new insights into DNA polymerase regulation, specifically the role played by contributions from SUMO and the Smc5/6 complex.

Asymmetric Processing of DNA Ends at a Double-Strand Break Leads to Unconstrained Dynamics and Ectopic Translocation

Multiple pathways regulate the repair of double-strand breaks (DSBs) to suppress potentially dangerous ectopic recombination. Both sequence and chromatin context are thought to influence pathway choice between non-homologous end-joining (NHEJ) and homology-driven recombination. To test the effect of repetitive sequences on break processing, we have inserted TG-rich repeats on one side of an inducible DSB at the budding yeast MAT locus on chromosome III. Five clustered Rap1 sites within a break-proximal TG repeat are sufficient to block Mre11-Rad50-Xrs2 recruitment, impair resection, and favor elongation by telomerase. The two sides of the break lose end-to-end tethering and show enhanced, uncoordinated movement. Only the TG-free side is resected and shifts to the nuclear periphery. In contrast to persistent DSBs without TG repeats that are repaired by imprecise NHEJ, nearly all survivors of repeat-proximal DSBs repair the break by a homology-driven, non-reciprocal translocation from ChrIII-R to ChrVII-L. This suppression of imprecise NHEJ at TG-repeat-flanked DSBs requires the Uls1 translocase activity.

Intricate SUMO-based control of the homologous recombination machinery

The homologous recombination (HR) machinery plays multiple roles in genome maintenance. Best studied in the context of DNA double-stranded break (DSB) repair, recombination enzymes can cleave, pair, and unwind DNA molecules, and collaborate with regulatory proteins to execute multiple DNA processing steps before generating specific repair products. HR proteins also help to cope with problems arising from DNA replication, modulating impaired replication forks or filling DNA gaps. Given these important roles, it is not surprising that each HR step is subject to complex regulation to adjust repair efficiency and outcomes as well as to limit toxic intermediates. Recent studies have revealed intricate regulation of all steps of HR by the protein modifier SUMO, which has been increasingly recognized for its broad influence in nuclear functions. This review aims to connect established roles of SUMO with its newly identified effects on recombinational repair and stimulate further thought on many unanswered questions.

The S phase checkpoint promotes the Smc5/6 complex dependent SUMOylation of Pol2, the catalytic subunit of DNA polymerase ε

Replication fork stalling and accumulation of single-stranded DNA trigger the S phase checkpoint, a signalling cascade that, in budding yeast, leads to the activation of the Rad53 kinase. Rad53 is essential in maintaining cell viability, but its targets of regulation are still partially unknown. Here we show that Rad53 drives the hyper-SUMOylation of Pol2, the catalytic subunit of DNA polymerase ε, principally following replication forks stalling induced by nucleotide depletion. Pol2 is the main target of SUMOylation within the replisome and its modification requires the SUMO-ligase Mms21, a subunit of the Smc5/6 complex. Moreover, the Smc5/6 complex co-purifies with Pol ε, independently of other replisome components. Finally, we map Pol2 SUMOylation to a single site within the N-terminal catalytic domain and identify a SUMO-interacting motif at the C-terminus of Pol2. These data suggest that the S phase checkpoint regulate Pol ε during replication stress through Pol2 SUMOylation and SUMO-binding ability.

SUMO-Chain-Regulated Proteasomal Degradation Timing Exemplified in DNA Replication Initiation

Similar to ubiquitin, SUMO forms chains, but the identity of SUMO-chain-modified factors and the purpose of this modification remain largely unknown. Here, we identify the budding yeast SUMO protease Ulp2, able to disassemble SUMO chains, as a DDK interactor enriched at replication origins that promotes DNA replication initiation. Replication-engaged DDK is SUMOylated on chromatin, becoming a degradation-prone substrate when Ulp2 no longer protects it against SUMO chain assembly. Specifically, SUMO chains channel DDK for SUMO-targeted ubiquitin ligase Slx5/Slx8-mediated and Cdc48 segregase-assisted proteasomal degradation. Importantly, the SUMOylation-defective ddk-KR mutant rescues inefficient replication onset and MCM activation in cells lacking Ulp2, suggesting that SUMO chains time DDK degradation. Using two unbiased proteomic approaches, we further identify subunits of the MCM helicase and other factors as SUMO-chain-modified degradation-prone substrates of Ulp2 and Slx5/Slx8. We thus propose SUMO-chain/Ulp2-protease-regulated proteasomal degradation as a mechanism that times the availability of functionally engaged SUMO-modified protein pools during replication and beyond.

Viperin catalyzes methionine oxidation to promote protein expression and function of helicases

Helicases play pivotal roles in fundamental biological processes, and posttranslational modifications regulate the localization, function, and stability of helicases. Here, we report that methionine oxidation of representative helicases, including DNA and RNA helicases of viral (ORF44 of KSHV) and cellular (MCM7 and RIG-I) origin, promotes their expression and functions. Cellular viperin, a major antiviral interferon-stimulated gene whose functions beyond host defense remain largely unknown, catalyzes the methionine oxidation of these helicases. Moreover, biochemical studies entailing loss-of-function mutations of helicases and a pharmacological inhibitor interfering with lipid metabolism and, hence, decreasing viperin activity indicate that methionine oxidation potently increases the stability and enzyme activity of these helicases that are critical for DNA replication and immune activation. Our work uncovers a pivotal role of viperin in catalyzing the methionine oxidation of helicases that are implicated in diverse fundamental biological processes.

Flap endonuclease overexpression drives genome instability and DNA damage hypersensitivity in a PCNA-dependent manner

Overexpression of the flap endonuclease FEN1 has been observed in a variety of cancer types and is a marker for poor prognosis. To better understand the cellular consequences of FEN1 overexpression we utilized a model of its Saccharomyces cerevisiae homolog, RAD27. In this system, we discovered that flap endonuclease overexpression impedes replication fork progression and leads to an accumulation of cells in mid-S phase. This was accompanied by increased phosphorylation of the checkpoint kinase Rad53 and histone H2A-S129. RAD27 overexpressing cells were hypersensitive to treatment with DNA damaging agents, and defective in ubiquitinating the replication clamp proliferating cell nuclear antigen (PCNA) at lysine 164. These effects were reversed when the interaction between overexpressed Rad27 and PCNA was ablated, suggesting that the observed phenotypes were linked to problems in DNA replication. RAD27 overexpressing cells also exhibited an unexpected dependence on the SUMO ligases SIZ1 and MMS21 for viability. Importantly, we found that overexpression of FEN1 in human cells also led to phosphorylation of CHK1, CHK2, RPA32 and histone H2AX, all markers of genome instability. Our data indicate that flap endonuclease overexpression is a driver of genome instability in yeast and human cells that impairs DNA replication in a manner dependent on its interaction with PCNA.

Molecular Basis for Control of Diverse Genome Stability Factors by the Multi-BRCT Scaffold Rtt107

BRCT domains support myriad protein-protein interactions involved in genome maintenance. Although di-BRCT recognition of phospho-proteins is well known to support the genotoxic response, whether multi-BRCT domains can acquire distinct structures and functions is unclear. Here we present the tetra-BRCT structures from the conserved yeast protein Rtt107 in free and ligand-bound forms. The four BRCT repeats fold into a tetrahedral structure that recognizes unmodified ligands using a bi-partite mechanism, suggesting repeat origami enabling function acquisition. Functional studies show that Rtt107 binding of partner proteins of diverse activities promotes genome replication and stability in both distinct and concerted manners. A unified theme is that tetra- and di-BRCT domains of Rtt107 collaborate to recruit partner proteins to chromatin. Our work thus illustrates how a master regulator uses two types of BRCT domains to recognize distinct genome factors and direct them to chromatin for constitutive genome protection.

Post-Translational Modifications of the Mini-Chromosome Maintenance Proteins in DNA Replication

The eukaryotic mini-chromosome maintenance (MCM) complex, composed of MCM proteins 2-7, is the core component of the replisome that acts as the DNA replicative helicase to unwind duplex DNA and initiate DNA replication. MCM10 tightly binds the cell division control protein 45 homolog (CDC45)/MCM2-7/ DNA replication complex Go-Ichi-Ni-San (GINS) (CMG) complex that stimulates CMG helicase activity. The MCM8-MCM9 complex may have a non-essential role in activating the pre-replicative complex in the gap 1 (G1) phase by recruiting cell division cycle 6 (CDC6) to the origin recognition complex (ORC). Each MCM subunit has a distinct function achieved by differential post-translational modifications (PTMs) in both DNA replication process and response to replication stress. Such PTMs include phosphorylation, ubiquitination, small ubiquitin-like modifier (SUMO)ylation, O-N-acetyl-D-glucosamine (GlcNAc)ylation, and acetylation. These PTMs have an important role in controlling replication progress and genome stability. Because MCM proteins are associated with various human diseases, they are regarded as potential targets for therapeutic development. In this review, we summarize the different PTMs of the MCM proteins, their involvement in DNA replication and disease development, and the potential therapeutic implications.

SUMOylation in Glioblastoma: A Novel Therapeutic Target

Protein SUMOylation is a dynamic post-translational modification which is involved in a diverse set of physiologic processes throughout the cell. Of note, SUMOylation also plays a role in the pathobiology of a myriad of cancers, one of which is glioblastoma (GBM). Accordingly, herein, we review core aspects of SUMOylation as it relates to GBM and in so doing highlight putative methods/modalities capable of therapeutically engaging the pathway for treatment of this deadly neoplasm.

SUMOylation of the nuclear pore complex basket is involved in sensing cellular stresses

The nuclear pore complex (NPC) is the major conduit for nucleocytoplasmic transport and serves as a platform for gene regulation and DNA repair. Several nucleoporins undergo ubiquitylation and SUMOylation, and these modifications play an important role in nuclear pore dynamics and plasticity. Here, we perform a detailed analysis of these post-translational modifications of yeast nuclear basket proteins under normal growth conditions as well as upon cellular stresses, with a focus on SUMOylation. We find that the balance between the dynamics of SUMOylation and deSUMOylation of Nup60 and Nup2 at the NPC differs substantially, particularly in G1 and S phase. While Nup60 is the unique target of genotoxic stress within the nuclear basket that probably belongs to the SUMO-mediated DNA damage response pathway, both Nup2 and Nup60 show a dramatic increase in SUMOylation upon osmotic stress, with Nup2 SUMOylation being enhanced in Nup60 SUMO-deficient mutant yeast strains. Taken together, our data reveal that there are several levels of crosstalk between nucleoporins, and that the post-translational modifications of the NPC serve in sensing cellular stress signals.

Summary: Post-translational modifications, and in particular SUMOylation, of the nuclear basket subcomplex of the nuclear pore complex serve in its function as a sensor for mediating cellular stress signals.