A dynamic model for replication protein A (RPA) function in DNA processing pathways

The N-terminus of RPA large subunit and its spatial position are important for the 5'->3' resection of DNA double-strand breaks

The first step of homology-dependent repair of DNA double-strand breaks (DSBs) is the resection of the 5' strand to generate 3' ss-DNA. Of the two major nucleases responsible for resection, EXO1 has intrinsic 5'->3' directionality, but DNA2 does not. DNA2 acts with RecQ helicases such as the Werner syndrome protein (WRN) and the heterotrimeric eukaryotic ss-DNA binding protein RPA. We have found that the N-terminus of the RPA large subunit (RPA1N) interacts with both WRN and DNA2 and is essential for stimulating WRN's 3'->5' helicase activity and DNA2's 5'->3' ss-DNA exonuclease activity. A mutant RPA complex that lacks RPA1N is unable to support resection in Xenopus egg extracts and human cells. Furthermore, relocating RPA1N to the middle subunit but not to the small subunit causes severe defects in stimulating DNA2 and WRN and in supporting resection. Together, these findings suggest that RPA1N and its spatial position are critical for restricting the directionality of the WRN-DNA2 resection pathway.

Maternal BCAS2 protects genomic integrity in mouse early embryonic development

Mammalian early embryos maintain accurate genome integrity for proper development within a programmed timeline despite constant assaults on their DNA by replication, DNA demethylation and genetic defects transmitted from germ cells. However, how genome integrity is safeguarded during mammalian early embryonic development remains unclear. BCAS2 (breast carcinoma amplified sequence 2), a core component of the PRP19 complex involved in pre-mRNA splicing, plays an important role in the DNA damage response through the RPA complex, a key regulator in the maintenance of genome integrity. Currently, the physiological role of BCAS2 in mammals is unknown. We now report that BCAS2 responds to endogenous and exogenous DNA damage in mouse zygotes. Maternal depletion of BCAS2 compromises the DNA damage response in early embryos, leading to developmental arrest at the two- to four-cell stage accompanied by the accumulation of damaged DNA and micronuclei. Furthermore, BCAS2 mutants that are unable to bind RPA1 fail in DNA repair during the zygotic stage. In addition, phosphorylated RPA2 cannot localise to the DNA damage sites in mouse zygotes with disrupted maternal BCAS2. These data suggest that BCAS2 might function through the RPA complex during DNA repair in zygotes. Together, our results reveal that maternal BCAS2 maintains the genome integrity of early embryos and is essential for female mouse fertility.

PTEN regulates RPA1 and protects DNA replication forks

Tumor suppressor PTEN regulates cellular activities and controls genome stability through multiple mechanisms. In this study, we report that PTEN is necessary for the protection of DNA replication forks against replication stress. We show that deletion of PTEN leads to replication fork collapse and chromosomal instability upon fork stalling following nucleotide depletion induced by hydroxyurea. PTEN is physically associated with replication protein A 1 (RPA1) via the RPA1 C-terminal domain. STORM and iPOND reveal that PTEN is localized at replication sites and promotes RPA1 accumulation on replication forks. PTEN recruits the deubiquitinase OTUB1 to mediate RPA1 deubiquitination. RPA1 deletion confers a phenotype like that observed in PTEN knockout cells with stalling of replication forks. Expression of PTEN and RPA1 shows strong correlation in colorectal cancer. Heterozygous disruption of RPA1 promotes tumorigenesis in mice. These results demonstrate that PTEN is essential for DNA replication fork protection. We propose that RPA1 is a target of PTEN function in fork protection and that PTEN maintains genome stability through regulation of DNA replication.

Managing Single-Stranded DNA during Replication Stress in Fission Yeast

Replication fork stalling generates a variety of responses, most of which cause an increase in single-stranded DNA. ssDNA is a primary signal of replication distress that activates cellular checkpoints. It is also a potential source of genome instability and a substrate for mutation and recombination. Therefore, managing ssDNA levels is crucial to chromosome integrity. Limited ssDNA accumulation occurs in wild-type cells under stress. In contrast, cells lacking the replication checkpoint cannot arrest forks properly and accumulate large amounts of ssDNA. This likely occurs when the replication fork polymerase and helicase units are uncoupled. Some cells with mutations in the replication helicase (mcm-ts) mimic checkpoint-deficient cells, and accumulate extensive areas of ssDNA to trigger the G2-checkpoint. Another category of helicase mutant (mcm4-degron) causes fork stalling in early S-phase due to immediate loss of helicase function. Intriguingly, cells realize that ssDNA is present, but fail to detect that they accumulate ssDNA, and continue to divide. Thus, the cellular response to replication stalling depends on checkpoint activity and the time that replication stress occurs in S-phase. In this review we describe the signs, signals, and symptoms of replication arrest from an ssDNA perspective. We explore the possible mechanisms for these effects. We also advise the need for caution when detecting and interpreting data related to the accumulation of ssDNA.

The Replication Checkpoint Prevents Two Types of Fork Collapse without Regulating Replisome Stability

The ATR replication checkpoint ensures that stalled forks remain stable when replisome movement is impeded. Using an improved iPOND protocol combined with SILAC mass spectrometry, we characterized human replisome dynamics in response to fork stalling. Our data provide a quantitative picture of the replisome and replication stress response proteomes in 32 experimental conditions. Importantly, rather than stabilize the replisome, the checkpoint prevents two distinct types of fork collapse. Unsupervised hierarchical clustering of protein abundance on nascent DNA is sufficient to identify protein complexes and place newly identified replisome-associated proteins into functional pathways. As an example, we demonstrate that ZNF644 complexes with the G9a/GLP methyltransferase at replication forks and is needed to prevent replication-associated DNA damage. Our data reveal how the replication checkpoint preserves genome integrity, provide insights into the mechanism of action of ATR inhibitors, and will be a useful resource for replication, DNA repair, and chromatin investigators.

Mycobacterium tuberculosis RecG protein but not RuvAB or RecA protein is efficient at remodeling the stalled replication forks: implications for multiple mechanisms of replication restart in mycobacteria

Aberrant DNA replication, defects in the protection, and restart of stalled replication forks are major causes of genome instability in all organisms. Replication fork reversal is emerging as an evolutionarily conserved physiological response for restart of stalled forks. Escherichia coli RecG, RuvAB, and RecA proteins have been shown to reverse the model replication fork structures in vitro. However, the pathways and the mechanisms by which Mycobacterium tuberculosis, a slow growing human pathogen, responds to different types of replication stress and DNA damage are unclear. Here, we show that M. tuberculosis RecG rescues E. coli ΔrecG cells from replicative stress. The purified M. tuberculosis RecG (MtRecG) and RuvAB (MtRuvAB) proteins catalyze fork reversal of model replication fork structures with and without a leading strand single-stranded DNA gap. Interestingly, single-stranded DNA-binding protein suppresses the MtRecG- and MtRuvAB-mediated fork reversal with substrates that contain lagging strand gap. Notably, our comparative studies with fork structures containing template damage and template switching mechanism of lesion bypass reveal that MtRecG but not MtRuvAB or MtRecA is proficient in driving the fork reversal. Finally, unlike MtRuvAB, we find that MtRecG drives efficient reversal of forks when fork structures are tightly bound by protein. These results provide direct evidence and valuable insights into the underlying mechanism of MtRecG-catalyzed replication fork remodeling and restart pathways in vivo.

Hydrocarbon stapled peptides as modulators of biological function

Peptide-based drug discovery has experienced a significant upturn within the past decade since the introduction of chemical modifications and unnatural amino acids has allowed for overcoming some of the drawbacks associated with peptide therapeutics. Strengthened by such features, modified peptides become capable of occupying a niche that emerges between the two major classes of today's therapeutics-small molecules (<500 Da) and biologics (>5000 Da). Stabilized α-helices have proven particularly successful at impairing disease-relevant PPIs previously considered "undruggable." Among those, hydrocarbon stapled α-helical peptides have emerged as a novel class of potential peptide therapeutics. This review provides a comprehensive overview of the development and applications of hydrocarbon stapled peptides discussing the benefits and limitations of this technique.

Analysis of difference of association between polymorphisms in the XRCC5, RPA3 and RTEL1 genes and glioma, astrocytoma and glioblastoma

BACKGROUND

Gliomas are the most common aggressive brain tumors and have many complex pathological types. Previous reports have discovered that genetic mutations are associated with the risk of glioma. However, it is unclear whether uniform genetic mutations exist difference between glioma and its two pathological types in the Han Chinese population.

MATERIALS AND METHODS

We evaluated 20 SNPs of 703 glioma cases (338 astrocytoma cases, 122 glioblastoma cases) and 635 controls in a Han Chinese population using χ(2) test and genetic model analysis.

RESULTS

In three case-control studies, we found rs9288516 in XRCC5 gene showed a decreased risk of glioma (OR, 0.85; 95% CI, 0.73-0.99; P = 0.042) and glioblastoma (OR, 0.70; 95% CI, 0.52-0.92; P = 0.001) in the allele model. We identified rs414805 in RPA3 gene showed an increased risk of glioblastoma in allele model (OR, 1.38; 95% CI, 1.00-1.89; P = 0.047) and dominant model (OR, 1.57; 95% CI, 1.05-2.35; P = 0.027), analysis respectively. Meanwhile, rs2297440 in RTEL1 gene showed an increased risk of glioma (OR, 1.30; 95% CI, 1.10-1.54; P = 0.002) and astrocytoma (OR, 1.26; 95% CI, 1.02-1.54; P = 0.029) in the allele model. In addition, we also observed a haplotype of "GCT" in the RTEL1 gene with an increased risk of astrocytoma (P = 0.005).

CONCLUSIONS

Polymorphisms in the XRCC5, RPA3 and RTEL1 genes, combinating with previous reaserches, are associated with glioma developing. However, those genes mutations may play different roles in the glioma, astrocytoma and glioblastoma, respectively.

REV7 counteracts DNA double-strand break resection and affects PARP inhibition

Error-free repair of DNA double-strand breaks (DSBs) is achieved by homologous recombination (HR), and BRCA1 is an important factor for this repair pathway. In the absence of BRCA1-mediated HR, the administration of PARP inhibitors induces synthetic lethality of tumour cells of patients with breast or ovarian cancers. Despite the benefit of this tailored therapy, drug resistance can occur by HR restoration. Genetic reversion of BRCA1-inactivating mutations can be the underlying mechanism of drug resistance, but this does not explain resistance in all cases. In particular, little is known about BRCA1-independent restoration of HR. Here we show that loss of REV7 (also known as MAD2L2) in mouse and human cell lines re-establishes CTIP-dependent end resection of DSBs in BRCA1-deficient cells, leading to HR restoration and PARP inhibitor resistance, which is reversed by ATM kinase inhibition. REV7 is recruited to DSBs in a manner dependent on the H2AX-MDC1-RNF8-RNF168-53BP1 chromatin pathway, and seems to block HR and promote end joining in addition to its regulatory role in DNA damage tolerance. Finally, we establish that REV7 blocks DSB resection to promote non-homologous end-joining during immunoglobulin class switch recombination. Our results reveal an unexpected crucial function of REV7 downstream of 53BP1 in coordinating pathological DSB repair pathway choices in BRCA1-deficient cells.

Replication stress in Mammalian cells and its consequences for mitosis

The faithful transmission of genetic information to daughter cells is central to maintaining genomic stability and relies on the accurate and complete duplication of genetic material during each cell cycle. However, the genome is routinely exposed to endogenous and exogenous stresses that can impede the progression of replication. Such replication stress can be an early cause of cancer or initiate senescence. Replication stress, which primarily occurs during S phase, results in consequences during mitosis, jeopardizing chromosome segregation and, in turn, genomic stability. The traces of replication stress can be detected in the daughter cells during G1 phase. Alterations in mitosis occur in two types: 1) local alterations that correspond to breaks, rearrangements, intertwined DNA molecules or non-separated sister chromatids that are confined to the region of the replication dysfunction; 2) genome-wide chromosome segregation resulting from centrosome amplification (although centrosomes do not contain DNA), which amplifies the local replication stress to the entire genome. Here, we discuss the endogenous causes of replication perturbations, the mechanisms of replication fork restart and the consequences for mitosis, chromosome segregation and genomic stability.

Functional dynamics in replication protein A DNA binding and protein recruitment domains

Replication Protein A (RPA) is an essential scaffold for many DNA processing machines; its function relies on its modular architecture. Here, we report (15)N-nuclear magnetic resonance heteronuclear relaxation analysis to characterize the movements of single-stranded (ss) DNA binding and protein interaction modules in the RPA70 subunit. Our results provide direct evidence for coordination of the motion of the tandem RPA70AB ssDNA binding domains. Moreover, binding of ssDNA substrate is found to cause dramatic reorientation and full coupling of inter-domain motion. In contrast, the RPA70N protein interaction domain remains structurally and dynamically independent of RPA70AB regardless of binding of ssDNA. This autonomy of motion between the 70N and 70AB modules supports a model in which the two binding functions of RPA are mediated fully independently, but remain differentially coordinated depending on the length of their flexible tethers. A critical role for linkers between the globular domains in determining the functional dynamics of RPA is proposed.

Mechanochemical regulations of RPA's binding to ssDNA

Replication protein A (RPA) is a ubiquitous eukaryotic single-stranded DNA (ssDNA) binding protein that serves to protect ssDNA from degradation and annealing, and as a template for recruitment of many downstream factors in virtually all DNA transactions in cell. During many of these transactions, DNA is tethered and is likely subject to force. Previous studies of RPA's binding behavior on ssDNA were conducted in the absence of force; therefore the RPA-ssDNA conformations regulated by force remain unclear. Here, using a combination of atomic force microscopy imaging and mechanical manipulation of single ssDNA tethers, we show that force mediates a switch of the RPA bound ssDNA from amorphous aggregation to a much more regular extended conformation. Further, we found an interesting non-monotonic dependence of the binding affinity on monovalent salt concentration in the presence of force. In addition, we discovered that zinc in micromolar concentrations drives ssDNA to a unique, highly stiff and more compact state. These results provide new mechanochemical insights into the influences and the mechanisms of action of RPA on large single ssDNA.

Characterization of the interaction between Rfa1 and Rad24 in Saccharomyces cerevisiae

Maintaining the integrity of the genome requires the high fidelity duplication of the genome and the ability of the cell to recognize and repair DNA lesions. The heterotrimeric single stranded DNA (ssDNA) binding complex Replication Protein A (RPA) is central to multiple DNA processes, which are coordinated by RPA through its ssDNA binding function and through multiple protein-protein interactions. Many RPA interacting proteins have been reported through large genetic and physical screens; however, the number of interactions that have been further characterized is limited. To gain a better understanding of how RPA functions in DNA replication, repair, and cell cycle regulation and to identify other potential functions of RPA, a yeast two hybrid screen was performed using the yeast 70 kDa subunit, Replication Factor A1 (Rfa1), as a bait protein. Analysis of 136 interaction candidates resulted in the identification of 37 potential interacting partners, including the cell cycle regulatory protein and DNA damage clamp loader Rad24. The Rfa1-Rad24 interaction is not dependent on ssDNA binding. However, this interaction appears affected by DNA damage. The regions of both Rfa1 and Rad24 important for this interaction were identified, and the region of Rad24 identified is distinct from the region reported to be important for its interaction with Rfc2 5. This suggests that Rad24-Rfc2-5 (Rad24-RFC) recruitment to DNA damage substrates by RPA occurs, at least partially, through an interaction between the N terminus of Rfa1 and the C terminus of Rad24. The predicted structure and location of the Rad24 C-terminus is consistent with a model in which RPA interacts with a damage substrate, loads Rad24-RFC at the 5’ junction, and then releases the Rad24-RFC complex to allow for proper loading and function of the DNA damage clamp.

Simian virus Large T antigen interacts with the N-terminal domain of the 70 kD subunit of Replication Protein A in the same mode as multiple DNA damage response factors

Simian virus 40 (SV40) serves as an important model organism for studying eukaryotic DNA replication. Its helicase, Large T-antigen (Tag), is a multi-functional protein that interacts with multiple host proteins, including the ubiquitous ssDNA binding protein Replication Protein A (RPA). Tag recruits RPA, actively loads it onto the unwound DNA, and together they promote priming of the template. Although interactions of Tag with RPA have been mapped, no interaction between Tag and the N-terminal protein interaction domain of the RPA 70kDa subunit (RPA70N) has been reported. Here we provide evidence of direct physical interaction of Tag with RPA70N and map the binding sites using a series of pull-down and mutational experiments. In addition, a monoclonal anti-Tag antibody, the epitope of which overlaps with the binding site, blocks the binding of Tag to RPA70N. We use NMR chemical shift perturbation analysis to show that Tag uses the same basic cleft in RPA70N as multiple of DNA damage response proteins. Mutations in the binding sites of both RPA70N and Tag demonstrate that specific charge reversal substitutions in either binding partner strongly diminish the interaction. These results expand the known repertoire of contacts between Tag and RPA, which mediate the many critical roles of Tag in viral replication.

Diphenylpyrazoles as replication protein a inhibitors

Replication Protein A is the primary eukaryotic ssDNA binding protein that has a central role in initiating the cellular response to DNA damage. RPA recruits multiple proteins to sites of DNA damage via the N-terminal domain of the 70 kDa subunit (RPA70N). Here we describe the optimization of a diphenylpyrazole carboxylic acid series of inhibitors of these RPA-protein interactions. We evaluated substituents on the aromatic rings as well as the type and geometry of the linkers used to combine fragments, ultimately leading to submicromolar inhibitors of RPA70N protein-protein interactions.

A proposal: Evolution of PCNA's role as a marker of newly replicated DNA

Processivity clamps that hold DNA polymerases to DNA for processivity were the first proteins known to encircle the DNA duplex. At the time, polymerase processivity was thought to be the only function of ring shaped processivity clamps. But studies from many laboratories have identified numerous proteins that bind and function with sliding clamps. Among these processes are mismatch repair and nucleosome assembly. Interestingly, there exist polymerases that are highly processive and do not require clamps. Hence, DNA polymerase processivity does not intrinsically require that sliding clamps evolved for this purpose. We propose that polymerases evolved to require clamps as a way of ensuring that clamps are deposited on newly replicated DNA. These clamps are then used on the newly replicated daughter strands, for processes important to genomic integrity, such as mismatch repair and the assembly of nucleosomes to maintain epigenetic states of replicating cells during development.

The DNA damage response and checkpoint adaptation in Saccharomyces cerevisiae: distinct roles for the replication protein A2 (Rfa2) N-terminus

In response to DNA damage, two general but fundamental processes occur in the cell: (1) a DNA lesion is recognized and repaired, and (2) concomitantly, the cell halts the cell cycle to provide a window of opportunity for repair to occur. An essential factor for a proper DNA-damage response is the heterotrimeric protein complex Replication Protein A (RPA). Of particular interest is hyperphosphorylation of the 32-kDa subunit, called RPA2, on its serine/threonine-rich amino (N) terminus following DNA damage in human cells. The unstructured N-terminus is often referred to as the phosphorylation domain and is conserved among eukaryotic RPA2 subunits, including Rfa2 in Saccharomyces cerevisiae. An aspartic acid/alanine-scanning and genetic interaction approach was utilized to delineate the importance of this domain in budding yeast. It was determined that the Rfa2 N-terminus is important for a proper DNA-damage response in yeast, although its phosphorylation is not required. Subregions of the Rfa2 N-terminus important for the DNA-damage response were also identified. Finally, an Rfa2 N-terminal hyperphosphorylation-mimetic mutant behaves similarly to another Rfa1 mutant (rfa1-t11) with respect to genetic interactions, DNA-damage sensitivity, and checkpoint adaptation. Our data indicate that post-translational modification of the Rfa2 N-terminus is not required for cells to deal with "repairable" DNA damage; however, post-translational modification of this domain might influence whether cells proceed into M-phase in the continued presence of unrepaired DNA lesions as a "last-resort" mechanism for cell survival.

A tight control of Rif1 by Oct4 and Smad3 is critical for mouse embryonic stem cell stability

Prolonged culture of embryonic stem cells (ESCs) leads them to adopt embryonal carcinoma cell features, creating enormous dangers for their further application. The mechanism involved in ESC stability has not, however, been extensively studied. We previously reported that SMAD family member 3 (Smad3) has an important role in maintaining mouse ESC stability, as depletion of Smad3 results in cancer cell-like properties in ESCs and Smad3-/- ESCs are prone to grow large, malignant teratomas. To understand how Smad3 contributes to ESC stability, we performed microarray analysis to compare the transcriptome of wild-type and Smad3-/- ESCs. We found that Rif1 (RAP1-associated protein 1), a factor important for genomic stability, is significantly upregulated in Smad3-/- ESCs. The expression level of Rif1 needs to be tightly controlled in ESCs, as a low level of Rif1 is associated with ESC differentiation, but a high level of Rif1 is linked to ESC transformation. In ESCs, Oct4 activates Rif1, whereas Smad3 represses its expression. Oct4 recruits Smad3 to bind to Rif1 promoter, but Smad3 joining facilitates the loading of a polycomb complex that generates a repressive epigenetic modification on Rif1 promoter, and thus maintains the expression of Rif1 at a proper level in ESCs. Interestingly, Rif1 short hairpin RNA (shRNA)-transduced Smad3-/- ESCs showed less malignant properties than the control shRNA-transduced Smad3-/- ESCs, suggesting a critical role of Rif1 in maintaining the stability of ESCs during proliferation.

Cell cycle specific distribution of killin: evidence for negative regulation of both DNA and RNA synthesis

p53 tumor-suppressor gene is a master transcription factor which controls cell cycle progression and apoptosis. killin was discovered as one of the p53 target genes implicated in S-phase control coupled to cell death. Due to its extreme proximity to pten tumor-suppressor gene on human chromosome 10, changes in epigenetic modification of killin have also been linked to Cowden syndrome as well as other human cancers. Previous studies revealed that Killin is a high-affinity DNA-binding protein with preference to single-stranded DNA, and it inhibits DNA synthesis in vitro and in vivo. Here, co-localization studies of RFP-Killin with either GFP-PCNA or endogenous single-stranded DNA binding protein RPA during S-phase show that Killin always adopts a mutually exclusive punctuated nuclear expression pattern with the 2 accessory proteins in DNA replication. In contrast, when cells are not in S-phase, RFP-Killin largely congregates in the nucleolus where rRNA transcription normally occurs. Both of these cell cycle specific localization patterns of RFP-Killin are stable under high salt condition, consistent with Killin being tightly associated with nucleic acids within cell nuclei. Together, these cell biological results provide a molecular basis for Killin in competitively inhibiting the formation of DNA replication forks during S-phase, as well as potentially negatively regulate RNA synthesis during other cell cycle phases.

Immunofluorescence-based methods to monitor DNA end resection

Double-strand breaks (DSBs) are the most deleterious among all types of DNA damage that can occur in the cell. These breaks arise from both endogenous (e.g., DNA replication stress) and exogenous insults (e.g., ionizing radiation). DSBs are principally repaired by one of two major pathways: nonhomologous end joining (NHEJ) or homologous recombination (HR). NHEJ is an error-prone process that can occur in all phases of the cell cycle, while HR is limited to the S and G2 phases of the cell cycle when a sister chromatid is available as a template for error-free repair. The first step in HR is "DNA end resection," a process during which the broken DNA end is converted into a long stretch of 3'-ended single-stranded DNA (ssDNA). In recent years, DNA end resection has been identified as a pivotal step that controls "repair pathway choice," i.e., the appropriate choice between NHEJ and HR for DSB repair. Therefore, methods to quantitatively or semiquantitatively assess DNA end resection have gained importance in laboratories working on DNA repair. In this chapter, we describe two simple immunofluorescence-based techniques to monitor DNA end resection in mammalian cells. The first technique involves immuno-detection of replication protein A (RPA), an ssDNA-binding protein that binds to resected DNA. The second technique involves labeling of genomic DNA with 5-bromo-2'-deoxyuridine (BrdU) that can be detected by anti-BrdU antibody only after the DNA becomes single stranded due to resection. These methods are not complicated, do not involve sophisticated instrumentation or reporter constructs, and can be applied to most mammalian cell lines and, therefore, should be of broad utility as simple ways of monitoring DNA end resection in vivo.

High-affinity DNA-binding domains of replication protein A (RPA) direct SMARCAL1-dependent replication fork remodeling

SMARCAL1 catalyzes replication fork remodeling to maintain genome stability. It is recruited to replication forks via an interaction with replication protein A (RPA), the major ssDNA-binding protein in eukaryotic cells. In addition to directing its localization, RPA also activates SMARCAL1 on some fork substrates but inhibits it on others, thereby conferring substrate specificity to SMARCAL1 fork-remodeling reactions. We investigated the mechanism by which RPA regulates SMARCAL1. Our results indicate that although an interaction between SMARCAL1 and RPA is essential for SMARCAL1 activation, the location of the interacting surface on RPA is not. Counterintuitively, high-affinity DNA binding of RPA DNA-binding domain (DBD) A and DBD-B near the fork junction makes it easier for SMARCAL1 to remodel the fork, which requires removing RPA. We also found that RPA DBD-C and DBD-D are not required for SMARCAL1 regulation. Thus, the orientation of the high-affinity RPA DBDs at forks dictates SMARCAL1 substrate specificity.

Characteristics and concepts of dynamic hub proteins in DNA processing machinery from studies of RPA

DNA replication, damage response and repair require the coordinated action of multi-domain proteins operating within dynamic multi-protein machines that act upon the DNA substrate. These modular proteins contain flexible linkers of various lengths, which enable changes in the spatial distribution of the globular domains (architecture) that harbor their essential biochemical functions. This mobile architecture is uniquely suited to follow the evolving substrate landscape present over the course of the specific process performed by the multi-protein machinery. A fundamental advance in understanding of protein machinery is the realization of the pervasive role of dynamics. Not only is the machine undergoing dynamic transformations, but the proteins themselves are flexible and constantly adapting to the progression through the steps of the overall process. Within this dynamic context the activity of the constituent proteins must be coordinated, a role typically played by hub proteins. A number of important characteristics of modular proteins and concepts about the operation of dynamic machinery have been discerned. These provide the underlying basis for the action of the machinery that reads DNA, and responds to and repairs DNA damage. Here, we introduce a number of key characteristics and concepts, including the modularity of the proteins, linkage of weak binding sites, direct competition between sites, and allostery, using the well recognized hub protein replication protein A (RPA).

RNA-processing proteins regulate Mec1/ATR activation by promoting generation of RPA-coated ssDNA

Eukaryotic cells respond to DNA double-strand breaks (DSBs) by activating a checkpoint that depends on the protein kinases Tel1/ATM and Mec1/ATR. Mec1/ATR is activated by RPA-coated single-stranded DNA (ssDNA), which arises upon nucleolytic degradation (resection) of the DSB. Emerging evidences indicate that RNA-processing factors play critical, yet poorly understood, roles in genomic stability. Here, we provide evidence that the Saccharomyces cerevisiae RNA decay factors Xrn1, Rrp6 and Trf4 regulate Mec1/ATR activation by promoting generation of RPA-coated ssDNA. The lack of Xrn1 inhibits ssDNA generation at the DSB by preventing the loading of the MRX complex. By contrast, DSB resection is not affected in the absence of Rrp6 or Trf4, but their lack impairs the recruitment of RPA, and therefore of Mec1, to the DSB. Rrp6 and Trf4 inactivation affects neither Rad51/Rad52 association nor DSB repair by homologous recombination (HR), suggesting that full Mec1 activation requires higher amount of RPA-coated ssDNA than HR-mediated repair. Noteworthy, deep transcriptome analyses do not identify common misregulated gene expression that could explain the observed phenotypes. Our results provide a novel link between RNA processing and genome stability.

Leishmania braziliensis replication protein A subunit 1: molecular modelling, protein expression and analysis of its affinity for both DNA and RNA

Background

Replication factor A (RPA) is a single-strand DNA binding protein involved in DNA replication, recombination and repair processes. It is composed by the subunits RPA-1, RPA-2 and RPA-3; the major DNA-binding activity resides in the subunit 1 of the heterotrimeric RPA complex. In yeast and higher eukaryotes, besides the three basic structural DNA-binding domains, the RPA-1 subunit contains an N-terminal region involved in protein-protein interactions with a fourth DNA-binding domain. Remarkably, the N-terminal extension is absent in the RPA-1 of the pathogenic protozoan Leishmania (Leishmania) amazonensis; however, the protein maintains its ability to bind ssDNA. In a recent work, we identify Leishmania (Viannia) braziliensis RPA-1 by its specific binding to the untranslated regions of the HSP70 mRNAs, suggesting that this protein might be also an RNA-binding protein.

Methods

Both rLbRPA-1 purified by His-tag affinity chromatography as well as the in vitro transcribed L. braziliensis 3′ HSP70-II UTR were used to perform pull down assays to asses nucleic acid binding properties. Also, homology modeling was carried out to construct the LbRPA-1 tridimensional structure to search relevant amino acid residues to bind nucleic acids.

Results

In this work, after obtaining the recombinant L. braziliensis RPA-1 protein under native conditions, competitive and non-competitive pull-down assays confirmed the single-stranded DNA binding activity of this protein and demonstrated its interaction with the 3′ UTR from the HSP70-II mRNA. As expected, this protein exhibits a high affinity for ssDNA, but we have found that RPA-1 interacts also with RNA. Additionally, we carried out a structural analysis of L. braziliensis RPA-1 protein using the X-ray diffraction structure of Ustilago maydis homologous protein as a template. Our results indicate that, in spite of the evolutionary divergence between both organisms, the structure of these two RPA-1 proteins seems to be highly conserved.

Conclusion

The LbRPA-1 protein is a ssDNA binding protein, but also it shows affinity in vitro for the HSP70 mRNA; this finding supports a possible in vivo role in the HSP70 mRNA metabolism. On the other hand, the three dimensional model of Leishmania RPA-1 serves as a starting point for both functional analysis and its exploration as a chemotherapeutic target to combat leishmaniasis.

Phosphorylation and cellular function of the human Rpa2 N-terminus in the budding yeast Saccharomyces cerevisiae

Maintenance of genome integrity is critical for proper cell growth. This occurs through accurate DNA replication and repair of DNA lesions. A key factor involved in both DNA replication and the DNA damage response is the heterotrimeric single-stranded DNA (ssDNA) binding complex Replication Protein A (RPA). Although the RPA complex appears to be structurally conserved throughout eukaryotes, the primary amino acid sequence of each subunit can vary considerably. Examination of sequence differences along with the functional interchangeability of orthologous RPA subunits or regions could provide insight into important regions and their functions. This might also allow for study in simpler systems. We determined that substitution of yeast Replication Factor A (RFA) with human RPA does not support yeast cell viability. Exchange of a single yeast RFA subunit with the corresponding human RPA subunit does not function due to lack of inter-species subunit interactions. Substitution of yeast Rfa2 with domains/regions of human Rpa2 important for Rpa2 function (i.e., the N-terminus and the loop 3-4 region) supports viability in yeast cells, and hybrid proteins containing human Rpa2 N-terminal phospho-mutations result in similar DNA damage phenotypes to analogous yeast Rfa2 N-terminal phospho-mutants. Finally, the human Rpa2 N-terminus (NT) fused to yeast Rfa2 is phosphorylated in a manner similar to human Rpa2 in human cells, indicating that conserved kinases recognize the human domain in yeast. The implication is that budding yeast represents a potential model system for studying not only human Rpa2 N-terminal phosphorylation, but also phosphorylation of Rpa2 N-termini from other eukaryotic organisms.

RPA-coated single-stranded DNA as a platform for post-translational modifications in the DNA damage response

The Replication Protein A (RPA) complex is an essential regulator of eukaryotic DNA metabolism. RPA avidly binds to single-stranded DNA (ssDNA) through multiple oligonucleotide/oligosaccharide-binding folds and coordinates the recruitment and exchange of genome maintenance factors to regulate DNA replication, recombination and repair. The RPA-ssDNA platform also constitutes a key physiological signal which activates the master ATR kinase to protect and repair stalled or collapsed replication forks during replication stress. In recent years, the RPA complex has emerged as a key target and an important regulator of post-translational modifications in response to DNA damage, which is critical for its genome guardian functions. Phosphorylation and SUMOylation of the RPA complex, and more recently RPA-regulated ubiquitination, have all been shown to control specific aspects of DNA damage signaling and repair by modulating the interactions between RPA and its partners. Here, we review our current understanding of the critical functions of the RPA-ssDNA platform in the maintenance of genome stability and its regulation through an elaborate network of covalent modifications.

RPA-1 from Leishmania amazonensis (LaRPA-1) structurally differs from other eukaryote RPA-1 and interacts with telomeric DNA via its N-terminal OB-fold domain

Replication protein A-1 (RPA-1) is a single-stranded DNA-binding protein involved in DNA metabolism. We previously demonstrated the interaction between LaRPA-1 and telomeric DNA. Here, we expressed and purified truncated mutants of LaRPA-1 and used circular dichroism measurements and molecular dynamics simulations to demonstrate that the tertiary structure of LaRPA-1 differs from human and yeast RPA-1. LaRPA-1 interacts with telomeric ssDNA via its N-terminal OB-fold domain, whereas RPA from higher eukaryotes show different binding modes to ssDNA. Our results show that LaRPA-1 is evolutionary distinct from other RPA-1 proteins and can potentially be used for targeting trypanosomatid telomeres.

Replication protein A prevents promiscuous annealing between short sequence homologies: Implications for genome integrity

Replication protein A (RPA) is the main eukaryotic single-stranded DNA (ssDNA) binding protein, having essential roles in all DNA metabolic reactions involving ssDNA. RPA binds ssDNA with high affinity, thereby preventing the formation of secondary structures and protecting ssDNA from the action of nucleases, and directly interacts with other DNA processing proteins. Here, we discuss recent results supporting the idea that one function of RPA is to prevent annealing between short repeats that can lead to chromosome rearrangements by microhomology-mediated end joining or the formation of hairpin structures that are substrates for structure-selective nucleases. We suggest that replication fork catastrophe caused by depletion of RPA could result from cleavage of secondary structures by nucleases, and that failure to cleave hairpin structures formed at DNA ends could lead to gene amplification. These studies highlight the important role RPA plays in maintaining genome integrity.

Chemical inhibitor targeting the replication protein A-DNA interaction increases the efficacy of Pt-based chemotherapy in lung and ovarian cancer

Platinum-based chemotherapeutics exert their therapeutic efficacy via the formation of DNA adducts which interfere with DNA replication, transcription and cell division and ultimately induce cell death. Repair and tolerance of these Pt-DNA lesions by nucleotide excision repair (NER) and homologous recombination (HR) can substantially reduce the effectiveness of therapy. Inhibition of these repair pathways, therefore, holds the potential to sensitize cancer cells to Pt treatment and increase clinical efficacy. Replication Protein A (RPA) plays essential roles in both NER and HR, along with its role in DNA replication and DNA damage checkpoint activation. Each of these functions is, in part, mediated by RPA binding to single-stranded DNA (ssDNA). Here we report the synthesis and characterization of novel derivatives of RPA small molecule inhibitors and their activity in models of epithelial ovarian cancer (EOC) and non-small cell lung cancer (NSCLC). We have synthesized analogs of our previously reported RPA inhibitor TDRL-505 and determined the structure-activity relationships. These data led us to the identification of TDRL-551, which exhibited a greater than 2-fold increase in in vitro activity. TDRL-551 showed synergy with Pt in tissue culture models of EOC and in vivo efficacy, as a single agent and in combination with platinum, in a NSCLC xenograft model. These data demonstrate the utility of RPA inhibition in EOC and NSCLC and the potential in developing novel anticancer therapeutics that target RPA-DNA interactions.

Architecture and ssDNA interaction of the Timeless-Tipin-RPA complex

The Timeless-Tipin (Tim-Tipin) complex, also referred to as the fork protection complex, is involved in coordination of DNA replication. Tim-Tipin is suggested to be recruited to replication forks via Replication Protein A (RPA) but details of the interaction are unknown. Here, using cryo-EM and biochemical methods, we characterized complex formation of Tim-Tipin, RPA and single-stranded DNA (ssDNA). Tim-Tipin and RPA form a 258 kDa complex with a 1:1:1 stoichiometry. The cryo-EM 3D reconstruction revealed a globular architecture of the Tim-Tipin-RPA complex with a ring-like and a U-shaped domain covered by a RPA lid. Interestingly, RPA in the complex adopts a horse shoe-like shape resembling its conformation in the presence of long ssDNA (>30 nucleotides). Furthermore, the recruitment of the Tim-Tipin-RPA complex to ssDNA is modulated by the RPA conformation and requires RPA to be in the more compact 30 nt ssDNA binding mode. The dynamic formation and disruption of the Tim-Tipin-RPA-ssDNA complex implicates the RPA-based recruitment of Tim-Tipin to the replication fork.

Quality control of homologous recombination

Exogenous and endogenous genotoxic agents, such as ionizing radiation and numerous chemical agents, cause DNA double-strand breaks (DSBs), which are highly toxic and lead to genomic instability or tumorigenesis if not repaired accurately and efficiently. Cells have over evolutionary time developed certain repair mechanisms in response to DSBs to maintain genomic integrity. Major DSB repair mechanisms include non-homologous end joining and homologous recombination (HR). Using sister homologues as templates, HR is a high-fidelity repair pathway that can rejoin DSBs without introducing mutations. However, HR execution without appropriate guarding may lead to more severe gross genome rearrangements. Here we review current knowledge regarding the factors and mechanisms required for accomplishment of accurate HR.

Characterization of the cryptic AV3 promoter of ageratum yellow vein virus in prokaryotic and eukaryotic systems

A cryptic prokaryotic promoter, designated AV3 promoter, has been previously identified in certain begomovirus genus, including ageratum yellow vein virus isolate NT (AYVV-NT). In this study, we demonstrated that the core nucleotides in the putative -10 and -35 boxes are necessary but not sufficient for promoter activity in Escherichia coli, and showed that AYVV-NT AV3 promoter could specifically interact with single-stranded DNA-binding protein and sigma 70 of E. coli involved in transcription. Several AYVV-NT-encoded proteins were found to increase the activity of AV3 promoter. The transcription start sites downstream to AV3 promoter were mapped to nucleotide positions 803 or 805 in E. coli, and 856 in Nicotiana benthamiana. The eukaryotic activity of AV3 promoter and the translatability of a short downstream open reading frame were further confirmed by using a green fluorescent protein reporter construct in yeast (Saccharomyces cerevisiae) cells. These results suggested that AV3 promoter might be a remnant of evolution that retained cryptic activity at present.

Replication protein A: single-stranded DNA's first responder: dynamic DNA-interactions allow replication protein A to direct single-strand DNA intermediates into different pathways for synthesis or repair

Replication protein A (RPA), the major single-stranded DNA-binding protein in eukaryotic cells, is required for processing of single-stranded DNA (ssDNA) intermediates found in replication, repair, and recombination. Recent studies have shown that RPA binding to ssDNA is highly dynamic and that more than high-affinity binding is needed for function. Analysis of DNA binding mutants identified forms of RPA with reduced affinity for ssDNA that are fully active, and other mutants with higher affinity that are inactive. Single molecule studies showed that while RPA binds ssDNA with high affinity, the RPA complex can rapidly diffuse along ssDNA and be displaced by other proteins that act on ssDNA. Finally, dynamic DNA binding allows RPA to prevent error-prone repair of double-stranded breaks and promote error-free repair. Together, these findings suggest a new paradigm where RPA acts as a first responder at sites with ssDNA, thereby actively coordinating DNA repair and DNA synthesis.

Regulation of Rfa2 phosphorylation in response to genotoxic stress in Candida albicans

Successful pathogens must be able to swiftly respond to and repair DNA damages inflicted by the host defence. The replication protein A (RPA) complex plays multiple roles in DNA damage response and is regulated by phosphorylation. However, the regulators of RPA phosphorylation remain unclear. Here, we investigated Rfa2 phosphorylation in the pathogenic fungus Candida albicans. Rfa2, a RFA subunit, is phosphorylated when DNA replication is inhibited by hydroxyurea and dephosphorylated during the recovery. By screening a phosphatase mutant library, we found that Pph3 associates with different regulatory subunits to differentially control Rfa2 dephosphorylation in stressed and unstressed cells. Site-directed mutagenesis revealed T11, S18, S29, and S30 being critical for Rfa2 phosphorylation in response to genotoxic insult. We obtained evidence that the genome integrity checkpoint kinase Mec1 and the cyclin-dependent kinase Clb2-Cdc28 mediate Rfa2 phosphorylation. Although cells expressing either a phosphomimetic or a non-phosphorylatable version of Rfa2 had defects, the latter exhibited greater sensitivity to genotoxic challenge, failure to repair DNA damages and to deactivate Rad53-mediated checkpoint pathways in a dosage-dependent manner. These mutants were also less virulent in mice. Our results provide important new insights into the regulatory mechanism and biological significance of Rfa2 phosphorylation in C. albicans.

Different replication protein A complexes of Arabidopsis thaliana have different DNA-binding properties as a function of heterotrimer composition

The heterotrimeric RPA (replication protein A) protein complex has single-stranded DNA-binding functions that are important for all DNA processing pathways in eukaryotic cells. In Arabidopsis thaliana, which has five homologs of the RPA1 subunit and two homologs each of RPA2 and RPA3, in theory 20 RPA complexes could form. Using Escherichia coli as a heterologous expression system and analysing the results of the co-purification of the different subunits, we conclude that AtRPA1a interacts with the AtRPA2b subunit, and AtRPA1b interacts with AtRPA2a. Additionally either AtRPA3a or AtRPA3b is part of the complexes. As shown by electrophoretic mobility shift assays, all of the purified AtRPA complexes bind single-stranded DNA, but differences in DNA binding, especially with respect to modified DNA, could be revealed for all four of the analyzed RPA complexes. Thus, the RPA3 subunits influence the DNA-binding properties of the complexes differently despite their high degree of similarity of 82%. The data support the idea that in plants a subfunctionalization of RPA homologs has occurred and that different complexes act preferentially in different pathways.

Diffusion of human replication protein A along single-stranded DNA

Replication protein A (RPA) is a eukaryotic single-stranded DNA (ssDNA) binding protein that plays critical roles in most aspects of genome maintenance, including replication, recombination and repair. RPA binds ssDNA with high affinity, destabilizes DNA secondary structure and facilitates binding of other proteins to ssDNA. However, RPA must be removed from or redistributed along ssDNA during these processes. To probe the dynamics of RPA-DNA interactions, we combined ensemble and single-molecule fluorescence approaches to examine human RPA (hRPA) diffusion along ssDNA and find that an hRPA heterotrimer can diffuse rapidly along ssDNA. Diffusion of hRPA is functional in that it provides the mechanism by which hRPA can transiently disrupt DNA hairpins by diffusing in from ssDNA regions adjacent to the DNA hairpin. hRPA diffusion was also monitored by the fluctuations in fluorescence intensity of a Cy3 fluorophore attached to the end of ssDNA. Using a novel method to calibrate the Cy3 fluorescence intensity as a function of hRPA position on the ssDNA, we estimate a one-dimensional diffusion coefficient of hRPA on ssDNA of D1~5000nt(2) s(-1) at 37°C. Diffusion of hRPA while bound to ssDNA enables it to be readily repositioned to allow other proteins access to ssDNA.

Structure of RPA32 bound to the N-terminus of SMARCAL1 redefines the binding interface between RPA32 and its interacting proteins

Replication protein A subunit RPA32 contains a C-terminal domain that interacts with a variety of DNA damage response proteins including SMARCAL1, Tipin, UNG2 and XPA. We have solved the high-resolution crystal structure of RPA32 C-terminal domain (RPA32C) in complex with a 26-amino-acid peptide derived from the N-terminus of SMARCAL1 (SMARCAL1N). The RPA32C-SMARCAL1N structure reveals a 1 : 1 binding stoichiometry and displays a well-ordered binding interface. SMARCAL1N adopts a long α-helical conformation with the highly conserved 11 residues aligned on one face of the α-helix showing extensive interactions with the RPA32C domain. Extensive mutagenesis experiments were performed to corroborate the interactions observed in crystal structure. Moreover, the α1/α2 loop of the RPA32C domain undergoes a conformational rearrangement upon SMARCAL1N binding. NMR study has further confirmed that the RPA32C-SMARCAL1N interaction induces conformational changes in RPA32C. Isothermal titration calorimetry studies have also demonstrated that the conserved α-helical motif defined in the current study is required for sufficient binding of RPA32C. Taken together, our study has provided convincing structural information that redefines the common recognition pattern shared by RPA32C interacting proteins.

DATABASE

The atomic coordinates of RPA32C in complex with 26-aa SMARCAL1 (SMARCAL1N) peptide have been deposited at the Protein Data Bank with accession code 4MQV.

STRUCTURED DIGITAL ABSTRACT

RPA32 and SMARCAL1 bind by isothermal titration calorimetry(1, 2, 3, 4, 5, 6, 7, 8, 9) RPA32 and SMARCAL1 bind by molecular sieving (View interaction) RPA32 and SMARCAL1 bind by x-ray crystallography (View interaction) Tipin and RPA32 bind by isothermal titration calorimetry (1, 2) RPA32 and UNG2 bind by isothermal titration calorimetry (1, 2, 3) SMARCAL1 and RPA32 bind by nuclear magnetic resonance (View interaction) UNG2 and RPA32 bind by nuclear magnetic resonance (View interaction) Tipin and RPA32 bind by nuclear magnetic resonance (View interaction).

Structural analysis of replication protein A recruitment of the DNA damage response protein SMARCAL1

SWI/SNF-related, matrix-associated, actin-dependent regulator of chromatin, subfamily A-like1 (SMARCAL1) is a recently identified DNA damage response protein involved in remodeling stalled replication forks. The eukaryotic single-strand DNA binding protein replication protein A (RPA) recruits SMARCAL1 to stalled forks in vivo and facilitates regression of forks containing leading strand gaps. Both activities are mediated by a direct interaction between an RPA binding motif (RBM) at the N-terminus of SMARCAL1 and the C-terminal winged-helix domain of the RPA 32 kDa subunit (RPA32C). Here we report a biophysical and structural characterization of the SMARCAL1–RPA interaction. Isothermal titration calorimetry and circular dichroism spectroscopy revealed that RPA32C binds SMARCAL1-RBM with a K_d of 2.5 μM and induces a disorder-to-helix transition. The crystal structure of RPA32C was refined to 1.4 Å resolution, and the SMARCAL1-RBM binding site was mapped on the structure on the basis of nuclear magnetic resonance chemical shift perturbations. Conservation of the interaction surface to other RBM-containing proteins allowed construction of a model for the RPA32C/SMARCAL1-RBM complex. The implications of our results are discussed with respect to the recruitment of SMARCAL1 and other DNA damage response and repair proteins to stalled replication forks.

Discovery of a potent stapled helix peptide that binds to the 70N domain of replication protein A

Stapled helix peptides can serve as useful tools for inhibiting protein-protein interactions but can be difficult to optimize for affinity. Here we describe the discovery and optimization of a stapled helix peptide that binds to the N-terminal domain of the 70 kDa subunit of replication protein A (RPA70N). In addition to applying traditional optimization strategies, we employed a novel approach for efficiently designing peptides containing unnatural amino acids. We discovered hot spots in the target protein using a fragment-based screen, identified the amino acid that binds to the hot spot, and selected an unnatural amino acid to incorporate, based on the structure-activity relationships of small molecules that bind to this site. The resulting stapled helix peptide potently and selectively binds to RPA70N, does not disrupt ssDNA binding, and penetrates cells. This peptide may serve as a probe to explore the therapeutic potential of RPA70N inhibition in cancer.

Structural basis of SOSS1 complex assembly and recognition of ssDNA

The SOSS1 complex comprising SOSSA, SOSSB1, and SOSSC senses single-stranded DNA (ssDNA) and promotes repair of DNA double-strand breaks (DSBs). But how SOSS1 is assembled and recognizes ssDNA remains elusive. The crystal structure of the N-terminal half of SOSSA (SOSSAN) in complex with SOSSB1 and SOSSC showed that SOSSAN serves as a scaffold to bind both SOSSB1 and SOSSC for assembly of the SOSS1 complex. The structures of SOSSAN/B1 in complex with a 12 nt ssDNA and SOSSAN/B1/C in complex with a 35 nt ssDNA showed that SOSSB1 interacts with both SOSSAN and ssDNA via two distinct surfaces. Recognition of ssDNA with a length of up to nine nucleotides is mediated solely by SOSSB1, whereas neither SOSSC nor SOSSAN are critical for ssDNA binding. These results reveal the structural basis of SOSS1 assembly and provide a framework for further study of the mechanism governing longer ssDNA recognition by the SOSS1 complex during DSB repair.

ATR prohibits replication catastrophe by preventing global exhaustion of RPA

ATR, activated by replication stress, protects replication forks locally and suppresses origin firing globally. Here, we show that these functions of ATR are mechanistically coupled. Although initially stable, stalled forks in ATR-deficient cells undergo nucleus-wide breakage after unscheduled origin firing generates an excess of single-stranded DNA that exhausts the nuclear pool of RPA. Partial reduction of RPA accelerated fork breakage, and forced elevation of RPA was sufficient to delay such "replication catastrophe" even in the absence of ATR activity. Conversely, unscheduled origin firing induced breakage of stalled forks even in cells with active ATR. Thus, ATR-mediated suppression of dormant origins shields active forks against irreversible breakage via preventing exhaustion of nuclear RPA. This study elucidates how replicating genomes avoid destabilizing DNA damage. Because cancer cells commonly feature intrinsically high replication stress, this study also provides a molecular rationale for their hypersensitivity to ATR inhibitors.

Chaperoning HMGA2 protein protects stalled replication forks in stem and cancer cells

Maintaining genome integrity requires the accurate and complete replication of chromosomal DNA. This is of the utmost importance for embryonic stem cells (ESCs), which differentiate into cells of all lineages, including germ cells. However, endogenous and exogenous factors frequently induce stalling of replication forks in every cell cycle, which can trigger mutations and chromosomal instabilities. We show here that the oncofetal, nonhistone chromatin factor HMGA2 equips cells with a highly effective first-line defense mechanism against endonucleolytic collapse of stalled forks. This fork-stabilizing function most likely employs scaffold formation at branched DNA via multiple DNA-binding domains. Moreover, HMGA2 works independently of other human factors in two heterologous cell systems to prevent DNA strand breaks. This fork chaperone function seemingly evolved to preserve ESC genome integrity. It is hijacked by tumor (stem) cells to also guard their genomes against DNA-damaging agents widely used to treat cancer patients.

Targeting homologous recombination-mediated DNA repair in cancer

INTRODUCTION

DNA is the target of many traditional non-specific chemotherapeutic drugs. New drugs or therapeutic approaches with a more rational and targeted component are mandatory to improve the success of cancer therapy. The homologous recombination (HR) pathway is an attractive target for the development of inhibitors because cancer cells rely heavily on HR for repair of DNA double-strand breaks resulting from chemotherapeutic treatments. Additionally, the discovery that poly(ADP)ribose polymerase-1 inhibitors selectively kill cells with genetic defects in HR has spurned an even greater interest in inhibitors of HR.

AREAS COVERED

HR drives the repair of broken DNA via numerous protein-mediated sequential DNA manipulations. Due to extensive number of steps and proteins involved, the HR pathway provides a rich pool of potential drug targets. This review discusses the latest developments concerning the strategies being explored to inhibit HR. Particular attention is given to the identification of small molecule inhibitors of key HR proteins, including the BRCA proteins and RAD51.

EXPERT OPINION

Current HR inhibitors are providing the basis for pharmaceutical development of more potent and specific inhibitors to be applied in mono- or combinatorial therapy regimes, while novel targets will be uncovered by experiments aimed to gain a deeper mechanistic understanding of HR and its subpathways.

Concentration-dependent exchange of replication protein A on single-stranded DNA revealed by single-molecule imaging

Replication protein A (RPA) is a ubiquitous eukaryotic single-stranded DNA (ssDNA) binding protein necessary for all aspects of DNA metabolism involving an ssDNA intermediate, including DNA replication, repair, recombination, DNA damage response and checkpoint activation, and telomere maintenance [1], [2], [3]. The role of RPA in most of these reactions is to protect the ssDNA until it can be delivered to downstream enzymes. Therefore a crucial feature of RPA is that it must bind very tightly to ssDNA, but must also be easily displaced from ssDNA to allow other proteins to gain access to the substrate. Here we use total internal reflection fluorescence microscopy and nanofabricated DNA curtains to visualize the behavior of Saccharomyces cerevisiae RPA on individual strands of ssDNA in real-time. Our results show that RPA remains bound to ssDNA for long periods of time when free protein is absent from solution. In contrast, RPA rapidly dissociates from ssDNA when free RPA is present in solution allowing rapid exchange between the free and bound states. In addition, the S. cerevisiae DNA recombinase Rad51 and E. coli single-stranded binding protein (SSB) also promote removal of RPA from ssDNA. These results reveal an unanticipated exchange between bound and free RPA suggesting a binding mechanism that can confer exceptionally slow off rates, yet also enables rapid displacement through a direct exchange mechanism that is reliant upon the presence of free ssDNA-binding proteins in solution. Our results indicate that RPA undergoes constant microscopic dissociation under all conditions, but this is only manifested as macroscopic dissociation (i.e. exchange) when free proteins are present in solution, and this effect is due to mass action. We propose that the dissociation of RPA from ssDNA involves a partially dissociated intermediate, which exposes a small section of ssDNA allowing other proteins to access to the DNA.

Single-stranded DNA-binding proteins: multiple domains for multiple functions

The recognition of single-stranded DNA (ssDNA) is integral to myriad cellular functions. In eukaryotes, ssDNA is present stably at the ends of chromosomes and at some promoter elements. Furthermore, it is formed transiently by several cellular processes including telomere synthesis, transcription, and DNA replication, recombination, and repair. To coordinate these diverse activities, a variety of proteins have evolved to bind ssDNA in a manner specific to their function. Here, we review the recognition of ssDNA through the analysis of high-resolution structures of proteins in complex with ssDNA. This functionally diverse set of proteins arises from a limited set of structural motifs that can be modified and arranged to achieve distinct activities, including a range of ligand specificities. We also investigate the ways in which these domains interact in the context of large multidomain proteins/complexes. These comparisons reveal the structural features that define the range of functions exhibited by these proteins.

Persistently stalled replication forks inhibit nucleotide excision repair in trans by sequestering Replication protein A

Rev3, the catalytic subunit of DNA polymerase ζ, is essential for translesion synthesis of cytotoxic DNA photolesions, whereas the Rev1 protein plays a noncatalytic role in translesion synthesis. Here, we reveal that mammalian Rev3(-/-) and Rev1(-/-) cell lines additionally display a nucleotide excision repair (NER) defect, specifically during S phase. This defect is correlated with the normal recruitment but protracted persistence at DNA damage sites of factors involved in an early stage of NER, while repair synthesis is affected. Remarkably, the NER defect becomes apparent only at 2 h post-irradiation indicating that Rev3 affects repair synthesis only indirectly, rather than performing an enzymatic role in NER. We provide evidence that the NER defect is caused by scarceness of Replication protein A (Rpa) available to NER, resulting from its sequestration at stalled replication forks. Also the induction of replicative stress using hydroxyurea precludes the accumulation of Rpa at photolesion sites, both in Rev3(-/-) and in wild-type cells. These data support a model in which the limited Rpa pool coordinates replicative stress and NER, resulting in increased cytotoxicity of ultraviolet light when replicative stress exceeds a threshold.

Activation of the prereplication complex is blocked by mimosine through reactive oxygen species-activated ataxia telangiectasia mutated (ATM) protein without DNA damage

Mimosine is an effective cell synchronization reagent used for arresting cells in late G1 phase. However, the mechanism underlying mimosine-induced G1 cell cycle arrest remains unclear. Using highly synchronous cell populations, we show here that mimosine blocks S phase entry through ATM activation. HeLa S3 cells are exposed to thymidine for 15 h, released for 9 h by washing out the thymidine, and subsequently treated with 1 mM mimosine for a further 15 h (thymidine → mimosine). In contrast to thymidine-induced S phase arrest, mimosine treatment synchronizes >90% of cells at the G1-S phase boundary by inhibiting the transition of the prereplication complex to the preinitiation complex. Mimosine treatment activates ataxia telangiectasia mutated (ATM)/ataxia telangiectasia and Rad3-related (ATR)-mediated checkpoint signaling without inducing DNA damage. Inhibition of ATM activity is found to induce mimosine-arrested cells to enter S phase. In addition, ATM activation by mimosine treatment is mediated by reactive oxygen species (ROS). These results suggest that, upon mimosine treatment, ATM blocks S phase entry in response to ROS, which prevents replication fork stalling-induced DNA damage.