Replication protein A: a heterotrimeric, single-stranded DNA-binding protein required for eukaryotic DNA metabolism

Elp1 facilitates RAD51-mediated homologous recombination repair via translational regulation

Background

RAD51-dependent homologous recombination (HR) is one of the most important pathways for repairing DNA double-strand breaks (DSBs), and its regulation is crucial to maintain genome integrity. Elp1 gene encodes IKAP/ELP1, a core subunit of the Elongator complex, which has been implicated in translational regulation. However, how ELP1 contributes to genome maintenance is unclear.

Methods

To investigate the function of Elp1, Elp1-deficient mouse embryonic fibroblasts (MEFs) were generated. Metaphase chromosome spreading, immunofluorescence, and comet assays were used to access chromosome abnormalities and DSB formation. Functional roles of Elp1 in MEFs were evaluated by cell viability, colony forming capacity, and apoptosis assays. HR-dependent DNA repair was assessed by reporter assay, immunofluorescence, and western blot. Polysome profiling was used to evaluate translational efficiency. Differentially expressed proteins and signaling pathways were identified using a label-free liquid chromatography–tandem mass spectrometry (LC–MS/MS) proteomics approach.

Results

Here, we report that Elp1 depletion enhanced genomic instability, manifested as chromosome breakage and genotoxic stress-induced genomic DNA fragmentation upon ionizing radiation (IR) exposure. Elp1-deficient cells were hypersensitive to DNA damage and exhibited impaired cell proliferation and defective HR repair. Moreover, Elp1 depletion reduced the formation of IR-induced RAD51 foci and decreased RAD51 protein levels. Polysome profiling analysis revealed that ELP1 regulated RAD51 expression by promoting its translation in response to DNA damage. Notably, the requirement for ELP1 in DSB repair could be partially rescued in Elp1-deficient cells by reintroducing RAD51, suggesting that Elp1-mediated HR-directed repair of DSBs is RAD51-dependent. Finally, using proteome analyses, we identified several proteins involved in cancer pathways and DNA damage responses as being differentially expressed upon Elp1 depletion.

Conclusions

Our study uncovered a molecular mechanism underlying Elp1-mediated regulation of HR activity and provides a novel link between translational regulation and genome stability.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12929-021-00773-z.

Modulating CRISPR/Cas9 genome-editing activity by small molecules

Clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9)-mediated genome engineering has become a standard procedure for creating genetic and epigenetic changes of DNA molecules in basic biology, biotechnology, and medicine. However, its versatile applications have been hampered by its overall low precise gene modification efficiency and uncontrollable prolonged Cas9 activity. Therefore, overcoming these problems could broaden the therapeutic use of CRISPR/Cas9-based technologies. Here, we review small molecules with the clinical potential to precisely modulate CRISPR/Cas9-mediated genome-editing activity and discuss their mechanisms of action. Based on these data, we suggest that direct-acting small molecules for Cas9 are more suitable for precisely regulating Cas9 activity. These findings provide useful information for the identification of novel small-molecule enhancers and inhibitors of Cas9 and Cas9-associated endonucleases.

The Relevance of G-Quadruplexes for DNA Repair

DNA molecules can adopt a variety of alternative structures. Among these structures are G-quadruplex DNA structures (G4s), which support cellular function by affecting transcription, translation, and telomere maintenance. These structures can also induce genome instability by stalling replication, increasing DNA damage, and recombination events. G-quadruplex-driven genome instability is connected to tumorigenesis and other genetic disorders. In recent years, the connection between genome stability, DNA repair and G4 formation was further underlined by the identification of multiple DNA repair proteins and ligands which bind and stabilize said G4 structures to block specific DNA repair pathways. The relevance of G4s for different DNA repair pathways is complex and depends on the repair pathway itself. G4 structures can induce DNA damage and block efficient DNA repair, but they can also support the activity and function of certain repair pathways. In this review, we highlight the roles and consequences of G4 DNA structures for DNA repair initiation, processing, and the efficiency of various DNA repair pathways.

R-loops trigger the release of cytoplasmic ssDNAs leading to chronic inflammation upon DNA damage

How DNA damage leads to chronic inflammation and tissue degeneration with aging remains to be fully resolved. Here, we show that DNA damage leads to cellular senescence, fibrosis, loss-of-tissue architecture, and chronic pancreatitis in mice with an inborn defect in the excision repair cross complementation group 1 (Ercc1) gene. We find that DNA damage-driven R-loops causally contribute to the active release and buildup of single-stranded DNAs (ssDNAs) in the cytoplasm of cells triggering a viral-like immune response in progeroid and naturally aged pancreata. To reduce the proinflammatory load, we developed an extracellular vesicle (EV)-based strategy to deliver recombinant S1 or ribonuclease H nucleases in inflamed Ercc1^−/− pancreatic cells. Treatment of Ercc1^−/− animals with the EV-delivered nuclease cargo eliminates DNA damage-induced R-loops and cytoplasmic ssDNAs alleviating chronic inflammation. Thus, DNA damage-driven ssDNAs causally contribute to tissue degeneration, Ercc1^−/− paving the way for novel rationalized intervention strategies against age-related chronic inflammation.

Replication protein A large subunit (RPA1a) limits chiasma formation during rice meiosis

Replication protein A (RPA), a single-stranded DNA-binding protein, plays essential role in homologous recombination. However, because deletion of RPA causes embryonic lethality in mammals, the exact function of RPA in meiosis remains unclear. In this study, we generated an rpa1a mutant using CRISPR/Cas9 technology and explored its function in rice (Oryza sativa) meiosis. In rpa1a, 12 bivalents were formed at metaphase I, just like in wild-type, but chromosome fragmentations were consistently observed at anaphase I. Fluorescence in situ hybridization assays indicated that these fragmentations were due to the failure of the recombination intermediates to resolve. Importantly, the mutant had a highly elevated chiasma number, and loss of RPA1a could completely restore the 12 bivalent formations in the zmm (for ZIP1-4, MSH4/5, and MER3) mutant background. Protein-protein interaction assays showed that RPA1a formed a complex with the methyl methansulfonate and UV sensitive 81 (and the Fanconi anemia complementation group M-Bloom syndrome protein homologs (RECQ4A)-Topoisomerase3α-RecQ-mediated genome instability 1 complex to regulate chiasma formation and processing of the recombination intermediates. Thus, our data establish a pivotal role for RPA1a in promoting the accurate resolution of recombination intermediates and in limiting redundant chiasma formation during rice meiosis.

hSSB2 (NABP1) is required for the recruitment of RPA during the cellular response to DNA UV damage

Maintenance of genomic stability is critical to prevent diseases such as cancer. As such, eukaryotic cells have multiple pathways to efficiently detect, signal and repair DNA damage. One common form of exogenous DNA damage comes from ultraviolet B (UVB) radiation. UVB generates cyclobutane pyrimidine dimers (CPD) that must be rapidly detected and repaired to maintain the genetic code. The nucleotide excision repair (NER) pathway is the main repair system for this type of DNA damage. Here, we determined the role of the human Single-Stranded DNA Binding protein 2, hSSB2, in the response to UVB exposure. We demonstrate that hSSB2 levels increase in vitro and in vivo after UVB irradiation and that hSSB2 rapidly binds to chromatin. Depletion of hSSB2 results in significantly decreased Replication Protein A (RPA32) phosphorylation and impaired RPA32 localisation to the site of UV-induced DNA damage. Delayed recruitment of NER protein Xeroderma Pigmentosum group C (XPC) was also observed, leading to increased cellular sensitivity to UVB. Finally, hSSB2 was shown to have affinity for single-strand DNA containing a single CPD and for duplex DNA with a two-base mismatch mimicking a CPD moiety. Altogether our data demonstrate that hSSB2 is involved in the cellular response to UV exposure.

New Methodologies to Study DNA Repair Processes in Space and Time Within Living Cells

DNA repair requires a coordinated effort from an array of factors that play different roles in the DNA damage response from recognizing and signaling the presence of a break, creating a repair competent environment, and physically repairing the lesion. Due to the rapid nature of many of these events, live-cell microscopy has become an invaluable method to study this process. In this review we outline commonly used tools to induce DNA damage under the microscope and discuss spatio-temporal analysis tools that can bring added information regarding protein dynamics at sites of damage. In particular, we show how to go beyond the classical analysis of protein recruitment curves to be able to assess the dynamic association of the repair factors with the DNA lesions as well as the target-search strategies used to efficiently find these lesions. Finally, we discuss how the use of mathematical models, combined with experimental evidence, can be used to better interpret the complex dynamics of repair proteins at DNA lesions.

Molecular mechanisms of chemo- and radiotherapy resistance and the potential implications for cancer treatment

Cancer is a leading cause of death worldwide. Surgery is the primary treatment approach for cancer, but the survival rate is very low due to the rapid progression of the disease and presence of local and distant metastasis at diagnosis. Adjuvant chemotherapy and radiotherapy are important components of the multidisciplinary approaches for cancer treatment. However, resistance to radiotherapy and chemotherapy may result in treatment failure or even cancer recurrence. Radioresistance in cancer is often caused by the repair response to radiation-induced DNA damage, cell cycle dysregulation, cancer stem cells (CSCs) resilience, and epithelial-mesenchymal transition (EMT). Understanding the molecular alterations that lead to radioresistance may provide new diagnostic markers and therapeutic targets to improve radiotherapy efficacy. Patients who develop resistance to chemotherapy drugs cannot benefit from the cytotoxicity induced by the prescribed drug and will likely have a poor outcome with these treatments. Chemotherapy often shows a low response rate due to various drug resistance mechanisms. This review focuses on the molecular mechanisms of radioresistance and chemoresistance in cancer and discusses recent developments in therapeutic strategies targeting chemoradiotherapy resistance to improve treatment outcomes.

Identification of an Apis cerana zinc finger protein 41 gene and its involvement in the oxidative stress response

Zinc finger proteins (ZFPs) are a class of transcription factors that contain zinc finger domains and play important roles in growth, aging, and responses to abiotic and biotic stresses. These proteins activate or inhibit gene transcription by binding to single-stranded DNA or RNA and through RNA/DNA bidirectional binding and protein-protein interactions. However, few studies have focused on the oxidation resistance functions of ZFPs in insects, particularly Apis cerana. In the current study, we identified a ZFP41 gene from A. cerana, AcZFP41, and verified its function in oxidative stress responses. Real-time quantitative polymerase chain reaction showed that the transcription level of AcZFP41 was upregulated to different degrees during exposure to oxidative stress, including that induced by extreme temperature, UV radiation, or pesticides. In addition, the silencing of AcZFP41 led to changes in the expression patterns of some known antioxidant genes. Moreover, the activities of the antioxidant enzymes catalase (CAT), superoxide dismutase (SOD), peroxidase (POD), and glutathione S-transferase (GST) in AcZFP41-silenced honeybees were higher than those in a control group. In summary, the data indicate that AcZFP41 is involved in the oxidative stress response. The results provide a theoretical basis for further studies of zinc finger proteins and improve our understanding of the antioxidant mechanisms of honeybees.

Structure of a meiosis-specific complex central to BRCA2 localization at recombination sites

Meiotic cells invoke breast cancer susceptibility gene 2 (BRCA2) to repair programmed double-stranded DNA breaks and accomplish homologous recombination. The meiosis-specific protein MEILB2 facilitates BRCA2 recruitment to meiotic recombination sites. Here, we combine crystallography, biochemical analysis and a mouse meiosis model to reveal a robust architecture that ensures meiotic BRCA2 recruitment. The crystal structure of the MEILB2-BRCA2 complex reveals how two MEILB2 homodimers sandwich two chains of BRCA2 to afford a 4:2 architecture. The sandwich lacks close contact between the two MEILB2 dimers or the two BRCA2 chains. Instead, the two halves of each BRCA2 chain bridge two MEILB2 subunits from different homodimers to form the MEILB2-BRCA2-MEILB2 sandwich. Several identical residues from the two MEILB2 subunits are employed to engage the BRCA2 halves, justifying their strict conservation. Mutational analysis of the interface reveals a synergistic mechanism for MEILB2-BRCA2 recruitment during meiosis. Overall, these studies demonstrate how BRCA2 efficiently localizes in the cell to facilitate meiosis.

The male germline-specific protein MAPS is indispensable for pachynema progression and fertility

Meiosis is a specialized cell division that creates haploid germ cells from diploid progenitors. Through differential RNA expression analyses, we previously identified a number of mouse genes that were dramatically elevated in spermatocytes, relative to their very low expression in spermatogonia and somatic organs. Here, we investigated in detail 1700102P08Rik, one of these genes, and independently conclude that it encodes a male germline-specific protein, in agreement with a recent report. We demonstrated that it is essential for pachynema progression in spermatocytes and named it male pachynema-specific (MAPS) protein. Mice lacking Maps (Maps ^-/- ) suffered from pachytene arrest and spermatocyte death, leading to male infertility, whereas female fertility was not affected. Interestingly, pubertal Maps ^-/- spermatocytes were arrested at early pachytene stage, accompanied by defects in DNA double-strand break (DSB) repair, crossover formation, and XY body formation. In contrast, adult Maps ^-/- spermatocytes only exhibited partially defective crossover but nonetheless were delayed or failed in progression from early to mid- and late pachytene stage, resulting in cell death. Furthermore, we report a significant transcriptional dysregulation in autosomes and XY chromosomes in both pubertal and adult Maps ^-/- pachytene spermatocytes, including failed meiotic sex chromosome inactivation (MSCI). Further experiments revealed that MAPS overexpression in vitro dramatically decreased the ubiquitination levels of cellular proteins. Conversely, in Maps ^-/- pachytene cells, protein ubiquitination was dramatically increased, likely contributing to the large-scale disruption in gene expression in pachytene cells. Thus, MAPS is a protein essential for pachynema progression in male mice, possibly in mammals in general.

Yeast Bromodomain Factor 1 and Its Human Homolog TAF1 Play Conserved Roles in Promoting Homologous Recombination

Histone acetylation is a key histone post-translational modification that shapes chromatin structure, dynamics, and function. Bromodomain (BRD) proteins, the readers of acetyl-lysines, are located in the center of the histone acetylation-signaling network. How they regulate DNA repair and genome stability remains poorly understood. Here, a conserved function of the yeast Bromodomain Factor 1 (Bdf1) and its human counterpart TAF1 is reported in promoting DNA double-stranded break repair by homologous recombination (HR). Depletion of either yeast BDF1 or human TAF1, or disruption of their BRDs impairs DNA end resection, Replication Protein A (RPA) and Rad51 loading, and HR repair, causing genome instability and hypersensitivity to DNA damage. Mechanistically, it is shown that Bdf1 preferentially binds the DNA damage-induced histone H4 acetylation (H4Ac) via the BRD motifs, leading to its chromatin recruitment. Meanwhile, Bdf1 physically interacts with RPA, and this interaction facilitates RPA loading in the chromatin context and the subsequent HR repair. Similarly, TAF1 also interacts with H4Ac or RPA. Thus, Bdf1 and TAF1 appear to share a conserved mechanism in linking the HR repair to chromatin acetylation in preserving genome integrity.

Transcription/Replication Conflicts in Tumorigenesis and Their Potential Role as Novel Therapeutic Targets in Multiple Myeloma

Simple Summary

Multiple myeloma is a hematologic cancer characterized by the accumulation of malignant plasma cells in the bone marrow. It remains a mostly incurable disease due to the inability to overcome refractory disease and drug-resistant relapse. Oncogenic transformation of PC in multiple myeloma is thought to occur within the secondary lymphoid organs. However, the precise molecular events leading to myelomagenesis remain obscure. Here, we identified genes involved in the prevention and the resolution of conflicts between the replication and transcription significantly overexpressed during the plasma cell differentiation process and in multiple myeloma cells. We discussed the potential role of these factors in myelomagenesis and myeloma biology. The specific targeting of these factors might constitute a new therapeutic strategy in multiple myeloma.

Abstract

Plasma cells (PCs) have an essential role in humoral immune response by secretion of antibodies, and represent the final stage of B lymphocytes differentiation. During this differentiation, the pre-plasmablastic stage is characterized by highly proliferative cells that start to secrete immunoglobulins (Igs). Thus, replication and transcription must be tightly regulated in these cells to avoid transcription/replication conflicts (TRCs), which could increase replication stress and lead to genomic instability. In this review, we analyzed expression of genes involved in TRCs resolution during B to PC differentiation and identified 41 genes significantly overexpressed in the pre-plasmablastic stage. This illustrates the importance of mechanisms required for adequate processing of TRCs during PCs differentiation. Furthermore, we identified that several of these factors were also found overexpressed in purified PCs from patients with multiple myeloma (MM) compared to normal PCs. Malignant PCs produce high levels of Igs concomitantly with cell cycle deregulation. Therefore, increasing the TRCs occurring in MM cells could represent a potent therapeutic strategy for MM patients. Here, we describe the potential roles of TRCs resolution factors in myelomagenesis and discuss the therapeutic interest of targeting the TRCs resolution machinery in MM.

Remote Homology Detection Identifies a Eukaryotic RPA DBD-C-like DNA Binding Domain as a Conserved Feature of Archaeal Rpa1-Like Proteins

The eukaryotic single-stranded DNA binding factor replication protein A (RPA) is essential for DNA replication, repair and recombination. RPA is a heterotrimer containing six related OB folds and a winged helix-turn-helix (wH) domain. The OB folds are designated DBD-A through DBD-F, with DBD-A through DBD-D being directly involved in ssDNA binding. DBD-C is located at the C-terminus of the RPA1 protein and has a distinctive structure that includes an integral C4 zinc finger, while the wH domain is found at the C-terminus of the RPA2 protein. Previously characterised archaeal RPA proteins fall into a number of classes with varying numbers of OB folds, but one widespread class includes proteins that contain a C4 or C3H zinc finger followed by a 100–120 amino acid C-terminal region reported to lack detectable sequence or structural similarity. Here, the sequences spanning this zinc finger and including the C-terminal region are shown to comprise a previously unrecognised DBD-C-like OB fold, confirming the evolutionary relatedness of this group of archaeal RPA proteins to eukaryotic RPA1. The evolutionary relationship between eukaryotic and archaeal RPA is further underscored by the presence of RPA2-like proteins comprising an OB fold and C-terminal winged helix (wH) domain in multiple species and crucially, suggests that several biochemically characterised archaeal RPA proteins previously thought to exist as monomers are likely to be RPA1-RPA2 heterodimers.

Heterogeneous nuclear ribonucleoprotein E1 binds polycytosine DNA and monitors genome integrity

hnRNP E1 binds polycytosine tracts of DNA and monitors genome integrity.

Heterogeneous nuclear ribonucleoprotein E1 (hnRNP E1) is a tumor suppressor protein that binds site- and structure-specifically to RNA sequences to regulate mRNA stability, facilitate alternative splicing, and suppress protein translation on several metastasis-associated mRNAs. Here, we show that hnRNP E1 binds polycytosine-rich DNA tracts present throughout the genome, including those at promoters of several oncogenes and telomeres and monitors genome integrity. It binds DNA in a site- and structure-specific manner. hnRNP E1-knockdown cells displayed increased DNA damage signals including γ-H2AX at its binding sites and also showed increased mutations. UV and hydroxyurea treatment of hnRNP E1-knockdown cells exacerbated the basal DNA damage signals with increased cell cycle arrest, activation of checkpoint proteins, and monoubiquitination of proliferating cell nuclear antigen despite no changes in deubiquitinating enzymes. DNA damage caused by genotoxin treatment localized to hnRNP E1 binding sites. Our work suggests that hnRNP E1 facilitates functions of DNA integrity proteins at polycytosine tracts and monitors DNA integrity at these sites.

OB-Folds and Genome Maintenance: Targeting Protein-DNA Interactions for Cancer Therapy

Genome stability and maintenance pathways along with their requisite proteins are critical for the accurate duplication of genetic material, mutation avoidance, and suppression of human diseases including cancer. Many of these proteins participate in these pathways by binding directly to DNA, and a subset employ oligonucleotide/oligosaccharide binding folds (OB-fold) to facilitate the protein-DNA interactions. OB-fold motifs allow for sequence independent binding to single-stranded DNA (ssDNA) and can serve to position specific proteins at specific DNA structures and then, via protein-protein interaction motifs, assemble the machinery to catalyze the replication, repair, or recombination of DNA. This review provides an overview of the OB-fold structural organization of some of the most relevant OB-fold containing proteins for oncology and drug discovery. We discuss their individual roles in DNA metabolism, progress toward drugging these motifs and their utility as potential cancer therapeutics. While protein-DNA interactions were initially thought to be undruggable, recent reports of success with molecules targeting OB-fold containing proteins suggest otherwise. The potential for the development of agents targeting OB-folds is in its infancy, but if successful, would expand the opportunities to impinge on genome stability and maintenance pathways for more effective cancer treatment.

Rtt105 promotes high-fidelity DNA replication and repair by regulating the single-stranded DNA-binding factor RPA

Single-stranded DNA (ssDNA) covered with the heterotrimeric Replication Protein A (RPA) complex is a central intermediate of DNA replication and repair. How RPA is regulated to ensure the fidelity of DNA replication and repair remains poorly understood. Yeast Rtt105 is an RPA-interacting protein required for RPA nuclear import and efficient ssDNA binding. Here, we describe an important role of Rtt105 in high-fidelity DNA replication and recombination and demonstrate that these functions of Rtt105 primarily depend on its regulation of RPA. The deletion of RTT105 causes elevated spontaneous DNA mutations with large duplications or deletions mediated by microhomologies. Rtt105 is recruited to DNA double-stranded break (DSB) ends where it promotes RPA assembly and homologous recombination repair by gene conversion or break-induced replication. In contrast, Rtt105 attenuates DSB repair by the mutagenic single-strand annealing or alternative end joining pathway. Thus, Rtt105-mediated regulation of RPA promotes high-fidelity replication and recombination while suppressing repair by deleterious pathways. Finally, we show that the human RPA-interacting protein hRIP-α, a putative functional homolog of Rtt105, also stimulates RPA assembly on ssDNA, suggesting the conservation of an Rtt105-mediated mechanism.

RPA facilitates rescue of keratinocytes from UVB radiation damage through insulin-like growth factor-I signalling

UVBR-induced photolesions in genomic DNA of keratinocytes impair cellular functions and potentially determine the cell fate post-irradiation. The ability of insulin-like growth factor-I (IGF-I) to rescue epidermal keratinocytes after photodamage via apoptosis prevention and photolesion removal was recently demonstrated using in vitro two-dimensional and three-dimensional skin models. Given the limited knowledge of specific signalling cascades contributing to post-UVBR IGF-I effects, we used inhibitors to investigate the impact of blockade of various signalling mediators on IGF-I photoprotection. IGF-I treatment, in the presence of signalling inhibitors, particularly TDRL-505, which targets replication protein A (RPA), impaired activation of IGF-1R downstream signalling, diminished cyclobutane pyrimidine dimer removal, arrested growth, reduced cell survival and increased apoptosis. Further, the transient partial knockdown of RPA was found to abrogate IGF-I-mediated responses in keratinocytes, ultimately affecting photoprotection and, thereby, establishing that RPA is required for IGF-I function. Our findings thus elucidate the importance of RPA in linking the damage response activation, cell cycle regulation, repair and survival pathways, separately initiated by IGF-I upon UVBR-induced damage. This information is potentially imperative for the development of effective sunburn and photodamage repair strategies. This article has an associated First Person interview with the first author of the paper.

SETD2-mediated H3K14 trimethylation promotes ATR activation and stalled replication fork restart in response to DNA replication stress

Ataxia telangiectasia and Rad3 related (ATR) activation after replication stress involves a cascade of reactions, including replication protein A (RPA) complex loading onto single-stranded DNA and ATR activator loading onto chromatin. The contribution of histone modifications to ATR activation, however, is unclear. Here, we report that H3K14 trimethylation responds to replication stress by enhancing ATR activation. First, we confirmed that H3K14 monomethylation, dimethylation, and trimethylation all exist in mammalian cells, and that both SUV39H1 and SETD2 methyltransferases can catalyze H3K14 trimethylation in vivo and in vitro. Interestingly, SETD2-mediated H3K14 trimethylation markedly increases in response to replication stress induced with hydroxyurea, a replication stress inducer. Under these conditions, SETD2-mediated H3K14me3 recruited the RPA complex to chromatin via a direct interaction with RPA70. The increase in H3K14me3 levels was abolished, and RPA loading was attenuated when SETD2 was depleted or H3K14 was mutated. Rather, the cells were sensitive to replication stress such that the replication forks failed to restart, and cell-cycle progression was delayed. These findings help us understand how H3K14 trimethylation links replication stress with ATR activation.

Single-molecule imaging reveals replication fork coupled formation of G-quadruplex structures hinders local replication stress signaling

Guanine-rich DNA sequences occur throughout the human genome and can transiently form G-quadruplex (G4) structures that may obstruct DNA replication, leading to genomic instability. Here, we apply multi-color single-molecule localization microscopy (SMLM) coupled with robust data-mining algorithms to quantitatively visualize replication fork (RF)-coupled formation and spatial-association of endogenous G4s. Using this data, we investigate the effects of G4s on replisome dynamics and organization. We show that a small fraction of active replication forks spontaneously form G4s at newly unwound DNA immediately behind the MCM helicase and before nascent DNA synthesis. These G4s locally perturb replisome dynamics and organization by reducing DNA synthesis and limiting the binding of the single-strand DNA-binding protein RPA. We find that the resolution of RF-coupled G4s is mediated by an interplay between RPA and the FANCJ helicase. FANCJ deficiency leads to G4 accumulation, DNA damage at G4-associated replication forks, and silencing of the RPA-mediated replication stress response. Our study provides first-hand evidence of the intrinsic, RF-coupled formation of G4 structures, offering unique mechanistic insights into the interference and regulation of stable G4s at replication forks and their effect on RPA-associated fork signaling and genomic instability.

Probing E. coli SSB protein-DNA topology by reversing DNA backbone polarity

Escherichia coli single-strand (ss) DNA binding protein (SSB) is an essential protein that binds ssDNA intermediates formed during genome maintenance. SSB homotetramers bind ssDNA in two major modes, differing in occluded site size and cooperativity. The (SSB)35 mode in which ssDNA wraps, on average, around two subunits is favored at low [NaCl] and high SSB/DNA ratios and displays high unlimited, nearest-neighbor cooperativity forming long protein clusters. The (SSB)65 mode, in which ssDNA wraps completely around four subunits of the tetramer, is favored at higher [NaCl] (>200 mM) and displays limited low cooperativity. Crystal structures of E. coli SSB and Plasmodium falciparum SSB show ssDNA bound to the SSB subunits (OB folds) with opposite polarities of the sugar phosphate backbones. To investigate whether SSB subunits show a polarity preference for binding ssDNA, we examined EcSSB and PfSSB binding to a series of (dT)70 constructs in which the backbone polarity was switched in the middle of the DNA by incorporating a reverse-polarity (RP) phosphodiester linkage, either 3'-3' or 5'-5'. We find only minor effects on the DNA binding properties for these RP constructs, although (dT)70 with a 3'-3' polarity switch shows decreased affinity for EcSSB in the (SSB)65 mode and lower cooperativity in the (SSB)35 mode. However, (dT)70 in which every phosphodiester linkage is reversed does not form a completely wrapped (SSB)65 mode but, rather, binds EcSSB in the (SSB)35 mode with little cooperativity. In contrast, PfSSB, which binds ssDNA only in an (SSB)65 mode and with opposite backbone polarity and different topology, shows little effect of backbone polarity on its DNA binding properties. We present structural models suggesting that strict backbone polarity can be maintained for ssDNA binding to the individual OB folds if there is a change in ssDNA wrapping topology of the RP ssDNA.

Pathways and signatures of mutagenesis at targeted DNA nicks

Nicks are the most frequent form of DNA damage and a potential source of mutagenesis in human cells. By deep sequencing, we have identified factors and pathways that promote and limit mutagenic repair at a targeted nick in human cells. Mutations were distributed asymmetrically around the nick site. BRCA2 inhibited all categories of mutational events, including indels, SNVs and HDR. DNA2 and RPA promoted resection. DNA2 inhibited 1 bp deletions but contributed to longer deletions, as did REV7. POLQ stimulated SNVs. Parallel analysis of DSBs targeted to the same site identified similar roles for DNA2 and POLQ (but not REV7) in promoting deletions and for POLQ in stimulating SNVs. Insertions were infrequent at nicks, and most were 1 bp in length, as at DSBs. The translesion polymerase REV1 stimulated +1 insertions at one nick site but not another, illustrating the potential importance of sequence context in determining the outcome of mutagenic repair. These results highlight the potential for nicks to promote mutagenesis, especially in BRCA-deficient cells, and identify mutagenic signatures of DNA2, REV1, REV3, REV7 and POLQ.

RPA complexes in Caenorhabditis elegans meiosis; unique roles in replication, meiotic recombination and apoptosis

Replication Protein A (RPA) is a critical complex that acts in replication and promotes homologous recombination by allowing recombinase recruitment to processed DSB ends. Most organisms possess three RPA subunits (RPA1, RPA2, RPA3) that form a trimeric complex critical for viability. The Caenorhabditis elegans genome encodes RPA-1, RPA-2 and an RPA-2 paralog RPA-4. In our analysis, we determined that RPA-2 is critical for germline replication and normal repair of meiotic DSBs. Interestingly, RPA-1 but not RPA-2 is essential for somatic replication, in contrast to other organisms that require both subunits. Six different hetero- and homodimeric complexes containing permutations of RPA-1, RPA-2 and RPA-4 can be detected in whole animal extracts. Our in vivo studies indicate that RPA-1/4 dimer is less abundant in the nucleus and its formation is inhibited by RPA-2. While RPA-4 does not participate in replication or recombination, we find that RPA-4 inhibits RAD-51 filament formation and promotes apoptosis of a subset of damaged nuclei. Altogether these findings point to sub-functionalization and antagonistic roles of RPA complexes in C. elegans.

A comparative study of protein-ssDNA interactions

Single-stranded DNA-binding proteins (SSBs) play crucial roles in DNA replication, recombination and repair, and serve as key players in the maintenance of genomic stability. While a number of SSBs bind single-stranded DNA (ssDNA) non-specifically, the others recognize and bind specific ssDNA sequences. The mechanisms underlying this binding discrepancy, however, are largely unknown. Here, we present a comparative study of protein-ssDNA interactions by annotating specific and non-specific SSBs and comparing structural features such as DNA-binding propensities and secondary structure types of residues in SSB-ssDNA interactions, protein-ssDNA hydrogen bonding and π-π interactions between specific and non-specific SSBs. Our results suggest that protein side chain-DNA base hydrogen bonds are the major contributors to protein-ssDNA binding specificity, while π-π interactions may mainly contribute to binding affinity. We also found the enrichment of aspartate in the specific SSBs, a key feature in specific protein-double-stranded DNA (dsDNA) interactions as reported in our previous study. In addition, no significant differences between specific and non-specific groups with respect of conformational changes upon ssDNA binding were found, suggesting that the flexibility of SSBs plays a lesser role than that of dsDNA-binding proteins in conferring binding specificity.

Repression of a large number of genes requires interplay between homologous recombination and HIRA

During homologous recombination, Dbl2 protein is required for localisation of Fbh1, an F-box helicase that efficiently dismantles Rad51-DNA filaments. RNA-seq analysis of dbl2Δ transcriptome showed that the dbl2 deletion results in upregulation of more than 500 loci in Schizosaccharomyces pombe. Compared with the loci with no change in expression, the misregulated loci in dbl2Δ are closer to long terminal and long tandem repeats. Furthermore, the misregulated loci overlap with antisense transcripts, retrotransposons, meiotic genes and genes located in subtelomeric regions. A comparison of the expression profiles revealed that Dbl2 represses the same type of genes as the HIRA histone chaperone complex. Although dbl2 deletion does not alleviate centromeric or telomeric silencing, it suppresses the silencing defect at the outer centromere caused by deletion of hip1 and slm9 genes encoding subunits of the HIRA complex. Moreover, our analyses revealed that cells lacking dbl2 show a slight increase of nucleosomes at transcription start sites and increased levels of methylated histone H3 (H3K9me2) at centromeres, subtelomeres, rDNA regions and long terminal repeats. Finally, we show that other proteins involved in homologous recombination, such as Fbh1, Rad51, Mus81 and Rad54, participate in the same gene repression pathway.

RPA-mediated recruitment of Bre1 couples histone H2B ubiquitination to DNA replication and repair

The ubiquitin E3 ligase Bre1-mediated H2B monoubiquitination (H2Bub) is essential for proper DNA replication and repair in eukaryotes. Deficiency in H2Bub causes genome instability and cancer. How the Bre1-H2Bub pathway is evoked in response to DNA replication or repair remains unknown. Here, we identify that the single-stranded DNA (ssDNA) binding factor RPA acts as a key mediator that couples Bre1-mediated H2Bub to DNA replication and repair in yeast. We found that RPA interacts with Bre1 in vitro and in vivo, and this interaction is stimulated by ssDNA. This association ensures the recruitment of Bre1 to replication forks or DNA breaks but does not affect its E3 ligase activity. Disruption of the interaction abolishes the local enrichment of H2Bub, resulting in impaired DNA replication, response to replication stress, and repair by homologous recombination, accompanied by increased genome instability and DNA damage sensitivity. Notably, we found that RNF20, the human homolog of Bre1, interacts with RPA70 in a conserved mode. Thus, RPA functions as a master regulator for the spatial-temporal control of H2Bub chromatin landscape during DNA replication and recombination, extending the versatile roles of RPA in guarding genome stability.

PRIMPOL ready, set, reprime!

DNA replication forks are constantly challenged by DNA lesions induced by endogenous and exogenous sources. DNA damage tolerance mechanisms ensure that DNA replication continues with minimal effects on replication fork elongation either by using specialized DNA polymerases, which have the ability to replicate through the damaged template, or by skipping the damaged DNA, leaving it to be repaired after replication. These mechanisms are evolutionarily conserved in bacteria, yeast, and higher eukaryotes, and are paramount to ensure timely and faithful duplication of the genome. The Primase and DNA-directed Polymerase (PRIMPOL) is a recently discovered enzyme that possesses both primase and polymerase activities. PRIMPOL is emerging as a key player in DNA damage tolerance, particularly in vertebrate and human cells. Here, we review our current understanding of the function of PRIMPOL in DNA damage tolerance by focusing on the structural aspects that define its dual enzymatic activity, as well as on the mechanisms that control its chromatin recruitment and expression levels. We also focus on the latest findings on the mitochondrial and nuclear functions of PRIMPOL and on the impact of loss of these functions on genome stability and cell survival. Defining the function of PRIMPOL in DNA damage tolerance is becoming increasingly important in the context of human disease. In particular, we discuss recent evidence pointing at the PRIMPOL pathway as a novel molecular target to improve cancer cell response to DNA-damaging chemotherapy and as a predictive parameter to stratify patients in personalized cancer therapy.

Single-molecule visualization of human RECQ5 interactions with single-stranded DNA recombination intermediates

RECQ5 is one of five RecQ helicases found in humans and is thought to participate in homologous DNA recombination by acting as a negative regulator of the recombinase protein RAD51. Here, we use kinetic and single molecule imaging methods to monitor RECQ5 behavior on various nucleoprotein complexes. Our data demonstrate that RECQ5 can act as an ATP-dependent single-stranded DNA (ssDNA) motor protein and can translocate on ssDNA that is bound by replication protein A (RPA). RECQ5 can also translocate on RAD51-coated ssDNA and readily dismantles RAD51-ssDNA filaments. RECQ5 interacts with RAD51 through protein-protein contacts, and disruption of this interface through a RECQ5-F666A mutation reduces translocation velocity by ∼50%. However, RECQ5 readily removes the ATP hydrolysis-deficient mutant RAD51-K133R from ssDNA, suggesting that filament disruption is not coupled to the RAD51 ATP hydrolysis cycle. RECQ5 also readily removes RAD51-I287T, a RAD51 mutant with enhanced ssDNA-binding activity, from ssDNA. Surprisingly, RECQ5 can bind to double-stranded DNA (dsDNA), but it is unable to translocate. Similarly, RECQ5 cannot dismantle RAD51-bound heteroduplex joint molecules. Our results suggest that the roles of RECQ5 in genome maintenance may be regulated in part at the level of substrate specificity.

Generation of Fluorescent Versions of Saccharomyces cerevisiae RPA to Study the Conformational Dynamics of Its ssDNA-Binding Domains

Replication protein A (RPA) is an essential single-stranded DNA (ssDNA)-binding protein that sequesters ssDNA and protects it from nucleolytic degradation. The RPA-ssDNA nucleoprotein acts as a hub to recruit over two dozen DNA metabolic enzymes onto ssDNA to coordinate DNA replication, repair, and recombination. RPA functions as a heterotrimer composed of RPA70, RPA32, and RPA14 subunits and has multiple DNA-binding and protein-interaction domains. Several of these domains are connected by disordered linkers allowing RPA to adopt a wide variety of conformations on ssDNA. Here we describe a fluorescence-based tool to monitor the dynamics of select DNA-binding domains of RPA. Noncanonical amino acids are utilized to site-specifically engineer fluorescent probes in Saccharomyces cerevisiae RPA heterologously expressed in BL21 (DE3) and its derivatives. A procedure to synthesize 4-azido-L-phenylalanine (4AZP), a noncanonical amino acid, is also described. Sites for fluorophore positioning that produce a measurable change in fluorescence upon binding to ssDNA are detailed. This fluorescence enhancement through noncanonical amino acid (FEncAA) approach can also be applied to other DNA-binding proteins to investigate the dynamics of protein-nucleic acid interactions.

Transcriptome analysis of the circadian clock gene BMAL1 deletion with opposite carcinogenic effects

We have previously reported that the deletion of BMAL1 gene has opposite effects in respect to its contribution to the pathways that are effective in the multistage carcinogenesis process. BMAL1 deletion sensitized nearly normal breast epithelial (MCF10A) and invasive breast cancer cells (MDA-MB-231) to cisplatin- and doxorubicin-induced apoptosis, while this deletion also aggravated the invasive potential of MDA-MB-231 cells. However, the mechanistic relationship of the seemingly opposite contribution of BMAL1 deletion to carcinogenesis process is not known at genome-wide level. In this study, an RNA-seq approach was taken to uncover the differentially expressed genes (DEGs) and pathways after treating BMAL1 knockout (KO) or wild-type (WT) MDA-MB-231 cells with cisplatin and doxorubicin to initiate apoptosis. Gene set enrichment analysis with the DEGs demonstrated that enrichment in multiple genes/pathways contributes to sensitization to cisplatin- or doxorubicin-induced apoptosis in BMAL1-dependent manner. Additionally, our DEG analysis suggested that non-coding transcript RNA (such as lncRNA and processed pseudogenes) may have role in cisplatin- or doxorubicin-induced apoptosis. Protein-protein interaction network obtained from common DEGs in cisplatin and doxorubicin treatments revealed that GSK3β, NACC1, and EGFR are the principal genes regulating the response of the KO cells. Moreover, the analysis of DEGs among untreated BMAL1 KO and WT cells revealed that epithelial-mesenchymal transition genes are up-regulated in KO cells. As a negative control, we have also analyzed the DEGs following treatment with an endoplasmic reticulum (ER) stress-inducing agent, tunicamycin, which was affected by BMAL1 deletion minimally. Collectively, the present study suggests that BMAL1 regulates many genes/pathways of which the alteration in BMAL1 KO cells may shed light on pleotropic phenotype observed.

Quantifying the Affinity of Trypanosoma cruzi RPA-1 to the Single-Stranded DNA Overhang of the Telomere Using Surface Plasmon Resonance

Surface plasmon resonance (SPR) biosensors provide real-time binding affinity measurements between a pair of biomolecules, characterizing its interaction dynamics. An example of Trypanosoma cruzi's RPA-1 and a single-stranded DNA telomere sequence is presented with detailed guidelines and fundamentals for SPR technology.

Single-Stranded DNA-Binding Proteins in the Archaea

Single-stranded (ss) DNA-binding proteins are found in all three domains of life where they play vital roles in nearly all aspects of DNA metabolism by binding to and stabilizing exposed ssDNA and acting as platforms onto which DNA-processing activities can assemble. The ssDNA-binding factors SSB and RPA are extremely well conserved across bacteria and eukaryotes, respectively, and comprise one or more OB-fold ssDNA-binding domains. In the third domain of life, the archaea, multiple types of ssDNA-binding protein are found with a variety of domain architectures and subunit compositions, with OB-fold ssDNA-binding domains being a characteristic of most, but not all. This chapter summarizes current knowledge of the distribution, structure, and biological function of the archaeal ssDNA-binding factors, highlighting key features shared between clades and those that distinguish the proteins of different clades from one another. The likely cellular functions of the proteins are discussed and gaps in current knowledge identified.

RPA-1 from Leishmania sp.: Recombinant Protein Expression and Purification, Molecular Modeling, and Molecular Dynamics Simulations Protocols

RPA is a conserved heterotrimeric complex and the major single-stranded DNA (ssDNA)-binding protein heterotrimeric complex, which in eukaryotes is formed by the RPA-1, RPA-2, and RPA-3 subunits. The main structural feature of RPA is the presence of the oligonucleotide/oligosaccharide-binding fold (OB-fold) domains, responsible for ssDNA binding and protein:protein interactions. Among the RPA subunits, RPA-1 bears three of the four OB folds involved with RPA-ssDNA binding, although in some organisms RPA-2 can also bind ssDNA. The OB-fold domains are also present in telomere end-binding proteins (TEBP), essential for chromosome end protection. RPA-1 from Leishmania sp., as well as RPA-1 from trypanosomatids, a group of early-divergent protozoa, shows some structural differences compared to higher eukaryote RPA-1. Also, RPA-1 from Leishmania sp., similar to TEBPs, may exert telomeric protective functions. Remarkably, different pieces of evidence have pointed out that trypanosomatids may not have OB fold-containing TEBPs. Moreover, recent data indicate that trypanosomatid RPA-1 may be considered a TEBP since it shares with TEBPs conserved functional and structural features. However, it is still unknown whether the RPA-1 protective telomeric role is exclusive to trypanosomatids or is also present in other primitive eukaryotes. Here, we describe a protocol to obtain highly purified and biologically active Leishmania amazonensis recombinant RPA-1, and to perform molecular modeling and molecular dynamics simulations methods which could be probably applied to functional and structural studies of homologous proteins in other primitive eukaryotes.

The Essential, Ubiquitous Single-Stranded DNA-Binding Proteins

Maintenance of genomes is fundamental for all living organisms. The diverse processes related to genome maintenance entail the management of various intermediate structures, which may be deleterious if unresolved. The most frequent intermediate structures that result from the melting of the DNA duplex are single-stranded (ss) DNA stretches. These are thermodynamically less stable and can spontaneously fold into secondary structures, which may obstruct a variety of genome processes. In addition, ssDNA is more prone to breaking, which may lead to the formation of deletions or DNA degradation. Single-stranded DNA-binding proteins (SSBs) bind and stabilize ssDNA, preventing the abovementioned deleterious consequences and recruiting the appropriate machinery to resolve that intermediate molecule. They are present in all forms of life and are essential for their viability, with very few exceptions. Here we present an introductory chapter to a volume of the Methods in Molecular Biology dedicated to SSBs, in which we provide a general description of SSBs from various taxa.

Determinants of replication protein A subunit interactions revealed using a phosphomimetic peptide

Replication protein A (RPA) is a eukaryotic ssDNA-binding protein and contains three subunits: RPA70, RPA32, and RPA14. Phosphorylation of the N-terminal region of the RPA32 subunit plays an essential role in DNA metabolism in processes such as replication and damage response. Phosphorylated RPA32 (pRPA32) binds to RPA70 and possibly regulates the transient RPA70-Bloom syndrome helicase (BLM) interaction to inhibit DNA resection. However, the structural details and determinants of the phosphorylated RPA32-RPA70 interaction are still unknown. In this study, we provide molecular details of the interaction between RPA70 and a mimic of phosphorylated RPA32 (pmRPA32) using fluorescence polarization and NMR analysis. We show that the N-terminal domain of RPA70 (RPA70N) specifically participates in pmRPA32 binding, whereas the unphosphorylated RPA32 does not bind to RPA70N. Our NMR data revealed that RPA70N binds pmRPA32 using a basic cleft region. We also show that at least 6 negatively charged residues of pmRPA32 are required for RPA70N binding. By introducing alanine mutations into hydrophobic positions of pmRPA32, we found potential points of contact between RPA70N and the N-terminal half of pmRPA32. We used this information to guide docking simulations that suggest the orientation of pmRPA32 in complex with RPA70N. Our study demonstrates detailed features of the domain-domain interaction between RPA70 and RPA32 upon phosphorylation. This result provides insight into how phosphorylation tunes transient bindings between RPA and its partners in DNA resection.

A Structural Guide to the Bloom Syndrome Complex

The Bloom syndrome complex is a DNA damage repair machine. It consists of several protein components which are functional in isolation, but interdependent in cells for the maintenance of accurate homologous recombination. Mutations to any of the genes encoding these proteins cause numerous physical and developmental markers as well as phenotypes of genome instability, infertility, and cancer predisposition. Here we review the published structural and biochemical data on each of the components of the complex: the helicase BLM, the type IA topoisomerase TOP3A, and the OB-fold-containing RMI and RPA subunits. We describe how each component contributes to function, interacts with each other, and the DNA that it manipulates/repairs.

The ssDNA-binding protein MEIOB acts as a dosage-sensitive regulator of meiotic recombination

Meiotic recombination enables reciprocal exchange of genetic information between parental chromosomes and is essential for fertility. MEIOB, a meiosis-specific ssDNA-binding protein, regulates early meiotic recombination. Here we report that the human infertility-associated missense mutation (N64I) in MEIOB causes protein degradation and reduced crossover formation in mouse testes. Although the MEIOB N64I substitution is associated with human infertility, the point mutant mice are fertile despite meiotic defects. Meiob mutagenesis identifies serine 67 as a critical residue for MEIOB. Biochemically, these two mutations (N64I and S67 deletion) cause self-aggregation of MEIOB and sharply reduced protein half-life. Molecular genetic analyses of both point mutants reveal an important role for MEIOB in crossover formation in late meiotic recombination. Furthermore, we find that the MEIOB protein levels directly correlate with the severity of meiotic defects. Our results demonstrate that MEIOB regulates meiotic recombination in a dosage-dependent manner.

Haloferax volcanii-a model archaeon for studying DNA replication and repair

The tree of life shows the relationship between all organisms based on their common ancestry. Until 1977, it comprised two major branches: prokaryotes and eukaryotes. Work by Carl Woese and other microbiologists led to the recategorization of prokaryotes and the proposal of three primary domains: Eukarya, Bacteria and Archaea. Microbiological, genetic and biochemical techniques were then needed to study the third domain of life. Haloferax volcanii, a halophilic species belonging to the phylum Euryarchaeota, has provided many useful tools to study Archaea, including easy culturing methods, genetic manipulation and phenotypic screening. This review will focus on DNA replication and DNA repair pathways in H. volcanii, how this work has advanced our knowledge of archaeal cellular biology, and how it may deepen our understanding of bacterial and eukaryotic processes.

The Role of Small Noncoding RNA in DNA Double-Strand Break Repair

DNA damage is a common phenomenon promoted through a variety of exogenous and endogenous factors. The DNA damage response (DDR) pathway involves a wide range of proteins, and as was indicated, small noncoding RNAs (sncRNAs). These are double-strand break-induced RNAs (diRNAs) and DNA damage response small RNA (DDRNA). Moreover, RNA binding proteins (RBPs) and RNA modifications have also been identified to modulate diRNA and DDRNA function in the DDR process. Several theories have been formulated regarding the synthesis and function of these sncRNAs during DNA repair; nevertheless, these pathways’ molecular details remain unclear. Here, we review the current knowledge regarding the mechanisms of diRNA and DDRNA biosynthesis and discuss the role of sncRNAs in maintaining genome stability.

Mechanism of Dynamic Binding of Replication Protein A to ssDNA

Replication protein A (RPA) serves as a hub protein inside eukaryotic cells, where it coordinates crucial DNA metabolic processes and activates the DNA-damage response system. A characteristic feature of its action is to associate with single-stranded DNA (ssDNA) intermediates before handing them over to downstream proteins. The length of ssDNA intermediates differs for different pathways. This means that RPA must have mechanisms for selective processing of ssDNA intermediates based on their length, the knowledge of which is fundamental to elucidate when and how DNA repair and replication processes are symphonized. By employing extensive molecular dynamics simulations, we investigated the mechanism of binding of RPA to ssDNA of different lengths. We show that the binding involves dynamic equilibrium with a stable intermediate, the population of which increases with the length of ssDNA. The vital underlying factors are decoded through collective variable principal component analysis. It suggests a differently orchestrated set of interactions that define the action of RPA based on the length of ssDNA intermediates. We further estimated the association kinetics that matches excellently well with previous experimental studies and probed the diffusion mechanism of RPA to ssDNA. RPA diffuses on short ssDNA through progressive "bulge" formation. With long ssDNA, we observed a conformational change in ssDNA coupled with its binding to RPA in a cooperative fashion. This unanticipated binding mechanism successfully explains how the "short-lived", long ssDNA intermediates are processed quickly in vivo. This study thus reveals the molecular basis of several recent experimental observations related to RPA binding to ssDNA and provides novel insights into the RPA functioning in DNA repair and replication.

Dynamic elements of replication protein A at the crossroads of DNA replication, recombination, and repair

The heterotrimeric eukaryotic Replication protein A (RPA) is a master regulator of numerous DNA metabolic processes. For a long time, it has been viewed as an inert protector of ssDNA and a platform for assembly of various genome maintenance and signaling machines. Later, the modular organization of the RPA DNA binding domains suggested a possibility for dynamic interaction with ssDNA. This modular organization has inspired several models for the RPA-ssDNA interaction that aimed to explain how RPA, the high-affinity ssDNA binding protein, is replaced by the downstream players in DNA replication, recombination, and repair that bind ssDNA with much lower affinity. Recent studies, and in particular single-molecule observations of RPA-ssDNA interactions, led to the development of a new model for the ssDNA handoff from RPA to a specific downstream factor where not only stability and structural rearrangements but also RPA conformational dynamics guide the ssDNA handoff. Here we will review the current knowledge of the RPA structure, its dynamic interaction with ssDNA, and how RPA conformational dynamics may be influenced by posttranslational modification and proteins that interact with RPA, as well as how RPA dynamics may be harnessed in cellular decision making.

RADX condenses single-stranded DNA to antagonize RAD51 loading

RADX is a mammalian single-stranded DNA-binding protein that stabilizes telomeres and stalled replication forks. Cellular biology studies have shown that the balance between RADX and Replication Protein A (RPA) is critical for DNA replication integrity. RADX is also a negative regulator of RAD51-mediated homologous recombination at stalled forks. However, the mechanism of RADX acting on DNA and its interactions with RPA and RAD51 are enigmatic. Using single-molecule imaging of the key proteins in vitro, we reveal that RADX condenses ssDNA filaments, even when the ssDNA is coated with RPA at physiological protein ratios. RADX compacts RPA-coated ssDNA filaments via higher-order assemblies that can capture ssDNA in trans. Furthermore, RADX blocks RPA displacement by RAD51 and prevents RAD51 loading on ssDNA. Our results indicate that RADX is an ssDNA condensation protein that inhibits RAD51 filament formation and may antagonize other ssDNA-binding proteins on RPA-coated ssDNA.

InteBac: An integrated bacterial and baculovirus expression vector suite

The successful production of recombinant protein for biochemical, biophysical, and structural biological studies critically depends on the correct expression organism. Currently, the most commonly used expression organisms for structural studies are Escherichia coli (~70% of all PDB structures) and the baculovirus/ insect cell expression system (~5% of all PDB structures). While insect cell expression is frequently successful for large eukaryotic proteins, it is relatively expensive and time-consuming compared to E. coli expression. Frequently the decision to carry out a baculovirus project means restarting cloning from scratch. Here we describe an integrated system that allows simultaneous cloning into E. coli and baculovirus expression vectors using the same PCR products. The system offers a flexible array of N- and C-terminal affinity, solubilization and utility tags, and the speed allows expression screening to be completed in E. coli, before carrying out time and cost-intensive experiments in baculovirus. Importantly, we describe a means of rapidly generating polycistronic bacterial constructs based on the hugely successful biGBac system, making InteBac of particular interest for researchers working on recombinant protein complexes.

Roles of OB-Fold Proteins in Replication Stress

Accurate DNA replication is essential for maintaining genome stability. However, this stability becomes vulnerable when replication fork progression is stalled or slowed - a condition known as replication stress. Prolonged fork stalling can cause DNA damage, leading to genome instabilities. Thus, cells have developed several pathways and a complex set of proteins to overcome the challenge at stalled replication forks. Oligonucleotide/oligosaccharide binding (OB)-fold containing proteins are a group of proteins that play a crucial role in fork protection and fork restart. These proteins bind to single-stranded DNA with high affinity and prevent premature annealing and unwanted nuclease digestion. Among these OB-fold containing proteins, the best studied in eukaryotic cells are replication protein A (RPA) and breast cancer susceptibility protein 2 (BRCA2). Recently, another RPA-like protein complex CTC1-STN1-TEN1 (CST) complex has been found to counter replication perturbation. In this review, we discuss the latest findings on how these OB-fold containing proteins (RPA, BRCA2, CST) cooperate to safeguard DNA replication and maintain genome stability.

The novel male meiosis recombination regulator coordinates the progression of meiosis prophase I

Meiosis is a specialized cell division for producing haploid gametes in sexually reproducing organisms. In this study, we have independently identified a novel meiosis protein male meiosis recombination regulator (MAMERR)/4930432K21Rik and showed that it is indispensable for meiosis prophase I progression in male mice. Using super-resolution structured illumination microscopy, we found that MAMERR functions at the same double-strand breaks as the replication protein A and meiosis-specific with OB domains/spermatogenesis associated 22 complex. We generated a Mamerr-deficient mouse model by deleting exons 3-6 and found that most of Mamerr^-/- spermatocytes were arrested at pachynema and failed to progress to diplonema, although they exhibited almost intact synapsis and progression to the pachytene stage along with XY body formation. Further mechanistic studies revealed that the recruitment of DMC1/RAD51 and heat shock factor 2-binding protein in Mamerr^-/- spermatocytes was only mildly impaired with a partial reduction in double-strand break repair, whereas a substantial reduction in ubiquitination on the autosomal axes and on the XY body appeared as a major phenotype in Mamerr^-/- spermatocytes. We propose that MAMERR may participate in meiotic prophase I progression by regulating the ubiquitination of key meiotic proteins on autosomes and XY chromosomes, and in the absence of MAMERR, the repressed ubiquitination of key meiotic proteins leads to pachytene arrest and cell death.

Replication protein A binds RNA and promotes R-loop formation

Replication protein A (RPA), a major eukaryotic ssDNA-binding protein, is essential for all metabolic processes that involve ssDNA, including DNA replication, repair, and damage signaling. To perform its functions, RPA binds ssDNA tightly. In contrast, it was presumed that RPA binds RNA weakly. However, recent data suggest that RPA may play a role in RNA metabolism. RPA stimulates RNA-templated DNA repair in vitro and associates in vivo with R-loops, the three-stranded structures consisting of an RNA-DNA hybrid and the displaced ssDNA strand. R-loops are common in the genomes of pro- and eukaryotes, including humans, and may play an important role in transcription-coupled homologous recombination and DNA replication restart. However, the mechanism of R-loop formation remains unknown. Here, we investigated the RNA-binding properties of human RPA and its possible role in R-loop formation. Using gel-retardation and RNA/DNA competition assays, we found that RPA binds RNA with an unexpectedly high affinity (K_D ≈ 100 pm). Furthermore, RPA, by forming a complex with RNA, can promote R-loop formation with homologous dsDNA. In reconstitution experiments, we showed that human DNA polymerases can utilize RPA-generated R-loops for initiation of DNA synthesis, mimicking the process of replication restart in vivo These results demonstrate that RPA binds RNA with high affinity, supporting the role of this protein in RNA metabolism and suggesting a mechanism of genome maintenance that depends on RPA-mediated DNA replication restart.

ATR signaling in mammalian meiosis: From upstream scaffolds to downstream signaling

In germ cells undergoing meiosis, the induction of double strand breaks (DSBs) is required for the generation of haploid gametes. Defects in the formation, detection, or recombinational repair of DSBs often result in defective chromosome segregation and aneuploidies. Central to the ability of meiotic cells to properly respond to DSBs are DNA damage response (DDR) pathways mediated by DNA damage sensor kinases. DDR signaling coordinates an extensive network of DDR effectors to induce cell cycle arrest and DNA repair, or trigger apoptosis if the damage is extensive. Despite their importance, the functions of DDR kinases and effector proteins during meiosis remain poorly understood and can often be distinct from their known mitotic roles. A key DDR kinase during meiosis is ataxia telangiectasia and Rad3-related (ATR). ATR mediates key signaling events that control DSB repair, cell cycle progression, and meiotic silencing. These meiotic functions of ATR depend on upstream scaffolds and regulators, including the 9-1-1 complex and TOPBP1, and converge on many downstream effectors such as the checkpoint kinase CHK1. Here, we review the meiotic functions of the 9-1-1/TOPBP1/ATR/CHK1 signaling pathway during mammalian meiosis.

Association between serum anti‑ASXL2 antibody levels and acute ischemic stroke, acute myocardial infarction, diabetes mellitus, chronic kidney disease and digestive organ cancer, and their possible association with atherosclerosis and hypertension

The aim of the present study was to identify novel antibody markers for the early diagnosis of atherosclerosis in order to improve the prognosis of patients at risk for acute ischemic stroke (AIS) and acute myocardial infarction (AMI). A first screening involved the serological identification of antigens by recombinant cDNA expression cloning and identified additional sex combs-like 2 (ASXL2) as a target antigen recognized by serum IgG antibodies in the sera of patients with atherosclerosis. Antigens, including the recombinant glutathione S-transferasefused ASXL2 protein and its synthetic peptide were then prepared to examine serum antibody levels. Amplified luminescence proximity homoge-neous assay-linked immunosorbent assay, which incorporates glutathione-donor beads and anti-human-IgG-acceptor beads, revealed significantly higher serum antibody levels against the ASXL2 protein and its peptide in the patients with AIS, diabetes mellitus, AMI, chronic kidney disease, esophageal squamous cell carcinoma, or colorectal carcinoma compared with those in healthy donors. The ASXL2 antibody levels were well associated with hypertension complication, but not with sex, body mass index, habitual smoking, or alcohol intake. These results suggest that the serum ASXL2 antibody marker can discriminate between hypertension-induced atherosclerotic AIS and AMI, as well as a number of digestive organ cancers.

Structure specific recognition of telomeric repeats containing RNA by the RGG-box of hnRNPA1

The telomere repeats containing RNA (TERRA) is transcribed from the C-rich strand of telomere DNA and comprises of UUAGGG nucleotides repeats in humans. The TERRA RNA repeats can exist in single stranded, RNA-DNA hybrid and G-quadruplex forms in the cell. Interaction of TERRA RNA with hnRNPA1 has been proposed to play critical roles in maintenance of telomere DNA. hnRNPA1 contains an N-terminal UP1 domain followed by an RGG-box containing C-terminal region. RGG-motifs are emerging as key protein motifs that recognize the higher order nucleic acid structures as well as are known to promote liquid-liquid phase separation of proteins. In this study, we have shown that the RGG-box of hnRNPA1 specifically recognizes the TERRA RNA G-quadruplexes that have loops in their topology, whereas it does not interact with the single-stranded RNA. Our results show that the N-terminal UP1 domain in the presence of the RGG-box destabilizes the loop containing TERRA RNA G-quadruplex efficiently compared to the RNA G-quadruplex that lacks loops, suggesting that unfolding of G-quadruplex structures by UP1 is structure dependent. Furthermore, we have compared the telomere DNA and TERRA RNA G-quadruplex binding by the RGG-box of hnRNPA1 and discussed its implications in telomere DNA maintenance.