Mechanism of single-stranded DNA annealing by RAD52-RPA complex.

RAD52 is important for the repair of DNA double-stranded breaks1,2, mitotic DNA synthesis3-5 and alternative telomere length maintenance6,7. Central to these functions, RAD52 promotes the annealing of complementary single-stranded DNA (ssDNA)8,9 and provides an alternative to BRCA2/RAD51-dependent homologous recombination repair10. Inactivation of RAD52 in homologous-recombination-deficient BRCA1- or BRCA2-defective cells is synthetically lethal11,12, and aberrant expression of RAD52 is associated with poor cancer prognosis13,14. As a consequence, RAD52 is an attractive therapeutic target against homologous-recombination-deficient breast, ovarian and prostate cancers15-17. Here we describe the structure of RAD52 and define the mechanism of annealing. As reported previously18-20, RAD52 forms undecameric (11-subunit) ring structures, but these rings do not represent the active form of the enzyme. Instead, cryo-electron microscopy and biochemical analyses revealed that ssDNA annealing is driven by RAD52 open rings in association with replication protein-A (RPA). Atomic models of the RAD52-ssDNA complex show that ssDNA sits in a positively charged channel around the ring. Annealing is driven by the RAD52 N-terminal domains, whereas the C-terminal regions modulate the open-ring conformation and RPA interaction. RPA associates with RAD52 at the site of ring opening with critical interactions occurring between the RPA-interacting domain of RAD52 and the winged helix domain of RPA2. Our studies provide structural snapshots throughout the annealing process and define the molecular mechanism of ssDNA annealing by the RAD52-RPA complex.

Rad52 Oligomeric N-Terminal Domain Stabilizes Rad51 Nucleoprotein Filaments and Contributes to Their Protection against Srs2.

Homologous recombination (HR) depends on the formation of a nucleoprotein filament of the recombinase Rad51 to scan the genome and invade the homologous sequence used as a template for DNA repair synthesis. Therefore, HR is highly accurate and crucial for genome stability. Rad51 filament formation is controlled by positive and negative factors. In Saccharomyces cerevisiae, the mediator protein Rad52 catalyzes Rad51 filament formation and stabilizes them, mostly by counteracting the disruptive activity of the translocase Srs2. Srs2 activity is essential to avoid the formation of toxic Rad51 filaments, as revealed by Srs2-deficient cells. We previously reported that Rad52 SUMOylation or mutations disrupting the Rad52-Rad51 interaction suppress Rad51 filament toxicity because they disengage Rad52 from Rad51 filaments and reduce their stability. Here, we found that mutations in Rad52 N-terminal domain also suppress the DNA damage sensitivity of Srs2-deficient cells. Structural studies showed that these mutations affect the Rad52 oligomeric ring structure. Overall, in vivo and in vitro analyzes of these mutants indicate that Rad52 ring structure is important for protecting Rad51 filaments from Srs2, but can increase Rad51 filament stability and toxicity in Srs2-deficient cells. This stabilization function is distinct from Rad52 mediator and annealing activities.

Single molecule microscopy reveals key physical features of repair foci in living cells.

In response to double strand breaks (DSB), repair proteins accumulate at damaged sites, forming membrane-less sub-compartments or foci. Here we explored the physical nature of these foci, using single molecule microscopy in living cells. Rad52, the functional homolog of BRCA2 in yeast, accumulates at DSB sites and diffuses ~6 times faster within repair foci than the focus itself, exhibiting confined motion. The Rad52 confinement radius coincides with the focus size: foci resulting from 2 DSBs are twice larger in volume that the ones induced by a unique DSB and the Rad52 confinement radius scales accordingly. In contrast, molecules of the single strand binding protein Rfa1 follow anomalous diffusion similar to the focus itself or damaged chromatin. We conclude that while most Rfa1 molecules are bound to the ssDNA, Rad52 molecules are free to explore the entire focus reflecting the existence of a liquid droplet around damaged DNA.

A simple fluorescent assay for the discovery of protein-protein interaction inhibitors.

Due to the therapeutic potential of targeting protein-protein interactions (PPIs) there is a need for easily executed assays to perform high throughput screening (HTS) of inhibitors. We have developed and optimized an innovative and robust fluorescence-based assay for detecting PPI inhibitors, called FluorIA (Fluorescence-based protein-protein Interaction Assay). Targeting the PPI of RAD52 with replication protein A (RPA) was used as an example, and the FluorIA protocol design, optimization and successful application to HTS of large chemical libraries are described. Here enhanced green fluorescent protein (EGFP)-tagged RAD52 detected the PPI using full-length RPA heterotrimer coated, black microtiter plates and loss in fluorescence intensity identified small molecule inhibitors (SMIs) that displaced the EGFP-tagged RAD52. The FluorIA design and protocol can be adapted and applied to detect PPIs for other protein systems. This should push forward efforts to develop targeted therapeutics against protein complexes in pathological processes.

Rad52-Rad51 association is essential to protect Rad51 filaments against Srs2, but facultative for filament formation.

Homology search and strand exchange mediated by Rad51 nucleoprotein filaments are key steps of the homologous recombination process. In budding yeast, Rad52 is the main mediator of Rad51 filament formation, thereby playing an essential role. The current model assumes that Rad51 filament formation requires the interaction between Rad52 and Rad51. However, we report here that Rad52 mutations that disrupt this interaction do not affect γ-ray- or HO endonuclease-induced gene conversion frequencies. In vivo and in vitro studies confirmed that Rad51 filaments formation is not affected by these mutations. Instead, we found that Rad52-Rad51 association makes Rad51 filaments toxic in Srs2-deficient cells after exposure to DNA damaging agents, independently of Rad52 role in Rad51 filament assembly. Importantly, we also demonstrated that Rad52 is essential for protecting Rad51 filaments against dissociation by the Srs2 DNA translocase. Our findings open new perspectives in the understanding of the role of Rad52 in eukaryotes.

Catch the live show: Visualizing damaged DNA in vivo.

The health of an organism is intimately linked to its ability to repair damaged DNA. Importantly, DNA repair processes are highly dynamic. This highlights the necessity of characterizing DNA repair in live cells. Advanced genome editing and imaging approaches allow us to visualize damaged DNA and its associated factors in real time. Here, we summarize both established and recent methods that are used to induce DNA damage and visualize damaged DNA and its repair in live cells.

Human RAD52 interactions with replication protein A and the RAD51 presynaptic complex.

Rad52 is a highly conserved protein involved in the repair of DNA damage. Human RAD52 has been shown to mediate single-stranded DNA (ssDNA) and is synthetic lethal with mutations in other key recombination proteins. For this study, we used single-molecule imaging and ssDNA curtains to examine the binding interactions of human RAD52 with replication protein A (RPA)-coated ssDNA, and we monitored the fate of RAD52 during assembly of the presynaptic complex. We show that RAD52 binds tightly to the RPA-ssDNA complex and imparts an inhibitory effect on RPA turnover. We also found that during presynaptic complex assembly, most of the RPA and RAD52 was displaced from the ssDNA, but some RAD52-RPA-ssDNA complexes persisted as interspersed clusters surrounded by RAD51 filaments. Once assembled, the presence of RAD51 restricted formation of new RAD52-binding events, but additional RAD52 could bind once RAD51 dissociated from the ssDNA. Together, these results provide new insights into the behavior and dynamics of human RAD52 during presynaptic complex assembly and disassembly.

Structure of the human DNA-repair protein RAD52 containing surface mutations.

The Rad52 protein is a eukaryotic single-strand DNA-annealing protein that is involved in the homologous recombinational repair of DNA double-strand breaks. The isolated N-terminal half of the human RAD52 protein (RAD52(1-212)) forms an undecameric ring structure with a surface that is mostly positively charged. In the present study, it was found that RAD52(1-212) containing alanine mutations of the charged surface residues (Lys102, Lys133 and Glu202) is highly amenable to crystallization. The structure of the mutant RAD52(1-212) was solved at 2.4 Šresolution. The structure revealed an association between the symmetry-related RAD52(1-212) rings, in which a partially unfolded, C-terminal region of RAD52 extended into the DNA-binding groove of the neighbouring ring in the crystal. The alanine mutations probably reduced the surface entropy of the RAD52(1-212) ring and stabilized the ring-ring association observed in the crystal.

Roles of C-Terminal Region of Yeast and Human Rad52 in Rad51-Nucleoprotein Filament Formation and ssDNA Annealing.

Yeast Rad52 (yRad52) has two important functions at homologous DNA recombination (HR); annealing complementary single-strand DNA (ssDNA) molecules and recruiting Rad51 recombinase onto ssDNA (recombination mediator activity). Its human homolog (hRAD52) has a lesser role in HR, and apparently lacks mediator activity. Here we show that yRad52 can load human Rad51 (hRAD51) onto ssDNA complexed with yeast RPA in vitro. This is biochemically equivalent to mediator activity because it depends on the C-terminal Rad51-binding region of yRad52 and on functional Rad52-RPA interaction. It has been reported that the N-terminal two thirds of both yRad52 and hRAD52 is essential for binding to and annealing ssDNA. Although a second DNA binding region has been found in the C-terminal region of yRad52, its role in ssDNA annealing is not clear. In this paper, we also show that the C-terminal region of yRad52, but not of hRAD52, is involved in ssDNA annealing. This suggests that the second DNA binding site is required for the efficient ssDNA annealing by yRad52. We propose an updated model of Rad52-mediated ssDNA annealing.

SUMOylation of Rad52-Rad59 synergistically change the outcome of mitotic recombination.

Homologous recombination (HR) is essential for maintenance of genome stability through double-strand break (DSB) repair, but at the same time HR can lead to loss of heterozygosity and uncontrolled recombination can be genotoxic. The post-translational modification by SUMO (small ubiquitin-like modifier) has been shown to modulate recombination, but the exact mechanism of this regulation remains unclear. Here we show that SUMOylation stabilizes the interaction between the recombination mediator Rad52 and its paralogue Rad59 in Saccharomyces cerevisiae. Although Rad59 SUMOylation is not required for survival after genotoxic stress, it affects the outcome of recombination to promote conservative DNA repair. In some genetic assays, Rad52 and Rad59 SUMOylation act synergistically. Collectively, our data indicate that the described SUMO modifications affect the balance between conservative and non-conservative mechanisms of HR.

Biochemical characterization of plant Rad52 protein from rice (Oryza sativa).

DNA damage in living cells is repaired by two main pathways, homologous recombination (HR) and non-homologous end joining (NHEJ). Of all the genes promoting HR, Rad52 (Radiation sensitive 52) is an important gene which is found to be highly conserved across different species. It was believed that RAD52 is absent in plant systems until lately. However, recent genetic studies have shown the presence of RAD52 homologues in plants. Rad52 homologues in plant systems have not yet been characterized biochemically. In the current study, we bring out the biochemical properties of rice Rad52-2a protein. OsRad52-2a was over-expressed in Escherichia coli BL21 (DE3) cells and the protein was purified. The identity of purified OsRad52-2a protein was confirmed via peptide mass fingerprinting. Gel filtration and native PAGE analysis indicated that the OsRad52-2a protein in its native state probably formed an undecameric structure. Purified OsRad52-2a protein showed binding to single stranded DNA, double stranded DNA. Protein also mediated the renaturation of complementary single strands into duplex DNA in both agarose gel and FRET based assays. Put together, OsRad52-2a forms oligomeric structures and binds to ssDNA/dsDNA for mediating an important function like renaturation during homologous recombination. This study represents the first report on biochemical properties of OsRad52-2a protein from important crop like rice. This information will help in dissecting the recombination and repair machinery in plant systems.

Targeting BRCA1- and BRCA2-deficient cells with RAD52 small molecule inhibitors.

RAD52 is a member of the homologous recombination (HR) pathway that is important for maintenance of genome integrity. While single RAD52 mutations show no significant phenotype in mammals, their combination with mutations in genes that cause hereditary breast cancer and ovarian cancer like BRCA1, BRCA2, PALB2 and RAD51C are lethal. Consequently, RAD52 may represent an important target for cancer therapy. In vitro, RAD52 has ssDNA annealing and DNA strand exchange activities. Here, to identify small molecule inhibitors of RAD52 we screened a 372,903-compound library using a fluorescence-quenching assay for ssDNA annealing activity of RAD52. The obtained 70 putative inhibitors were further characterized using biochemical and cell-based assays. As a result, we identified compounds that specifically inhibit the biochemical activities of RAD52, suppress growth of BRCA1- and BRCA2-deficient cells and inhibit RAD52-dependent single-strand annealing (SSA) in human cells. We will use these compounds for development of novel cancer therapy and as a probe to study mechanisms of DNA repair.

Identification of a Small Molecule Inhibitor of RAD52 by Structure-Based Selection.

It has been reported that inhibition of RAD52 either by specific shRNA or a small peptide aptamer induced synthetic lethality in tumor cell lines carrying BRCA1 and BRCA2 inactivating mutations. Molecular docking was used to screen two chemical libraries: 1) 1,217 FDA approved drugs, and 2) 139,735 drug-like compounds to identify candidates for interacting with DNA binding domain of human RAD52. Thirty six lead candidate compounds were identified that were predicted to interfere with RAD52 -DNA binding. Further biological testing confirmed that 9 of 36 candidate compounds were able to inhibit the binding of RAD52 to single-stranded DNA in vitro. Based on molecular binding combined with functional assays, we propose a model in which the active compounds bind to a critical "hotspot" in RAD52 DNA binding domain 1. In addition, one of the 9 active compounds, adenosine 5'-monophosphate (A5MP), and also its mimic 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) 5' phosphate (ZMP) inhibited RAD52 activity in vivo and exerted synthetic lethality against BRCA1 and BRCA2-mutated carcinomas. These data suggest that active, inhibitory RAD52 binding compounds could be further refined for efficacy and safety to develop drugs inducing synthetic lethality in tumors displaying deficiencies in BRCA1/2-mediated homologous recombination.

Association of a functional RAD52 genetic variant locating in a miRNA binding site with risk of HBV-related hepatocellular carcinoma.

As an important member in homologous recombination repair, RAD52 plays a crucial part in maintaining genomic stability and prevent carcinogenesis. Several cancer susceptibility RAD52 single nucleotide polymorphisms (SNPs) have been identified previously. However, little or nothing has been known about the RAD52 SNPs and their functional significance in hepatitis B viruses (HBV)-related hepatocellular carcinoma (HCC). Therefore, we investigated the association between five RAD52 SNPs (rs1051669, rs10774474, rs11571378, rs7963551, and rs6489769) and HBV-related HCC risk as well as its biological function in vivo. Genotypes were determined in two independent case-control sets from two regions of China. Odds ratios (ORs) and 95% confidence intervals (CIs) were estimated by logistic regression. The allele-specific regulation on RAD52 expression by the functional genetic variant was examined with normal liver tissues. We found that only the RAD52 rs7963551 SNP was significantly associated with HCC risk, with the odds of having the rs7963551 CC genotype in patients was 0.59 (95% CI = 0.45-0.78, P = 1.5 × 10(-4), HCC cases versus chronic HBV carriers) or 0.65 (95% CI = 0.52-0.81, P = 1.1 × 10(-4), HCC cases versus healthy controls) compared with the AA genotype. In the genotype-phenotype correlation analyses of 44 human liver tissue samples, rs7963551 CC or AC was associated with a statistically significant increase of RAD52 mRNA expression, which are consistent to functional relevance of allelic regulation of RAD52 expression by rs7963551 SNP and miRNA let-7 in cancer cells. Our data demonstrated that RAD52 functional rs7963551 SNP contributes to susceptibility to developing HCC.

Functional analyses of the C-terminal half of the Saccharomyces cerevisiae Rad52 protein.

The Saccharomyces cerevisiae Rad52 protein is essential for efficient homologous recombination (HR). An important role of Rad52 in HR is the loading of Rad51 onto replication protein A-coated single-stranded DNA (ssDNA), which is referred to as the recombination mediator activity. In vitro, Rad52 displays additional activities, including self-association, DNA binding and ssDNA annealing. Although Rad52 has been a subject of extensive genetic, biochemical and structural studies, the mechanisms by which these activities are coordinated in the various roles of Rad52 in HR remain largely unknown. In the present study, we found that an isolated C-terminal half of Rad52 disrupted the Rad51 oligomer and formed a heterodimeric complex with Rad51. The Rad52 fragment inhibited the binding of Rad51 to double-stranded DNA, but not to ssDNA. The phenylalanine-349 and tyrosine-409 residues present in the C-terminal half of Rad52 were critical for the interaction with Rad51, the disruption of Rad51 oligomers, the mediator activity of the full-length protein and for DNA repair in vivo in the presence of methyl methanesulfonate. Our studies suggested that phenylalanine-349 and tyrosine-409 are key residues in the C-terminal half of Rad52 and probably play an important role in the mediator activity.

The C-terminal region of Rad52 is essential for Rad52 nuclear and nucleolar localization, and accumulation at DNA damage sites immediately after irradiation.

Rad52 plays essential roles in homologous recombination (HR) and repair of DNA double-strand breaks (DSBs) in Saccharomyces cerevisiae. However, in vertebrates, knockouts of the Rad52 gene show no hypersensitivity to agents that induce DSBs. Rad52 localizes in the nucleus and forms foci at a late stage following irradiation. Ku70 and Ku80, which play an essential role in nonhomologous DNA-end-joining (NHEJ), are essential for the accumulation of other core NHEJ factors, e.g., XRCC4, and a HR-related factor, e.g., BRCA1. Here, we show that the subcellular localization of EYFP-Rad52(1-418) changes dynamically during the cell cycle. In addition, EYFP-Rad52(1-418) accumulates rapidly at microirradiated sites and colocalizes with the DSB sensor protein Ku80. Moreover, the accumulation of EYFP-Rad52(1-418) at DSB sites is independent of the core NHEJ factors, i.e., Ku80 and XRCC4. Furthermore, we observed that EYFP-Rad52(1-418) localizes in nucleoli in CHO-K1 cells and XRCC4-deficient cells, but not in Ku80-deficient cells. We also found that Rad52 nuclear localization, nucleolar localization, and accumulation at DSB sites are dependent on eight amino acids (411-418) at the end of the C-terminal region of Rad52 (Rad52 CTR). Furthermore, basic amino acids on Rad52 CTR are highly conserved among mammalian, avian, and fish homologues, suggesting that Rad52 CTR is important for the regulation and function of Rad52 in vertebrates. These findings also suggest that the mechanism underlying the regulation of subcellular localization of Rad52 is important for the physiological function of Rad52 not only at a late stage following irradiation, but also at an early stage.

There and back again: new single-molecule insights in the motion of DNA repair proteins.

Cellular DNA repair machines are constantly at work supporting the integrity of our genomes. Numerous proteins cooperate to form a complex and adaptive system dedicated to detection and timely processing of DNA damage. The molecular underpinnings of how these proteins locate and discriminate DNA lesions, match homologous sequences, mend the DNA and attend to a replication in distress are of a paramount biomedical importance, but in many cases remain unclear. Combined with more conventional tools, single-molecule biochemistry has been stepping in to address the age-old problems in the DNA repair field. This review will address new insights into diffusive properties of three DNA repair systems: I will discuss the emerging model of how MutS homologues locate and respond to mismatches in the dsDNA; the mechanism by which RAD52 promotes annealing of complementary DNA strands coated with ssDNA binding protein RPA; and how the nucleoprotein filament formed by RecA recombinase on ssDNA searches for homology within duplex DNA. These three distinct DNA repair factors exemplify the dynamic nature of cellular DNA repair machines revealed by single-molecule studies.

Tyrosine phosphorylation enhances RAD52-mediated annealing by modulating its DNA binding.

RAD52 protein has an important role in homology-directed DNA repair by mediating RAD51 nucleoprotein filament formation on single-stranded DNA (ssDNA) protected by replication protein-A (RPA) and annealing of RPA-coated ssDNA. In human, cellular response to DNA damage includes phosphorylation of RAD52 by c-ABL kinase at tyrosine 104. To address how this phosphorylation modulates RAD52 function, we used an amber suppressor technology to substitute tyrosine 104 with chemically stable phosphotyrosine analogue (p-Carboxymethyl-L-phenylalanine, pCMF). The RAD52(Y104pCMF) retained ssDNA-binding activity characteristic of unmodified RAD52 but showed lower affinity for double-stranded DNA (dsDNA) binding. Single-molecule analyses revealed that RAD52(Y104pCMF) specifically targets and wraps ssDNA. While RAD52(Y104pCMF) is confined to ssDNA region, unmodified RAD52 readily diffuses into dsDNA region. The Y104pCMF substitution also increased the ssDNA annealing rate and allowed overcoming the inhibitory effect of dsDNA. We propose that phosphorylation at Y104 enhances ssDNA annealing activity of RAD52 by attenuating dsDNA binding. Implications of phosphorylation-mediated activation of RAD52 annealing activity are discussed.

Presynaptic filament dynamics in homologous recombination and DNA repair.

Homologous recombination (HR) is an essential genome stability mechanism used for high-fidelity repair of DNA double-strand breaks and for the recovery of stalled or collapsed DNA replication forks. The crucial homology search and DNA strand exchange steps of HR are catalyzed by presynaptic filaments-helical filaments of a recombinase enzyme bound to single-stranded DNA (ssDNA). Presynaptic filaments are fundamentally dynamic structures, the assembly, catalytic turnover, and disassembly of which must be closely coordinated with other elements of the DNA recombination, repair, and replication machinery in order for genome maintenance functions to be effective. Here, we reviewed the major dynamic elements controlling the assembly, activity, and disassembly of presynaptic filaments; some intrinsic such as recombinase ATP-binding and hydrolytic activities, others extrinsic such as ssDNA-binding proteins, mediator proteins, and DNA motor proteins. We examined dynamic behavior on multiple levels, including atomic- and filament-level structural changes associated with ATP binding and hydrolysis as evidenced in crystal structures, as well as subunit binding and dissociation events driven by intrinsic and extrinsic factors. We examined the biochemical properties of recombination proteins from four model systems (T4 phage, Escherichia coli, Saccharomyces cerevisiae, and Homo sapiens), demonstrating how their properties are tailored for the context-specific requirements in these diverse species. We proposed that the presynaptic filament has evolved to rely on multiple external factors for increased multilevel regulation of HR processes in genomes with greater structural and sequence complexity.