The human Rad52 protein exists as a heptameric ring

Monomers and short oligomers of human RAD52 promote single-strand annealing

Genome maintenance and stability rely on the repair of DNA double-strand breaks. Breaks can be repaired via the single-strand-annealing pathway mediated by the protein RAD52. RAD52 oligomerizes to rings that are thought to promote annealing. However, rings have only been observed at micromolar concentrations at which annealing activity is impaired. Thus, it is unclear which oligomeric form is responsible for annealing. We combined single-molecule mass photometry with biochemical assays to determine the in vitro oligomeric states of human RAD52. We found that RAD52 was mostly monomeric at lower nanomolar concentrations. With increasing concentration, RAD52 oligomerized and formed rings with a variable stoichiometry from heptamers to tridecamers consistent with an oligomerization model of noncooperative assembly coupled with preferential cyclization. Under conditions where hardly any rings were present, RAD52 already promoted single-strand annealing in vitro. Our findings indicate that in vitro single-strand annealing can be mediated by monomers and short oligomers of RAD52. The oligomerization model suggests that ring formation is similar to a phase transition whereby rings are a reservoir to replenish the monomer and short oligomer pool. This pool has a nearly constant concentration which may be optimal for annealing and would be independent, for example, of the amount of DNA damage, protein upregulation, or the cell cycle.

Prime Editing: Mechanistic Insights and DNA Repair Modulation

Prime editing is a genome editing technique that allows precise modifications of cellular DNA without relying on donor DNA templates. Recently, several different prime editor proteins have been published in the literature, relying on single- or double-strand breaks. When prime editing occurs, the DNA undergoes one of several DNA repair pathways, and these processes can be modulated with the use of inhibitors. Firstly, this review provides an overview of several DNA repair mechanisms and their modulation by known inhibitors. In addition, we summarize different published prime editors and provide a comprehensive overview of associated DNA repair mechanisms. Finally, we discuss the delivery and safety aspects of prime editing.

Biological functions of 5-methylcytosine RNA-binding proteins and their potential mechanisms in human cancers

The 5-methylcytosine (m5C) modification is a crucial epigenetic RNA modification, which is involved in the post-transcriptional regulation of genes. It plays an important role in various biological processes, including cell metabolism, growth, apoptosis, and tumorigenesis. By affecting the proliferation, migration, invasion, and drug sensitivity of tumor cells, m5C methylation modification plays a vital part in the initiation and progression of tumors and is closely associated with the poor tumor prognosis. m5C-related proteins are categorized into three functional groups: m5C methyltransferases (m5C writers), m5C demethylases (m5C erasers), and m5C methyl-binding proteins (m5C readers). This paper introduces several common methodologies for detecting m5C methylation; and reviews the molecular structure and biological functions of m5C readers, including ALYREF, YBX1, YBX2, RAD52, YTHDF2, FMRP, and SRSF2. It further summarizes their roles and regulatory mechanisms in tumors, offering novel targets and insights for tumor treatment.

Homologous Recombination and DNA Intermediates Analyzed by Electron Microscopy

Homologous recombination (HR) is a high-fidelity DNA repair pathway that uses a homologous DNA sequence as a template. Recombinase proteins are the central HR players in the three kingdoms of life. RecA/RadA/Rad51 assemble on ssDNA, generated after the processing of double-strand breaks or stalled replication forks into an active and dynamic presynaptic helical nucleofilament. Presynaptic filament formation is regulated by a series of partners of the recombinase, such as scRad52/hBRCA2 mediators or anti-recombinase proteins, to form an active machinery involved in homology search, pair-matching, and invasion within homologous sequences. During homology search, but also during strand invasion, the multiprotein complexes that form the nucleofilament induce the formation of a variety of DNA intermediate states. Here we present specific approaches to study and characterize the different DNA and DNA-protein intermediates formed during homologous recombination. The combination of powerful electron microscopy and sample preparation methods provides a better understanding of these proteins' molecular activity and their interactions.

A double-ring of human RAD52 remodels replication forks restricting fork reversal

Human RAD52 1,2 is a multifunctional DNA repair protein involved in several cellular events that support genome stability including protection of stalled DNA replication forks from excessive degradation 3-7 . In its gatekeeper role, RAD52 binds to and stabilizes stalled replication forks during replication stress protecting them from reversal by SMARCAL1 5 . The structural and molecular mechanism of the RAD52-mediated fork protection remains elusive. Here, using P1 nuclease sensitivity, biochemical and single-molecule analyses we show that RAD52 dynamically remodels replication forks through its strand exchange activity. The presence of the ssDNA binding protein RPA at the fork modulates the kinetics of the strand exchange without impeding the reaction outcome. Mass photometry and single-particle cryo-electron microscopy show that the replication fork promotes a unique nucleoprotein structure containing head-to-head arrangement of two undecameric RAD52 rings with an extended positively charged surface that accommodates all three arms of the replication fork. We propose that the formation and continuity of this surface is important for the strand exchange reaction and for competition with SMARCAL1.

One Sentence Summary

Using cryo-EM, biochemical and single-molecule approaches we show that the structure of stalled DNA replication fork promotes a unique two-ring organization of human RAD52 protein which remodels the fork via DNA strand exchange.

An integrative structural study of the human full-length RAD52 at 2.2 Å resolution

Human RAD52 (RAD52) is a DNA-binding protein involved in many DNA repair mechanisms and genomic stability maintenance. In the last few years, this protein was discovered to be a promising novel pharmacological target for anticancer strategies. Although the interest in RAD52 has exponentially grown in the previous decade, most information about its structure and mechanism still needs to be elucidated. Here, we report the 2.2 Å resolution cryo-EM reconstruction of the full-length RAD52 (FL-RAD52) protein. This allows us to describe the hydration shell of the N-terminal region of FL-RAD52, which is structured in an undecamer ring. Water molecules coordinate with protein residues to promote stabilization inside and among the protomers and within the inner DNA binding cleft to drive protein-DNA recognition. Additionally, through a multidisciplinary approach involving SEC-SAXS and computational methods, we comprehensively describe the highly flexible and dynamic organization of the C-terminal portion of FL-RAD52. This work discloses unprecedented structural details on the FL-RAD52, which will be critical for characterizing its mechanism of action and inhibitor development, particularly in the context of novel approaches to synthetic lethality and anticancer drug discovery.

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.

Half a century after their discovery: Structural insights into exonuclease and annealase proteins catalyzing recombineering

Recombineering is an essential tool for molecular biologists, allowing for the facile and efficient manipulation of bacterial genomes directly in cells without the need for costly and laborious in vitro manipulations involving restriction enzymes. The main workhorses behind recombineering are bacteriophage proteins that promote the single-strand annealing (SSA) homologous recombination pathway to repair double-stranded DNA breaks. While there have been several reviews examining recombineering methods and applications, comparatively few have focused on the mechanisms of the proteins that are the key players in the SSA pathway: a 5'→3' exonuclease and a single-strand annealing protein (SSAP or "annealase"). This review dives into the structures and functions of the two SSA recombination systems that were the first to be developed for recombineering in E. coli: the RecET system from E. coli Rac prophage and the λRed system from bacteriophage λ. By comparing the structures of the RecT and Redβ annealases, and the RecE and λExo exonucleases, we provide new insights into how the structures of these proteins dictate their function. Examining the sequence conservation of the λExo and RecE exonucleases gives more profound insights into their critical functional features. Ultimately, as recombineering accelerates and evolves in the laboratory, a better understanding of the mechanisms of the proteins behind this powerful technique will drive the development of improved and expanded capabilities in the future.

Human RAD52 stimulates the RAD51-mediated homology search

Homologous recombination (HR) is a DNA repair mechanism of double-strand breaks and blocked replication forks, involving a process of homology search leading to the formation of synaptic intermediates that are regulated to ensure genome integrity. RAD51 recombinase plays a central role in this mechanism, supported by its RAD52 and BRCA2 partners. If the mediator function of BRCA2 to load RAD51 on RPA-ssDNA is well established, the role of RAD52 in HR is still far from understood. We used transmission electron microscopy combined with biochemistry to characterize the sequential participation of RPA, RAD52, and BRCA2 in the assembly of the RAD51 filament and its activity. Although our results confirm that RAD52 lacks a mediator activity, RAD52 can tightly bind to RPA-coated ssDNA, inhibit the mediator activity of BRCA2, and form shorter RAD51-RAD52 mixed filaments that are more efficient in the formation of synaptic complexes and D-loops, resulting in more frequent multi-invasions as well. We confirm the in situ interaction between RAD51 and RAD52 after double-strand break induction in vivo. This study provides new molecular insights into the formation and regulation of presynaptic and synaptic intermediates by BRCA2 and RAD52 during human HR.

Analysis of the indispensable RAD51 cofactor BRCA2 in Naganishia liquefaciens, a Basidiomycota yeast

The BRCA2 tumor suppressor plays a critical role in homologous recombination by regulating RAD51, the eukaryotic homologous recombinase. We identified the BRCA2 homolog in a Basidiomycota yeast, Naganishia liquefaciens BRCA2 homologs are found in many Basidiomycota species but not in Ascomycota species. Naganishia BRCA2 (Brh2, for BRCA2 homolog) is about one-third the size of human BRCA2. Brh2 carries three potential BRC repeats with two oligonucleotide/oligosaccharide-binding domains. The homolog of DSS1, a small acidic protein serving as an essential partner of BRCA2 was also identified. The yeast two-hybrid assay shows the interaction of Brh2 with both Rad51 and Dss1. Unlike human BRCA2, Brh2 is not required for normal cell growth, whereas loss of Dss1 results in slow growth. The loss of Brh2 caused pronounced sensitivity to UV and ionizing radiation, and their HR ability, as assayed by gene-targeting efficiency, is compromised. These phenotypes are indistinguishable from those of the rad51 mutant, and the rad51 brh2 double mutant. Naganishia Brh2 is likely the BRCA2 ortholog that functions as an indispensable auxiliary factor for Rad51.

Positive and negative regulators of RAD51/DMC1 in homologous recombination and DNA replication

RAD51 recombinase plays a central role in homologous recombination (HR) by forming a nucleoprotein filament on single-stranded DNA (ssDNA) to catalyze homology search and strand exchange between the ssDNA and a homologous double-stranded DNA (dsDNA). The catalytic activity of RAD51 assembled on ssDNA is critical for the DNA-homology-mediated repair of DNA double-strand breaks in somatic and meiotic cells and restarting stalled replication forks during DNA replication. The RAD51-ssDNA complex also plays a structural role in protecting the regressed/reversed replication fork. Two types of regulators control RAD51 filament formation, stability, and dynamics, namely positive regulators, including mediators, and negative regulators, so-called remodelers. The appropriate balance of action by the two regulators assures genome stability. This review describes the roles of positive and negative RAD51 regulators in HR and DNA replication and its meiosis-specific homolog DMC1 in meiotic recombination. We also provide future study directions for a comprehensive understanding of RAD51/DMC1-mediated regulation in maintaining and inheriting genome integrity.

Twinkle-Catalyzed Toehold-Mediated DNA Strand Displacement Reaction

Strand exchange between homologous nucleic acid sequences is the basis for cellular DNA repair, recombination, and genome editing technologies. Specialized enzymes catalyze cellular strand exchange; however, the reaction occurs spontaneously when a single-stranded DNA toehold can dock the invader strand on the target DNA to initiate strand exchange through branch migration. Due to its precise response, the spontaneous toehold-mediated strand displacement (TMSD) reaction is widely employed in DNA nanotechnology. However, enzyme-free TMSD suffers from slow rates, resulting in slow response times. Here, we show that human mitochondrial DNA helicase Twinkle can accelerate TMSD up to 6000-fold. Mechanistic studies indicate that Twinkle accelerates TMSD by catalyzing the docking step, which typically limits spontaneous reactions. The catalysis occurs without ATP, and Twinkle-catalyzed TMSD rates remain sensitive to base-pair mismatches. The simple catalysis, tunability, and speed improvement of the catalyzed TMSD can be leveraged in nanotechnology, requiring sensitive detection and faster response times.

Yeast Rad52 is a homodecamer and possesses BRCA2-like bipartite Rad51 binding modes

Homologous recombination (HR) is an essential double-stranded DNA break repair pathway. In HR, Rad52 facilitates the formation of Rad51 nucleoprotein filaments on RPA-coated ssDNA. Here, we decipher how Rad52 functions using single-particle cryo-electron microscopy and biophysical approaches. We report that Rad52 is a homodecameric ring and each subunit possesses an ordered N-terminal and disordered C-terminal half. An intrinsic structural asymmetry is observed where a few of the C-terminal halves interact with the ordered ring. We describe two conserved charged patches in the C-terminal half that harbor Rad51 and RPA interacting motifs. Interactions between these patches regulate ssDNA binding. Surprisingly, Rad51 interacts with Rad52 at two different bindings sites: one within the positive patch in the disordered C-terminus and the other in the ordered ring. We propose that these features drive Rad51 nucleation onto a single position on the DNA to promote formation of uniform pre-synaptic Rad51 filaments in HR.

Homodecameric Rad52 promotes single-position Rad51 nucleation in homologous recombination

Homologous recombination (HR) is a pathway for the accurate repair of double-stranded DNA breaks. These breaks are resected to yield single-stranded DNA (ssDNA) that are coated by Replication Protein A (RPA). Saccharomyces cerevisiae Rad52 is a mediator protein that promotes HR by facilitating formation of Rad51 nucleoprotein filaments on RPA-coated ssDNA. Canonically, Rad52 has been described to function by displacing RPA to promote Rad51 binding. However, in vitro, Rad51 readily forms a filament by displacing RPA in the absence of Rad52. Yet, in vivo, Rad52 is essential for HR. Here, we resolve how Rad52 functions as a mediator using single-particle cryo-electron microscopy and biophysical approaches. We show that Rad52 functions as a homodecamer and catalyzes single-position nucleation of Rad51. The N-terminal half of Rad52 is a well-ordered ring, while the C-terminal half is disordered. An intrinsic asymmetry within Rad52 is observed, where one or a few of the C-terminal halves interact with the ordered N-terminal ring. Within the C-terminal half, we identify two conserved charged patches that harbor the Rad51 and RPA interacting motifs. Interactions between these two charged patches regulate a ssDNA binding. These features drive Rad51 binding to a single position on the Rad52 decameric ring. We propose a Rad52 catalyzed single-position nucleation model for the formation of pre-synaptic Rad51 filaments in HR.

Therapeutic disruption of RAD52-ssDNA complexation via novel drug-like inhibitors

RAD52 protein is a coveted target for anticancer drug discovery. Similar to poly-ADP-ribose polymerase (PARP) inhibitors, pharmacological inhibition of RAD52 is synthetically lethal with defects in genome caretakers BRCA1 and BRCA2 (∼25% of breast and ovarian cancers). Emerging structure activity relationships for RAD52 are complex, making it challenging to transform previously identified disruptors of the RAD52-ssDNA interaction into drug-like leads using traditional medicinal chemistry approaches. Using pharmacophoric informatics on the RAD52 complexation by epigallocatechin (EGC), and the Enamine in silico REAL database, we identified six distinct chemical scaffolds that occupy the same physical space on RAD52 as EGC. All six were RAD52 inhibitors (IC50 ∼23-1200 μM) with two of the compounds (Z56 and Z99) selectively killing BRCA-mutant cells and inhibiting cellular activities of RAD52 at micromolar inhibitor concentrations. While Z56 had no effect on the ssDNA-binding protein RPA and was toxic to BRCA-mutant cells only, Z99 inhibited both proteins and displayed toxicity towards BRCA-complemented cells. Optimization of the Z99 scaffold resulted in a set of more powerful and selective inhibitors (IC50 ∼1.3-8 μM), which were only toxic to BRCA-mutant cells. RAD52 complexation by Z56, Z99 and its more specific derivatives provide a roadmap for next generation of cancer therapeutics.

Novel Insights into RAD52's Structure, Function, and Druggability for Synthetic Lethality and Innovative Anticancer Therapies

In recent years, the RAD52 protein has been highlighted as a mediator of many DNA repair mechanisms. While RAD52 was initially considered to be a non-essential auxiliary factor, its inhibition has more recently been demonstrated to be synthetically lethal in cancer cells bearing mutations and inactivation of specific intracellular pathways, such as homologous recombination. RAD52 is now recognized as a novel and critical pharmacological target. In this review, we comprehensively describe the available structural and functional information on RAD52. The review highlights the pathways in which RAD52 is involved and the approaches to RAD52 inhibition. We discuss the multifaceted role of this protein, which has a complex, dynamic, and functional 3D superstructural arrangement. This complexity reinforces the need to further investigate and characterize RAD52 to solve a challenging mechanistic puzzle and pave the way for a robust drug discovery campaign.

The cryo-EM structure of full-length RAD52 protein contains an undecameric ring

The human RAD52 protein, which forms an oligomeric ring structure, is involved in DNA double-strand break repair. The N-terminal half of RAD52 is primarily responsible for self-oligomerisation and DNA binding. Crystallographic studies have revealed the detailed structure of the N-terminal half. However, only low-resolution structures have been reported for the full-length protein, and thus the structural role of the C-terminal half in self-oligomerisation has remained elusive. In this study, we determined the solution structure of the human RAD52 protein by cryo-electron microscopy (cryo-EM), at an average resolution of 3.5 Å. The structure revealed an undecameric ring that is nearly identical to the crystal structures of the N-terminal half. The cryo-EM map for the C-terminal half was poorly defined, indicating that the region is intrinsically disordered. The present cryo-EM structure provides important insights into the mechanistic roles played by the N-terminal and C-terminal halves of RAD52 during DNA double-strand break repair.

The Rad52 SSAP superfamily and new insight into homologous recombination

Recent structures of DNA-bound bacterial and phage recombinases provide insights into homologous recombination and suggest relation to the eukaryotic Rad52 and identification of a Rad52 single strand annealing protein (SSAP) superfamily.

The effect of repeat length on Marcal1-dependent single-strand annealing in Drosophila

Proper repair of DNA double-strand breaks is essential to the maintenance of genomic stability and avoidance of genetic disease. Organisms have many ways of repairing double-strand breaks, including the use of homologous sequences through homology-directed repair. While homology-directed repair is often error free, in single-strand annealing homologous repeats flanking a double-strand break are annealed to one another, leading to the deletion of one repeat and the intervening sequences. Studies in yeast have shown a relationship between the length of the repeat and single-strand annealing efficacy. We sought to determine the effects of homology length on single-strand annealing in Drosophila, as Drosophila uses a different annealing enzyme (Marcal1) than yeast. Using an in vivo single-strand annealing assay, we show that 50 base pairs are insufficient to promote single-strand annealing and that 500-2,000 base pairs are required for maximum efficiency. Loss of Marcal1 generally followed the same homology length trend as wild-type flies, with single-strand annealing frequencies reduced to about a third of wild-type frequencies regardless of homology length. Interestingly, we find a difference in single-strand annealing rates between 500-base pair homologies that align to the annealing target either nearer or further from the double-strand break, a phenomenon that may be explained by Marcal1 dynamics. This study gives insights into Marcal1 function and provides important information to guide the design of genome engineering strategies that use single-strand annealing to integrate linear DNA constructs into a chromosomal double-strand break.

A moving target for drug discovery: Structure activity relationship and many genome (de)stabilizing functions of the RAD52 protein

BRCA-ness phenotype, a signature of many breast and ovarian cancers, manifests as deficiency in homologous recombination, and as defects in protection and repair of damaged DNA replication forks. A dependence of such cancers on DNA repair factors less important for survival of BRCA-proficient cells, offers opportunities for development of novel chemotherapeutic interventions. The first drugs targeting BRCA-deficient cancers, poly-ADP-ribose polymerase (PARP) inhibitors have been approved for the treatment of advanced, chemotherapy resistant cancers in patients with BRCA1/2 germline mutations. Nine additional proteins that can be targeted to selectively kill BRCA-deficient cancer cells have been identified. Among them, a DNA repair protein RAD52 is an especially attractive target due to general tolerance of the RAD52 loss of function, and protective role of an inactivating mutation. Yet, the effective pharmacological inhibitors of RAD52 have not been forthcoming. In this review, we discuss advances in the state of our knowledge of the RAD52 structure, activities and cellular functions, with a specific focus on the features that make RAD52 an attractive, but difficult drug target.

Redβ177 annealase structure reveals details of oligomerization and λ Red-mediated homologous DNA recombination

The Redβ protein of the bacteriophage λ red recombination system is a model annealase which catalyzes single-strand annealing homologous DNA recombination. Here we present the structure of a helical oligomeric annealing intermediate of Redβ, consisting of N-terminal residues 1-177 bound to two complementary 27mer oligonucleotides, determined via cryogenic electron microscopy (cryo-EM) to a final resolution of 3.3 Å. The structure reveals a continuous binding groove which positions and stabilizes complementary DNA strands in a planar orientation to facilitate base pairing via a network of hydrogen bonding. Definition of the inter-subunit interface provides a structural basis for the propensity of Redβ to oligomerize into functionally significant long helical filaments, a trait shared by most annealases. Our cryo-EM structure and molecular dynamics simulations suggest that residues 133-138 form a flexible loop which modulates access to the binding groove. More than half a century after its discovery, this combination of structural and computational observations has allowed us to propose molecular mechanisms for the actions of the model annealase Redβ, a defining member of the Redβ/RecT protein family.

Redβ annealase catalyses single-strand annealing homologous DNA recombination. Here, the authors present a cryo-EM structure of a Redβ annealing intermediate bound to two complementary 27mer oligonucleotides.

Single-strand annealing: Molecular mechanisms and potential applications in CRISPR-Cas-based precision genome editing

BACKGROUND

Spontaneous double-stranded DNA breaks (DSBs) frequently occur within the genome of all living organisms and must be well repaired for survival. Recently, more important roles of the DSB repair pathways that were previously thought to be minor pathways, such as single-strand annealing (SSA), have been shown. Nevertheless, the biochemical mechanisms and applications of the SSA pathway in genome editing have not been updated.

PURPOSE AND SCOPE

Understanding the molecular mechanism of SSA is important to design potential applications in gene editing. This review provides insights into the recent progress of SSA studies and establishes a model for their potential applications in precision genome editing.

SUMMARY AND CONCLUSION

The SSA mechanism involved in DNA DSB repair appears to be activated by a complex signaling cascade starting with broken end sensing and 5'-3' resection to reveal homologous repeats on the 3' ssDNA overhangs that flank the DSB. Annealing the repeats would help to amend the discontinuous ends and restore the intact genome, resulting in the missing of one repeat and the intervening sequence between the repeats. We proposed a model for CRISPR-Cas-based precision insertion or replacement of DNA fragments to take advantage of the characteristics. The proposed model can add a tool to extend the choice for precision gene editing. Nevertheless, the model needs to be experimentally validated and optimized with SSA-favorable conditions for practical applications.

POLθ-mediated end joining is restricted by RAD52 and BRCA2 until the onset of mitosis

BRCA2-mutant cells are defective in homologous recombination, making them vulnerable to the inactivation of other pathways for the repair of DNA double-strand breaks (DSBs). This concept can be clinically exploited but is currently limited due to insufficient knowledge about how DSBs are repaired in the absence of BRCA2. We show that DNA polymerase θ (POLθ)-mediated end joining (TMEJ) repairs DSBs arising during the S phase in BRCA2-deficient cells only after the onset of the ensuing mitosis. This process is regulated by RAD52, whose loss causes the premature usage of TMEJ and the formation of chromosomal fusions. Purified RAD52 and BRCA2 proteins both block the DNA polymerase function of POLθ, suggesting a mechanism explaining their synthetic lethal relationships. We propose that the delay of TMEJ until mitosis ensures the conversion of originally one-ended DSBs into two-ended DSBs. Mitotic chromatin condensation might further serve to juxtapose correct break ends and limit chromosomal fusions.

XAB2 promotes Ku eviction from single-ended DNA double-strand breaks independently of the ATM kinase

Replication-associated single-ended DNA double-strand breaks (seDSBs) are repaired predominantly through RAD51-mediated homologous recombination (HR). Removal of the non-homologous end-joining (NHEJ) factor Ku from resected seDSB ends is crucial for HR. The coordinated actions of MRE11-CtIP nuclease activities orchestrated by ATM define one pathway for Ku eviction. Here, we identify the pre-mRNA splicing protein XAB2 as a factor required for resistance to seDSBs induced by the chemotherapeutic alkylator temozolomide. Moreover, we show that XAB2 prevents Ku retention and abortive HR at seDSBs induced by temozolomide and camptothecin, via a pathway that operates in parallel to the ATM-CtIP-MRE11 axis. Although XAB2 depletion preserved RAD51 focus formation, the resulting RAD51-ssDNA associations were unproductive, leading to increased NHEJ engagement in S/G2 and genetic instability. Overexpression of RAD51 or RAD52 rescued the XAB2 defects and XAB2 loss was synthetically lethal with RAD52 inhibition, providing potential perspectives in cancer therapy.

Guardians of the Genome: BRCA2 and Its Partners

The tumor suppressor BRCA2 functions as a central caretaker of genome stability, and individuals who carry BRCA2 mutations are predisposed to breast, ovarian, and other cancers. Recent research advanced our mechanistic understanding of BRCA2 and its various interaction partners in DNA repair, DNA replication support, and DNA double-strand break repair pathway choice. In this review, we discuss the biochemical and structural properties of BRCA2 and examine how these fundamental properties contribute to DNA repair and replication fork stabilization in living cells. We highlight selected BRCA2 binding partners and discuss their role in BRCA2-mediated homologous recombination and fork protection. Improved mechanistic understanding of how BRCA2 functions in genome stability maintenance can enable experimental evidence-based evaluation of pathogenic BRCA2 mutations and BRCA2 pseudo-revertants to support targeted therapy.

Mutational Analysis of Redβ Single Strand Annealing Protein: Roles of the 14 Lysine Residues in DNA Binding and Recombination In Vivo

Redβ is a 261 amino acid protein from bacteriophage λ that promotes a single-strand annealing (SSA) reaction for repair of double-stranded DNA (dsDNA) breaks. While there is currently no high-resolution structure available for Redβ, models of its DNA binding domain (residues 1-188) have been proposed based on homology with human Rad52, and a crystal structure of its C-terminal domain (CTD, residues 193-261), which binds to λ exonuclease and E. coli single-stranded DNA binding protein (SSB), has been determined. To evaluate these models, the 14 lysine residues of Redβ were mutated to alanine, and the variants tested for recombination in vivo and DNA binding and annealing in vitro. Most of the lysines within the DNA binding domain, including K36, K61, K111, K132, K148, K154, and K172, were found to be critical for DNA binding in vitro and recombination in vivo. By contrast, none of the lysines within the CTD, including K214, K245, K251, K253, and K258 were required for DNA binding in vitro, but two, K214 and K253, were critical for recombination in vivo, likely due to their involvement in binding to SSB. K61 was identified as a residue that is critical for DNA annealing, but not for initial ssDNA binding, suggesting a role in binding to the second strand of DNA incorporated into the complex. The K148A variant, which has previously been shown to be defective in oligomer formation, had the lowest affinity for ssDNA, and was the only variant that was completely non-cooperative, suggesting that ssDNA binding is coupled to oligomerization.

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.

Post-translational modification of factors involved in homologous recombination

DNA is the molecule that stores the chemical instructions necessary for life and its stability is therefore of the utmost importance. Despite this, DNA is damaged by both exogenous and endogenous factors at an alarming frequency. The most severe type of DNA damage is a double-strand break (DSB), in which a scission occurs in both strands of the double helix, effectively dividing a single normal chromosome into two pathological chromosomes. Homologous recombination (HR) is a universal DSB repair mechanism that solves this problem by identifying another region of the genome that shares high sequence similarity with the DSB site and using it as a template for repair. Rad51 possess the enzymatic activity that is essential for this repair but several auxiliary factors are required for Rad51 to fulfil its function. It is becoming increasingly clear that many HR factors are subjected to post-translational modification. Here, we review what is known about how these modifications affect HR. We first focus on cases where there is experimental evidence to support a function for the modification, then discuss speculative cases where a function can be inferred. Finally, we contemplate why such modifications might be necessary.

Identification of a RAD52 Inhibitor Inducing Synthetic Lethality in BRCA2-Deficient Cancer Cells

The breast cancer susceptibility gene 1/2 (BRCA1/2) is frequently mutated in many malignant tumors, such as breast cancer and ovarian cancer. Studies have demonstrated that inhibition of RAD52 gene function in BRCA2-deficient cancer causes synthetic lethality, suggesting a potential application of RAD52 in cancer-targeted therapy. In this study, we have performed a virtual screening by targeting the self-association domain (residues 85-159) of RAD52 with a library of 66,608 compounds and found one compound, C791-0064, that specifically inhibited the proliferation of BRCA2-deficient cancer cells. Our biochemical and cell-based experimental data suggested that C791-0064 specifically bound to RAD52 and disrupted the single-strand annealing activity of RAD52. Taken together, C791-0064 is a promising leading compound worthy of further exploitation in the context of BRCA-deficient targeted cancer therapy.

Variants of the human RAD52 gene confer defects in ionizing radiation resistance and homologous recombination repair in budding yeast

RAD52 is a structurally and functionally conserved component of the DNA double-strand break (DSB) repair apparatus from budding yeast to humans. We recently showed that expressing the human gene, HsRAD52 in rad52 mutant budding yeast cells can suppress both their ionizing radiation (IR) sensitivity and homologous recombination repair (HRR) defects. Intriguingly, we observed that HsRAD52 supports DSB repair by a mechanism of HRR that conserves genome structure and is independent of the canonical HR machinery. In this study we report that naturally occurring variants of HsRAD52, one of which suppresses the pathogenicity of BRCA2 mutations, were unable to suppress the IR sensitivity and HRR defects of rad52 mutant yeast cells, but fully suppressed a defect in DSB repair by single-strand annealing (SSA). This failure to suppress both IR sensitivity and the HRR defect correlated with an inability of HsRAD52 protein to associate with and drive an interaction between genomic sequences during DSB repair by HRR. These results suggest that HsRAD52 supports multiple, distinct DSB repair apparatuses in budding yeast cells and help further define its mechanism of action in HRR. They also imply that disruption of HsRAD52-dependent HRR in BRCA2-defective human cells may contribute to protection against tumorigenesis and provide a target for killing BRCA2-defective cancers.

Current Understanding of RAD52 Functions: Fundamental and Therapeutic Insights

In this Special Issue, we would like to focus on the various functions of the RAD52 helicase-like protein and the current implications of such findings for cancer treatment. Over the last few years, various laboratories have discovered particular activities of mammalian RAD52—both in S and M phase—that are distinct from the auxiliary role of yeast RAD52 in homologous recombination. At DNA double-strand breaks, RAD52 was demonstrated to spur alternative pathways to compensate for the loss of homologous recombination functions. At collapsed replication forks, RAD52 activates break-induced replication. In the M phase, RAD52 promotes the finalization of DNA replication. Its compensatory role in the resolution of DNA double-strand breaks has put RAD52 in the focus of synthetic lethal strategies, which is particularly relevant for cancer treatment.

Phosphoregulation of Rad51/Rad52 by CDK1 functions as a molecular switch for cell cycle-specific activation of homologous recombination

Homologous recombination is exquisitely activated only during specific cell phases. In the G₁ phase, homologous recombination activity is completely suppressed. According to previous reports, the activation of homologous recombination during specific cell phases depends on the kinase activity of cyclin-dependent kinase 1 (CDK1). However, the precise regulatory mechanism and target substrates of CDK1 for this regulation have not been completely determined. Here, we report that the budding yeast CDK1, Cdc28, phosphorylates the major homologous recombination regulators Rad51 and Rad52. This phosphorylation occurs in the G₂/M phase by Cdc28 in combination with G₂/M phase cyclins. Nonphosphorylatable mutations in Rad51 and Rad52 impair the DNA binding affinity of Rad51 and the affinity between Rad52 rings that leads to their interaction. Collectively, our data provide detailed insights into the regulatory mechanism of cell cycle-dependent homologous recombination activation in eukaryotic cells.

DSS1 interacts with and stimulates RAD52 to promote the repair of DSBs

The proper repair of deleterious DNA lesions such as double strand breaks prevents genomic instability and carcinogenesis. In yeast, the Rad52 protein mediates DSB repair via homologous recombination. In mammalian cells, despite the presence of the RAD52 protein, the tumour suppressor protein BRCA2 acts as the predominant mediator during homologous recombination. For decades, it has been believed that the RAD52 protein played only a back-up role in the repair of DSBs performing an error-prone single strand annealing (SSA). Recent studies have identified several new functions of the RAD52 protein and have drawn attention to its important role in genome maintenance. Here, we show that RAD52 activities are enhanced by interacting with a small and highly acidic protein called DSS1. Binding of DSS1 to RAD52 changes the RAD52 oligomeric conformation, modulates its DNA binding properties, stimulates SSA activity and promotes strand invasion. Our work introduces for the first time RAD52 as another interacting partner of DSS1 and shows that both proteins are important players in the SSA and BIR pathways of DSB repair.

Structure-Function Analysis Reveals the Singularity of Plant Mitochondrial DNA Replication Components: A Mosaic and Redundant System

Plants are sessile organisms, and their DNA is particularly exposed to damaging agents. The integrity of plant mitochondrial and plastid genomes is necessary for cell survival. During evolution, plants have evolved mechanisms to replicate their mitochondrial genomes while minimizing the effects of DNA damaging agents. The recombinogenic character of plant mitochondrial DNA, absence of defined origins of replication, and its linear structure suggest that mitochondrial DNA replication is achieved by a recombination-dependent replication mechanism. Here, I review the mitochondrial proteins possibly involved in mitochondrial DNA replication from a structural point of view. A revision of these proteins supports the idea that mitochondrial DNA replication could be replicated by several processes. The analysis indicates that DNA replication in plant mitochondria could be achieved by a recombination-dependent replication mechanism, but also by a replisome in which primers are synthesized by three different enzymes: Mitochondrial RNA polymerase, Primase-Helicase, and Primase-Polymerase. The recombination-dependent replication model and primers synthesized by the Primase-Polymerase may be responsible for the presence of genomic rearrangements in plant mitochondria.

RAD52 Functions in Homologous Recombination and Its Importance on Genomic Integrity Maintenance and Cancer Therapy

Genomes are continually subjected to DNA damage whether they are induced from intrinsic physiological processes or extrinsic agents. Double-stranded breaks (DSBs) are the most injurious type of DNA damage, being induced by ionizing radiation (IR) and cytotoxic agents used in cancer treatment. The failure to repair DSBs can result in aberrant chromosomal abnormalities which lead to cancer development. An intricate network of DNA damage signaling pathways is usually activated to eliminate these damages and to restore genomic stability. These signaling pathways include the activation of cell cycle checkpoints, DNA repair mechanisms, and apoptosis induction, also known as DNA damage response (DDR)-mechanisms. Remarkably, the homologous recombination (HR) is the major DSBs repairing pathway, in which RAD52 gene has a crucial repairing role by promoting the annealing of complementary single-stranded DNA and by stimulating RAD51 recombinase activity. Evidence suggests that variations in RAD52 expression can influence HR activity and, subsequently, influence the predisposition and treatment efficacy of cancer. In this review, we present several reports in which the down or upregulation of RAD52 seems to be associated with different carcinogenic processes. In addition, we discuss RAD52 inhibition in DDR-defective cancers as a possible target to improve cancer therapy efficacy.

RAD52 as a Potential Target for Synthetic Lethality-Based Anticancer Therapies

Alterations in DNA repair systems play a key role in the induction and progression of cancer. Tumor-specific defects in DNA repair mechanisms and activation of alternative repair routes create the opportunity to employ a phenomenon called "synthetic lethality" to eliminate cancer cells. Targeting the backup pathways may amplify endogenous and drug-induced DNA damage and lead to specific eradication of cancer cells. So far, the synthetic lethal interaction between BRCA1/2 and PARP1 has been successfully applied as an anticancer treatment. Although PARP1 constitutes a promising target in the treatment of tumors harboring deficiencies in BRCA1/2-mediated homologous recombination (HR), some tumor cells survive, resulting in disease relapse. It has been suggested that alternative RAD52-mediated HR can protect BRCA1/2-deficient cells from the accumulation of DNA damage and the synthetic lethal effect of PARPi. Thus, simultaneous inhibition of RAD52 and PARP1 might result in a robust dual synthetic lethality, effectively eradicating BRCA1/2-deficient tumor cells. In this review, we will discuss the role of RAD52 and its potential application in synthetic lethality-based anticancer therapies.

Emerging Roles of RAD52 in Genome Maintenance

The maintenance of genome integrity is critical for cell survival. Homologous recombination (HR) is considered the major error-free repair pathway in combatting endogenously generated double-stranded lesions in DNA. Nevertheless, a number of alternative repair pathways have been described as protectors of genome stability, especially in HR-deficient cells. One of the factors that appears to have a role in many of these pathways is human RAD52, a DNA repair protein that was previously considered to be dispensable due to a lack of an observable phenotype in knock-out mice. In later studies, RAD52 deficiency has been shown to be synthetically lethal with defects in BRCA genes, making RAD52 an attractive therapeutic target, particularly in the context of BRCA-deficient tumors.

Microhomology Selection for Microhomology Mediated End Joining in Saccharomyces cerevisiae

Microhomology-mediated end joining (MMEJ) anneals short, imperfect microhomologies flanking DNA breaks, producing repair products with deletions in a Ku- and RAD52-independent fashion. Puzzlingly, MMEJ preferentially selects certain microhomologies over others, even when multiple microhomologies are available. To define rules and parameters for microhomology selection, we altered the length, the position, and the level of mismatches to the microhomologies flanking homothallic switching (HO) endonuclease-induced breaks and assessed their effect on MMEJ frequency and the types of repair product formation. We found that microhomology of eight to 20 base pairs carrying no more than 20% mismatches efficiently induced MMEJ. Deletion of MSH6 did not impact MMEJ frequency. MMEJ preferentially chose a microhomology pair that was more proximal from the break. Interestingly, MMEJ events preferentially retained the centromere proximal side of the HO break, while the sequences proximal to the telomere were frequently deleted. The asymmetry in the deletional profile among MMEJ products was reduced when HO was induced on the circular chromosome. The results provide insight into how cells search and select microhomologies for MMEJ in budding yeast.

Structure and mechanism of the Red recombination system of bacteriophage λ

While much of this volume focuses on mammalian DNA repair systems that are directly involved in genome stability and cancer, it is important to still be mindful of model systems from prokaryotes. Herein we review the Red recombination system of bacteriophage λ, which consists of an exonuclease for resecting dsDNA ends, and a single-strand annealing protein (SSAP) for binding the resulting 3'-overhang and annealing it to a complementary strand. The genetics and biochemistry of Red have been studied for over 50 years, in work that has laid much of the foundation for understanding DNA recombination in higher eukaryotes. In fact, the Red exonuclease (λ exo) is homologous to Dna2, a nuclease involved in DNA end-resection in eukaryotes, and the Red annealing protein (Redβ) is homologous to Rad52, the primary SSAP in eukaryotes. While eukaryotic recombination involves an elaborate network of proteins that is still being unraveled, the phage systems are comparatively simple and streamlined, yet still encompass the fundamental features of recombination, namely DNA end-resection, homologous pairing (annealing), and a coupling between them. Moreover, the Red system has been exploited in powerful methods for bacterial genome engineering that are important for functional genomics and systems biology. However, several mechanistic aspects of Red, particularly the action of the annealing protein, remain poorly understood. This review will focus on the proteins of the Red recombination system, with particular attention to structural and mechanistic aspects, and how the lessons learned can be applied to eukaryotic systems.

Nonfilament-forming RecA dimer catalyzes homologous joint formation

Homologous recombination is essential to genome maintenance, and also to genome diversification. In virtually all organisms, homologous recombination depends on the RecA/Rad51-family recombinases, which catalyze ATP-dependent formation of homologous joints—critical intermediates in homologous recombination. RecA/Rad51 binds first to single-stranded (ss) DNA at a damaged site to form a spiral nucleoprotein filament, after which double-stranded (ds) DNA interacts with the filament to search for sequence homology and to form consecutive base pairs with ssDNA (‘pairing’). How sequence homology is recognized and what exact role filament formation plays remain unknown. We addressed the question of whether filament formation is a prerequisite for homologous joint formation. To this end we constructed a nonpolymerizing (np) head-to-tail-fused RecA dimer (npRecA dimer) and an npRecA monomer. The npRecA dimer bound to ssDNA, but did not form continuous filaments upon binding to DNA; it formed beads-on-string structures exclusively. Although its efficiency was lower, the npRecA dimer catalyzed the formation of D-loops (a type of homologous joint), whereas the npRecA monomer was completely defective. Thus, filament formation contributes to efficiency, but is not essential to sequence-homology recognition and pairing, for which a head-to-tail dimer form of RecA protomer is required and sufficient.

The effects of AtRad52 over-expression on homologous recombination in Arabidopsis

AtRad52 homologs are involved in DNA recombination and repair, but their precise functions in different homologous recombination (HR) pathways or in gene-targeting have not been analyzed. In order to facilitate our analyses, we generated an AtRad52-1A variant that had a stronger nuclear localization than the native gene thanks to the removal of the transit peptide for mitochondrial localization and to the addition of a nuclear localization signal. Over-expression of this variant increased HR in the nucleus, compared with the native AtRad52-1A: it increased intra-chromosomal recombination and synthesis-dependent strand-annealing HR repair rates; but conversely, it repressed the single-strand annealing pathway. The effect of AtRad52-1A over-expression on gene-targeting was tested with and without the expression of small RNAs generated from an RNAi construct containing homology to the target and donor sequences. True gene-targeting events at the Arabidopsis Cruciferin locus were obtained only when combining AtRad52-1A over-expression and target/donor-specific RNAi. This suggests that sequence-specific small RNAs might be involved in AtRad52-1A-mediated HR.

Structural Basis of Homology-Directed DNA Repair Mediated by RAD52

RAD52 mediates homologous recombination by annealing cDNA strands. However, the detailed mechanism of DNA annealing promoted by RAD52 has remained elusive. Here we report two crystal structures of human RAD52 single-stranded DNA (ssDNA) complexes that probably represent key reaction intermediates of RAD52-mediated DNA annealing. The first structure revealed a "wrapped" conformation of ssDNA around the homo-oligomeric RAD52 ring, in which the edges of the bases involved in base pairing are exposed to the solvent. The ssDNA conformation is close to B-form and appears capable of engaging in Watson-Crick base pairing with the cDNA strand. The second structure revealed a "trapped" conformation of ssDNA between two RAD52 rings. This conformation is stabilized by a different RAD52 DNA binding site, which promotes the accumulation of multiple RAD52 rings on ssDNA and the aggregation of ssDNA. These structures provide a structural framework for understanding the mechanism of RAD52-mediated DNA annealing.

Novel function of HATs and HDACs in homologous recombination through acetylation of human RAD52 at double-strand break sites

The p300 and CBP histone acetyltransferases are recruited to DNA double-strand break (DSB) sites where they induce histone acetylation, thereby influencing the chromatin structure and DNA repair process. Whether p300/CBP at DSB sites also acetylate non-histone proteins, and how their acetylation affects DSB repair, remain unknown. Here we show that p300/CBP acetylate RAD52, a human homologous recombination (HR) DNA repair protein, at DSB sites. Using in vitro acetylated RAD52, we identified 13 potential acetylation sites in RAD52 by a mass spectrometry analysis. An immunofluorescence microscopy analysis revealed that RAD52 acetylation at DSBs sites is counteracted by SIRT2- and SIRT3-mediated deacetylation, and that non-acetylated RAD52 initially accumulates at DSB sites, but dissociates prematurely from them. In the absence of RAD52 acetylation, RAD51, which plays a central role in HR, also dissociates prematurely from DSB sites, and hence HR is impaired. Furthermore, inhibition of ataxia telangiectasia mutated (ATM) protein by siRNA or inhibitor treatment demonstrated that the acetylation of RAD52 at DSB sites is dependent on the ATM protein kinase activity, through the formation of RAD52, p300/CBP, SIRT2, and SIRT3 foci at DSB sites. Our findings clarify the importance of RAD52 acetylation in HR and its underlying mechanism.

Compound F779-0434 causes synthetic lethality in BRCA2-deficient cancer cells by disrupting RAD52–ssDNA association

A novel compound named F779-0434 caused synthetic lethality in BRCA2-deficient cancer cells by disrupting RAD52–ssDNA associations.

Human RAD52 Captures and Holds DNA Strands, Increases DNA Flexibility, and Prevents Melting of Duplex DNA: Implications for DNA Recombination

Human RAD52 promotes annealing of complementary single-stranded DNA (ssDNA). In-depth knowledge of RAD52-DNA interaction is required to understand how its activity is integrated in DNA repair processes. Here, we visualize individual fluorescent RAD52 complexes interacting with single DNA molecules. The interaction with ssDNA is rapid, static, and tight, where ssDNA appears to wrap around RAD52 complexes that promote intra-molecular bridging. With double-stranded DNA (dsDNA), interaction is slower, weaker, and often diffusive. Interestingly, force spectroscopy experiments show that RAD52 alters the mechanics dsDNA by enhancing DNA flexibility and increasing DNA contour length, suggesting intercalation. RAD52 binding changes the nature of the overstretching transition of dsDNA and prevents DNA melting, which is advantageous for strand clamping during or after annealing. DNA-bound RAD52 is efficient at capturing ssDNA in trans. Together, these effects may help key steps in DNA repair, such as second-end capture during homologous recombination or strand annealing during RAD51-independent recombination reactions.

•

RAD52 binds ssDNA rapidly and tightly using wrapping and bridging modes

•

RAD52 binding to dsDNA is slower, weaker, and often diffusive

•

RAD52 changes dsDNA mechanics and intercalates into the double helix

•

RAD52 prevents DNA melting by clamping DNA strands

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.

DNA annealing by Redβ is insufficient for homologous recombination and the additional requirements involve intra- and inter-molecular interactions

Single strand annealing proteins (SSAPs) like Redβ initiate homologous recombination by annealing complementary DNA strands. We show that C-terminally truncated Redβ, whilst still able to promote annealing and nucleoprotein filament formation, is unable to mediate homologous recombination. Mutations of the C-terminal domain were evaluated using both single- and double stranded (ss and ds) substrates in recombination assays. Mutations of critical amino acids affected either dsDNA recombination or both ssDNA and dsDNA recombination indicating two separable functions, one of which is critical for dsDNA recombination and the second for recombination per se. As evaluated by co-immunoprecipitation experiments, the dsDNA recombination function relates to the Redα-Redβ protein-protein interaction, which requires not only contacts in the C-terminal domain but also a region near the N-terminus. Because the nucleoprotein filament formed with C-terminally truncated Redβ has altered properties, the second C-terminal function could be due to an interaction required for functional filaments. Alternatively the second C-terminal function could indicate a requirement for a Redβ-host factor interaction. These data further advance the model for Red recombination and the proposition that Redβ and RAD52 SSAPs share ancestral and mechanistic roots.

Reappearance from Obscurity: Mammalian Rad52 in Homologous Recombination

Homologous recombination (HR) plays an important role in maintaining genomic integrity. It is responsible for repair of the most harmful DNA lesions, DNA double-strand breaks and inter-strand DNA cross-links. HR function is also essential for proper segregation of homologous chromosomes in meiosis, maintenance of telomeres, and resolving stalled replication forks. Defects in HR often lead to genetic diseases and cancer. Rad52 is one of the key HR proteins, which is evolutionarily conserved from yeast to humans. In yeast, Rad52 is important for most HR events; Rad52 mutations disrupt repair of DNA double-strand breaks and targeted DNA integration. Surprisingly, in mammals, Rad52 knockouts showed no significant DNA repair or recombination phenotype. However, recent work demonstrated that mutations in human RAD52 are synthetically lethal with mutations in several other HR proteins including BRCA1 and BRCA2. These new findings indicate an important backup role for Rad52, which complements the main HR mechanism in mammals. In this review, we focus on the Rad52 activities and functions in HR and the possibility of using human RAD52 as therapeutic target in BRCA1 and BRCA2-deficient familial breast cancer and ovarian cancer.

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.