Brh2-Dss1 interplay enables properly controlled recombination in Ustilago maydis

Suppressors of Blm-deficiency identify three novel proteins that facilitate DNA repair in Ustilago maydis

To identify new molecular components of the Brh2-governed homologous recombination (HR)-network in the highly radiation-resistant fungus Ustilago maydis, we undertook a genetic screen for suppressors of blm-KR hydroxyurea (HU)-sensitivity. Twenty DNA-damage sensitive mutants were obtained, three of which showing slow-growth phenotypes. Focusing on the "normally" growing candidates we identified five mutations, two in previously well-defined genes (Rec2 and Rad51) and the remaining three in completely uncharacterized genes (named Rec3, Bls9 and Zdr1). A common feature among these novel factors is their prominent role in DNA repair. Rec3 contains the P-loop NTPase domain which is most similar to that found in U. maydis Rec2 protein, and like Rec2, Rec3 plays critical roles in induced allelic recombination, is crucial for completion of meiosis, and with regard to DNA repair Δrec3 and Δrec2 are epistatic to one another. Importantly, overexpression of Brh2 in Δrec3 can effectively restore DNA-damage resistance, indicating a close functional connection between Brh2 and Rec3. The Bls9 does not seem to have any convincing domains that would give a clue as to its function. Nevertheless, we present evidence that, besides being involved in DNA-repair, Bls9 is also necessary for HR between chromosome homologs. Moreover, Δbls9 showed epistasis with Δbrh2 with respect to killing by DNA-damaging agents. Both, Rec3 and Bls9, play an important role in protecting the genome from mutations. Zdr1 is Cys2-His2 zinc finger (C2H2-ZF) protein, whose loss does not cause a detectable change in HR. Also, the functions of both Bls9 and Zdr1 genes are dispensable in meiosis and sporulation. However, Zdr1 appears to have overlapping activities with Blm and Mus81 in protecting the organism from methyl methanesulfonate- and diepoxybutane-induced DNA-damage. Finally, while deletion of Rec3 and Zdr1 can suppress HU-sensitivity of blm-KR, Δgen1, and Δmus81 mutants, interestingly loss of Bls9 does not rescue HU-sensitivity of Δgen1.

Determinants governing BRC function evaluated by mutational analysis of Brh2 in Ustilago maydis

BRC is a short evolutionarily conserved sequence motif generally arranged in multiple tandem repeats that is present as a defining feature in members of the BRCA2 tumor suppressor protein family. From crystallographic studies of a co-complex, the human BRC4 was found to form a structural element that interacts with RAD51, a key component in the DNA repair machinery directed by homologous recombination. The BRC is distinguished by two tetrameric sequence modules with characteristic hydrophobic residues separated by an intervening spacer region marked by certain highly conserved residues forming a hydrophobic surface for interaction with RAD51. It is present as a single copy in Brh2 of Ustilago maydis, the only reported example of a fungal BRCA2 ortholog. By comparative sequence analysis, examples of BRCA2 orthologs were identified in other fungal phyla, some of which featured multiple tandem repeats like those found in mammals. An expeditious biological assay system was developed for evaluating the two-tetramer module model and assessing the importance of particular conserved amino acid residues of BRC contributing to Brh2 functionality in DNA repair. This work was aided by the finding that the human BRC4 repeat could substitute completely for the endogenous BRC element in Brh2, while the human BRC5 repeat could not. In a survey of point mutations of certain residues, certain BRC mutant variants termed antimorphs were identified that caused a DNA repair phenotype more severe than the null.

CRISPR/Cas9-Targeted Disruption of Two Highly Homologous Arabidopsis thaliana DSS1 Genes with Roles in Development and the Oxidative Stress Response

Global climate change has a detrimental effect on plant growth and health, causing serious losses in agriculture. Investigation of the molecular mechanisms of plant responses to various environmental pressures and the generation of plants tolerant to abiotic stress are imperative to modern plant science. In this paper, we focus on the application of the well-established technology CRISPR/Cas9 genome editing to better understand the functioning of the intrinsically disordered protein DSS1 in plant response to oxidative stress. The Arabidopsis genome contains two highly homologous DSS1 genes, AtDSS1(I) and AtDSS1(V). This study was designed to identify the functional differences between AtDSS1s, focusing on their potential roles in oxidative stress. We generated single dss1(I) and dss1(V) mutant lines of both Arabidopsis DSS1 genes using CRISPR/Cas9 technology. The homozygous mutant lines with large indels (dss1(I)del25 and dss1(V)ins18) were phenotypically characterized during plant development and their sensitivity to oxidative stress was analyzed. The characterization of mutant lines revealed differences in root and stem lengths, and rosette area size. Plants with a disrupted AtDSS1(V) gene exhibited lower survival rates and increased levels of oxidized proteins in comparison to WT plants exposed to oxidative stress induced by hydrogen peroxide. In this work, the dss1 double mutant was not obtained due to embryonic lethality. These results suggest that the DSS1(V) protein could be an important molecular component in plant abiotic stress response.

Yeast as a Model to Unravel New BRCA2 Functions in Cell Metabolism

Mutations in BRCA2 gene increase the risk for breast cancer and for other cancer types, including pancreatic and prostate cancer. Since its first identification as an oncosupressor in 1995, the best-characterized function of BRCA2 is in the repair of DNA double-strand breaks (DSBs) by homologous recombination. BRCA2 directly interacts with both RAD51 and single-stranded DNA, mediating loading of RAD51 recombinase to sites of single-stranded DNA. In the absence of an efficient homologous recombination pathway, DSBs accumulate resulting in genome instability, thus supporting tumorigenesis. Yet the precise mechanism by which BRCA2 exerts its tumor suppressor function remains unclear. BRCA2 has also been involved in other biological functions including protection of telomere integrity and stalled replication forks, cell cycle progression, transcriptional control and mitophagy. Recently, we and others have reported a role of BRCA2 in modulating cell death programs through a molecular mechanism conserved in yeast and mammals. Here we hypothesize that BRCA2 is a multifunctional protein which exerts specific functions depending on cell stress response pathway. Based on a differential RNA sequencing analysis carried out on yeast cells either growing or undergoing a regulated cell death process, either in the absence or in the presence of BRCA2, we suggest that BRCA2 causes central carbon metabolism reprogramming in response to death stimuli and encourage further investigation on the role of metabolic reprogramming in BRCA2 oncosuppressive function.

The Pathogenic R3052W BRCA2 Variant Disrupts Homology-Directed Repair by Failing to Localize to the Nucleus

The BRCA2 germline missense variant, R3052W, resides in the DNA binding domain and has been previously classified as a pathogenic allele. In this study, we sought to determine how R3052W alters the cellular functions of BRCA2 in the DNA damage response. The BRCA2 R3052W mutated protein exacerbates genome instability, is unable to rescue homology-directed repair, and fails to complement cell survival following exposure to PARP inhibitors and crosslinking drugs. Surprisingly, despite anticipated defects in DNA binding or RAD51-mediated DNA strand exchange, the BRCA2 R3052W protein mislocalizes to the cytoplasm precluding its ability to perform any DNA repair functions. Rather than acting as a simple loss-of-function mutation, R3052W behaves as a dominant negative allele, likely by sequestering RAD51 in the cytoplasm.

Role of BRCA2 DNA-binding and C-terminal domain in its mobility and conformation in DNA repair

Breast cancer type two susceptibility protein (BRCA2) is an essential protein in genome maintenance, homologous recombination (HR), and replication fork protection. Its function includes multiple interaction partners and requires timely localization to relevant sites in the nucleus. We investigated the importance of the highly conserved DNA-binding domain (DBD) and C-terminal domain (CTD) of BRCA2. We generated BRCA2 variants missing one or both domains in mouse embryonic stem (ES) cells and defined their contribution in HR function and dynamic localization in the nucleus, by single-particle tracking of BRCA2 mobility. Changes in molecular architecture of BRCA2 induced by binding partners of purified BRCA2 were determined by scanning force microscopy. BRCA2 mobility and DNA-damage-induced increase in the immobile fraction were largely unaffected by C-terminal deletions. The purified proteins missing CTD and/or DBD were defective in architectural changes correlating with reduced HR function in cells. These results emphasize BRCA2 activity at sites of damage beyond promoting RAD51 delivery.

BRCA1 and BRCA2 Tumor Suppressor Function in Meiosis

Meiosis is a specialized cell cycle that results in the production of haploid gametes for sexual reproduction. During meiosis, homologous chromosomes are connected by chiasmata, the physical manifestation of crossovers. Crossovers are formed by the repair of intentionally induced double strand breaks by homologous recombination and facilitate chromosome alignment on the meiotic spindle and proper chromosome segregation. While it is well established that the tumor suppressors BRCA1 and BRCA2 function in DNA repair and homologous recombination in somatic cells, the functions of BRCA1 and BRCA2 in meiosis have received less attention. Recent studies in both mice and the nematode Caenorhabditis elegans have provided insight into the roles of these tumor suppressors in a number of meiotic processes, revealing both conserved and organism-specific functions. BRCA1 forms an E3 ubiquitin ligase as a heterodimer with BARD1 and appears to have regulatory roles in a number of key meiotic processes. BRCA2 is a very large protein that plays an intimate role in homologous recombination. As women with no indication of cancer but carrying BRCA mutations show decreased ovarian reserve and accumulated oocyte DNA damage, studies in these systems may provide insight into why BRCA mutations impact reproductive success in addition to their established roles in cancer.

DSS1 and ssDNA regulate oligomerization of BRCA2

The tumor suppressor BRCA2 plays a key role in initiating homologous recombination by facilitating RAD51 filament formation on single-stranded DNA. The small acidic protein DSS1 is a crucial partner to BRCA2 in this process. In vitro and in cells (1,2), BRCA2 associates into oligomeric complexes besides also existing as monomers. A dimeric structure was further characterized by electron microscopic analysis (3), but the functional significance of the different BRCA2 assemblies remains to be determined. Here, we used biochemistry and electron microscopic imaging to demonstrate that the multimerization of BRCA2 is counteracted by DSS1 and ssDNA. When validating the findings, we identified three self-interacting regions and two types of self-association, the N-to-C terminal and the N-to-N terminal interactions. The N-to-C terminal self-interaction of BRCA2 is sensitive to DSS1 and ssDNA. The N-to-N terminal self-interaction is modulated by ssDNA. Our results define a novel role of DSS1 to regulate BRCA2 in an RPA-independent fashion. Since DSS1 is required for BRCA2 function in recombination, we speculate that the monomeric and oligomeric forms of BRCA2 might be active for different cellular events in recombinational DNA repair and replication fork stabilization.

Characterization of a potent dominant negative mutant variant of Rad51 in Ustilago maydis

Rad51 serves to maintain and protect integrity of the genome through its actions in DNA repair and replication fork protection. The active form of Rad51 is a nucleoprotein filament consisting of chains of protomer units arranged linearly along single-stranded DNA. In a mutant screen using Ustilago maydis as an experimental system we identified a novel variant of Rad51, in which an amino acid change near the protomer-protomer interaction interface confers a strong trans dominant inhibitory effect on resistance to DNA damaging agents and proficiency in homologous recombination. Modeling studies of the mutated residue D161Y suggested that steric interference with surrounding residues was the likely cause of the inhibitory effect. Changes of two nearby residues, predicted from the modeling to minimize steric clashes, mitigated the inhibition of DNA repair. Direct testing of purified Rad51^D161Y protein in defined biochemical reactions revealed it to be devoid of DNA-binding activity itself, but capable of interfering with Rad51^WT in formation and maintenance of nucleoprotein filaments on single-stranded DNA and in DNA strand exchange. Rad51^D161Y protein appears to be unable to self-associate in solution and defective in forming complexes with the U. maydis BRCA2 ortholog.

Approaches to Understanding the Mediator Function of Brh2 in Ustilago maydis

Primary components of the homologous recombination pathway in eukaryotes include Rad51 whose function is to search for DNA sequence homology and promote strand exchange, its mediator BRCA2, and Dss1, a key regulator of BRCA2. We seek to understand the role of BRCA2 in governing the activity of Rad51 and to learn how BRCA2 function is regulated by Dss1. We use the microbe Ustilago maydis as a model system for experimentation because it has a well-conserved BRCA2-homolog, Brh2, and is amenable to biochemical and molecular genetic manipulations and analysis. The powerful attributes of this system open the way for gaining insight into BRCA2's molecular mechanism through avenues not immediately approachable in the vertebrate systems. Here we provide protocols for preparing Brh2, Dss1, and Rad51 as reagents for use in biochemical assays to monitor function and present methods for transposon-based mutational analysis of Brh2 for use in genetic dissection of function.

Collaboration in the actions of Brh2 with resolving functions during DNA repair and replication stress in Ustilago maydis

Cells maintain a small arsenal of resolving functions to process and eliminate complex DNA intermediates that result as a consequence of homologous recombination and distressed replication. Ordinarily the homologous recombination system serves as a high-fidelity mechanism to restore the integrity of a damaged genome, but in the absence of the appropriate resolving function it can turn DNA intermediates resulting from replication stress into pathological forms that are toxic to cells. Here we have investigated how the nucleases Mus81 and Gen1 and the helicase Blm contribute to survival after DNA damage or replication stress in Ustilago maydis cells with crippled yet homologous recombination-proficient forms of Brh2, the BRCA2 ortholog and primary Rad51 mediator. We found collaboration among the factors. Notable were three findings. First, the ability of Gen1 to rescue hydroxyurea sensitivity of dysfunctional Blm requires the absence of Mus81. Second, the response of mutants defective in Blm and Gen1 to hydroxyurea challenge is markedly similar suggesting cooperation of these factors in the same pathway. Third, the repair proficiency of Brh2 mutant variants deleted of its N-terminal DNA binding region requires not only Rad52 but also Gen1 and Mus81. We suggest these factors comprise a subpathway for channeling repair when Brh2 is compromised in its interplay with DNA.

Contributions of recombination and repair proteins to telomere maintenance in telomerase-positive and negative Ustilago maydis

Homologous recombination and repair factors are known to promote both telomere replication and recombination-based telomere extension. Herein, we address the diverse contributions of several recombination/repair proteins to telomere maintenance in Ustilago maydis, a fungus that bears strong resemblance to mammals with respect to telomere regulation and recombination mechanisms. In telomerase-positive U. maydis, deletion of rad51 and blm separately caused shortened but stably maintained telomeres, whereas deletion of both engendered similar telomere loss, suggesting that the repair proteins help to resolve similar problems in telomere replication. In telomerase-negative cells, the loss of Rad51 or Brh2 caused accelerated senescence and failure to generate survivors on semi-solid medium. However, slow growing survivors can be isolated through continuous liquid culturing, and these survivors exhibit type II-like as well as ALT-like telomere features. In contrast, the trt1Δ blmΔ double mutant gives rise to survivors as readily as the trt1Δ single mutant, and like the single mutant survivors, exhibit almost exclusively type I-like telomere features. In addition, we observed direct physical interactions between Blm and two telomere-binding proteins, which may thus recruit or regulate Blm at telomeres. Our findings provide the basis for further analyzing the interplays between telomerase, telomere replication, and telomere recombination.

Dss1 Regulates Association of Brh2 with Rad51

Brh2, the BRCA2 ortholog in the fungus Ustilago maydis, mediates delivery of Rad51 to DNA during the course of homology-directed DNA repair. Rad51 interacts with Brh2 through the highly conserved BRC element and through a second region termed CRE located at the extreme carboxy terminus. Dss1, a small intrinsically unstructured protein that interacts with Brh2, is crucial for its activity in DNA repair, but the mechanism of this regulation is uncertain. In previous studies, we found that interaction of Brh2 with DNA was strongly modulated by association with Dss1. Here we report that CRE influences interaction of Dss1 with Brh2 and that Dss1 status markedly alters interaction of Brh2 with Rad51. While it appears that a single Rad51 protomer associates with Brh2 in complex with Dss1, loss of Dss1 is accompanied by a large increase in the number of Rad51 protomers that can associate with Brh2. Concomitant with this buildup of Rad51, Brh2 loses its ability to bind DNA. These observations suggest a feedback circuit in which release of Dss1 from Brh2 as it binds DNA triggers nucleation of a short Rad51 oligomer on Brh2, which in turn promotes dissociation of Brh2 from the DNA.

Unraveling Fungal Radiation Resistance Regulatory Networks through the Genome-Wide Transcriptome and Genetic Analyses of Cryptococcus neoformans

The basidiomycetous fungus Cryptococcus neoformans has been known to be highly radiation resistant and has been found in fatal radioactive environments such as the damaged nuclear reactor at Chernobyl. To elucidate the mechanisms underlying the radiation resistance phenotype of C. neoformans, we identified genes affected by gamma radiation through genome-wide transcriptome analysis and characterized their functions. We found that genes involved in DNA damage repair systems were upregulated in response to gamma radiation. Particularly, deletion of recombinase RAD51 and two DNA-dependent ATPase genes, RAD54 and RDH54, increased cellular susceptibility to both gamma radiation and DNA-damaging agents. A variety of oxidative stress response genes were also upregulated. Among them, sulfiredoxin contributed to gamma radiation resistance in a peroxiredoxin/thioredoxin-independent manner. Furthermore, we found that genes involved in molecular chaperone expression, ubiquitination systems, and autophagy were induced, whereas genes involved in the biosynthesis of proteins and fatty acids/sterols were downregulated. Most importantly, we discovered a number of novel C. neoformans genes, the expression of which was modulated by gamma radiation exposure, and their deletion rendered cells susceptible to gamma radiation exposure, as well as DNA damage insults. Among these genes, we found that a unique transcription factor containing the basic leucine zipper domain, named Bdr1, served as a regulator of the gamma radiation resistance of C. neoformans by controlling expression of DNA repair genes, and its expression was regulated by the evolutionarily conserved DNA damage response protein kinase Rad53. Taken together, the current transcriptome and functional analyses contribute to the understanding of the unique molecular mechanism of the radiation-resistant fungus C. neoformans.

Although there are no natural environments under intense radiation, some living organisms have been found to show high radiation resistance. Organisms harboring the ability of radiation resistance have unique regulatory networks to overcome this stress. Cryptococcus neoformans is one of the radiation-resistant fungi and is found in highly radioactive environments. However, it remains elusive how radiation-resistant eukaryotic microorganisms work differentially from radiation-sensitive ones. Here, we performed transcriptome analysis of C. neoformans to explore gene expression profiles after gamma radiation exposure and functionally characterized some of identified radiation resistance genes. Notably, we identified a novel regulator of radiation resistance, named Bdr1 (a bZIP TF for DNA damage response 1), which is a transcription factor (TF) that is not closely homologous to any known TF and is transcriptionally controlled by the Rad53 kinase. Therefore, our work could shed light on understanding not only the radiation response but also the radiation resistance mechanism of C. neoformans.

Homologous recombination and human health: the roles of BRCA1, BRCA2, and associated proteins

Homologous recombination (HR) is a major pathway for the repair of DNA double-strand breaks in mammalian cells, the defining step of which is homologous strand exchange directed by the RAD51 protein. The physiological importance of HR is underscored by the observation of genomic instability in HR-deficient cells and, importantly, the association of cancer predisposition and developmental defects with mutations in HR genes. The tumor suppressors BRCA1 and BRCA2, key players at different stages of HR, are frequently mutated in familial breast and ovarian cancers. Other HR proteins, including PALB2 and RAD51 paralogs, have also been identified as tumor suppressors. This review summarizes recent findings on BRCA1, BRCA2, and associated proteins involved in human disease with an emphasis on their molecular roles and interactions.

Quality control of homologous recombination

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

Dual DNA-binding domains shape the interaction of Brh2 with DNA

Brh2, the BRCA2 ortholog in the fungus Ustilago maydis, harbors two different DNA-binding domains, one located in the N-terminal region and the other located in the C-terminal region. Here we were interested in comparing the biochemical properties of Brh2 fragments, Brh2(NT) and Brh2(CT), respectively, harboring the two different DNA-binding regions to understand the mechanistic purpose of dual DNA-interaction domains. With oligonucleotide substrates to model different DNA conformations, it was found that the substrate specificity of Brh2(NT) and Brh2(CT) was almost indistinguishable although avidity was different depending on salt concentration. DNA annealing activity inherent in Brh2 was found to be attributable to Brh2(NT). Likewise, activity responsible for a second-end capture reaction modeling a later step in repair of DNA double-strand breaks was found attributable to Brh2(NT). Efficient annealing of DNA strands coated with RPA required full length Brh2 rather than Brh2(NT) suggesting Brh2(CT) contributes to the activity when RPA is present. Brh2(NT) and Brh2(CT) were both found capable of physically interacting with RPA. The results suggest that while the two DNA-binding regions of Brh2 appear functionally redundant in certain aspects of DNA repair, they differ in fundamental properties, and likely contribute in different ways to repair processes involving or arising from stalled DNA replication forks.

DSSylation, a novel protein modification targets proteins induced by oxidative stress, and facilitates their degradation in cells

Timely removal of oxidatively damaged proteins is critical for cells exposed to oxidative stresses; however, cellular mechanism for clearing oxidized proteins is not clear. Our study reveals a novel type of protein modification that may play a role in targeting oxidized proteins and remove them. In this process, DSS1 (deleted in split hand/split foot 1), an evolutionally conserved small protein, is conjugated to proteins induced by oxidative stresses in vitro and in vivo, implying oxidized proteins are DSS1 clients. A subsequent ubiquitination targeting DSS1-protein adducts has been observed, suggesting the client proteins are degraded through the ubiquitin-proteasome pathway. The DSS1 attachment to its clients is evidenced to be an enzymatic process modulated by an unidentified ATPase. We name this novel protein modification as DSSylation, in which DSS1 plays as a modifier, whose attachment may render target proteins a signature leading to their subsequent ubiquitination, thereby recruits proteasome to degrade them.

A cancer-associated BRCA2 mutation reveals masked nuclear export signals controlling localization

Germline missense mutations affecting a single BRCA2 allele predispose humans to cancer. Here we identify a protein-targeting mechanism that is disrupted by the cancer-associated mutation, BRCA2(D2723H), and that controls the nuclear localization of BRCA2 and its cargo, the recombination enzyme RAD51. A nuclear export signal (NES) in BRCA2 is masked by its interaction with a partner protein, DSS1, such that point mutations impairing BRCA2-DSS1 binding render BRCA2 cytoplasmic. In turn, cytoplasmic mislocalization of mutant BRCA2 inhibits the nuclear retention of RAD51 by exposing a similar NES in RAD51 that is usually obscured by the BRCA2-RAD51 interaction. Thus, a series of NES-masking interactions localizes BRCA2 and RAD51 in the nucleus. Notably, BRCA2(D2723H) decreases RAD51 nuclear retention even when wild-type BRCA2 is also present. Our findings suggest a mechanism for the regulation of the nucleocytoplasmic distribution of BRCA2 and RAD51 and its impairment by a heterozygous disease-associated mutation.

Brh2 and Rad51 promote telomere maintenance in Ustilago maydis, a new model system of DNA repair proteins at telomeres

Recent studies implicate a number of DNA repair proteins in mammalian telomere maintenance. However, because several key repair proteins in mammals are missing from the well-studied budding and fission yeast, their roles at telomeres cannot be modeled in standard fungi. In this report, we explored the dimorphic fungus Ustilago maydis as an alternative model for telomere research. This fungus, which belongs to the phylum Basidiomycota, has a telomere repeat unit that is identical to the mammalian repeat, as well as a constellation of DNA repair proteins that more closely mimic the mammalian collection. We showed that the two core components of homology-directed repair (HDR) in U. maydis, namely Brh2 and Rad51, both promote telomere maintenance in telomerase positive cells, just like in mammals. In addition, we found that Brh2 is localized to telomeres in vivo, suggesting that it acts directly at chromosome ends. We surveyed a series of mutants with DNA repair defects, and found many of them to have short telomeres. Our results indicate that factors involved in DNA repair are probably also needed for optimal telomere maintenance in U. maydis, and that this fungus is a useful alternative model system for telomere research.

DNA damage emergency: cellular garbage disposal to the rescue?

The proteasome is a cellular machine found in the cytosol, nucleus and on chromatin that performs much of the proteolysis in eukaryotic cells. Recent reports show it is enriched at sites of double-stranded DNA breaks (DSBs) in mammalian cells. What is it doing there? This review will address three possibilities suggested by recent reports: in degrading proteins after their ubiquitination at and eviction from chromatin; as a deubiquitinase, specific to the antagonism of ubiquitin conjugates generated as part of the signalling of a DSB; and as a functional component of DNA repair mechanism itself. These findings add complexity to the proteasome as a potential therapeutic target in cancer treatment.

Trypanosoma brucei BRCA2 acts in a life cycle-specific genome stability process and dictates BRC repeat number-dependent RAD51 subnuclear dynamics

Trypanosoma brucei survives in mammals through antigenic variation, which is driven by RAD51-directed homologous recombination of Variant Surface Glycoproteins (VSG) genes, most of which reside in a subtelomeric repository of >1000 silent genes. A key regulator of RAD51 is BRCA2, which in T. brucei contains a dramatic expansion of a motif that mediates interaction with RAD51, termed the BRC repeats. BRCA2 mutants were made in both tsetse fly-derived and mammal-derived T. brucei, and we show that BRCA2 loss has less impact on the health of the former. In addition, we find that genome instability, a hallmark of BRCA2 loss in other organisms, is only seen in mammal-derived T. brucei. By generating cells expressing BRCA2 variants with altered BRC repeat numbers, we show that the BRC repeat expansion is crucial for RAD51 subnuclear dynamics after DNA damage. Finally, we document surprisingly limited co-localization of BRCA2 and RAD51 in the T. brucei nucleus, and we show that BRCA2 mutants display aberrant cell division, revealing a function distinct from BRC-mediated RAD51 interaction. We propose that BRCA2 acts to maintain the huge VSG repository of T. brucei, and this function has necessitated the evolution of extensive RAD51 interaction via the BRC repeats, allowing re-localization of the recombinase to general genome damage when needed.

Dss1 release activates DNA binding potential in Brh2

Dss1 is an intrinsically unstructured polypeptide that partners with the much larger Brh2 protein, the BRCA2 ortholog in Ustilago maydis, to form a tight complex. Mutants lacking Dss1 have essentially the same phenotype as mutants defective in Brh2, implying that through physical interaction Dss1 serves as a positive activator of Brh2. Dss1 associates with Brh2 through an interaction surface in the carboxy-terminal region. Certain derivatives of Brh2 lacking this interaction surface remain highly competent in DNA repair as long as a DNA-binding domain is present. However, the Dss1-independent activity raises the question of what function might be met in the native protein by having Brh2 under Dss1 control. Using a set of Brh2 fusions and truncated derivatives, we show here that Dss1 is capable of exerting control when there is a cognate Dss1-interacting surface present. We find that association of Dss1 attenuates the DNA binding potential of Brh2 and that the amino-terminal domain of Brh2 helps evict Dss1 from its carboxy-terminal interaction surface. The findings presented here add to the notion that Dss1 serves in a regulatory capacity to dictate order in association of Brh2's amino-terminal and carboxy-terminal domains with DNA.

Identification of the deleted in split hand/split foot 1 protein as a novel biomarker for human cervical cancer

The morphological detection of early neoplastic transformation leading to cervical cancer remains problematic. In this work, we have identified deleted in split hand/split foot 1 protein (DSS1) as an early biomarker that is specifically upregulated in premalignant and malignant cervical epithelial cells, but is low or undetectable in non-malignant cells. DSS1 mRNA and protein levels are significantly increased in cultured human cervical carcinoma cell lines originating from primary and metastatic tumors. In fact, > 96% of patient tumor tissues were found to have cells with elevated DSS1 when compared with tumor-adjacent normal cells. In histological sections of cervical tissue containing either invasive cervical carcinoma or its precursor lesions, DSS1 was readily detected in the tumor cells. Steady-state DSS1 expression by immortalized cervical cancer cell lines was found to be necessary for maintenance of their transformed phenotype, since stable shRNA-mediated depletion of DSS1 in HeLa cells inhibited their proliferation and colony-forming activity in monolayer cultures and prevented division of these cells in soft agar. When DSS1 levels are reduced using shRNA, the cells ultimately undergo apoptosis via activation of p53 and the p53 downstream targets, and cleavage of apoptosis-associated proteins including CPP32/caspase-3, poly(ADP-ribose)polymerase and DNA-PKcs. In addition, silencing of DSS1 makes cervical cancer cells sensitive to cell death after treatment with cisplatin. We conclude that the DSS1 protein is critically involved in the maintenance of the transformed phenotype in cervical cancer cells, and that it might be a specific, robust and reliable marker for early detection, diagnosis and treatment.

Brh2 domain function distinguished by differential cellular responses to DNA damage and replication stress

Mutants of the fungus Ustilago maydis defective in the RecQ helicase Blm are highly sensitive to killing by the DNA replication stressor hydroxyurea. This sensitivity or toxicity is dependent on the homologous recombination (HR) system and apparently results from formation of dead-end HR DNA intermediates. HU toxicity can be suppressed by deletion of the gene encoding Brh2, the BRCA2 orthologue that serves to regulate HR by mediating Rad51 filament formation on single-stranded DNA. Brh2 harbours two different DNA-binding domains that contribute to HR function. DNA-binding activity from a single domain is sufficient to provide Brh2 functional activity in HR, but to enable HU-induced killing two functional DNA-binding domains must be present. Despite this stringent requirement for dual functioning domains, the source of DNA-binding domains is less critical in that heterologous domains can substitute for the native endogenous ones. The results suggest a model in which the nature of the DNA lesion is an important determinant in the functional response of Brh2 action.

Plasticity of BRCA2 function in homologous recombination: genetic interactions of the PALB2 and DNA binding domains

The breast cancer suppressor BRCA2 is essential for the maintenance of genomic integrity in mammalian cells through its role in DNA repair by homologous recombination (HR). Human BRCA2 is 3,418 amino acids and is comprised of multiple domains that interact with the RAD51 recombinase and other proteins as well as with DNA. To gain insight into the cellular function of BRCA2 in HR, we created fusions consisting of various BRCA2 domains and also introduced mutations into these domains to disrupt specific protein and DNA interactions. We find that a BRCA2 fusion peptide deleted for the DNA binding domain and active in HR is completely dependent on interaction with the PALB2 tumor suppressor for activity. Conversely, a BRCA2 fusion peptide deleted for the PALB2 binding domain is dependent on an intact DNA binding domain, providing a role for this conserved domain in vivo; mutagenesis suggests that both single-stranded and double-stranded DNA binding activities in the DNA binding domain are required for its activity. Given that PALB2 itself binds DNA, these results suggest alternative mechanisms to deliver RAD51 to DNA. In addition, the BRCA2 C terminus contains both RAD51-dependent and -independent activities which are essential to HR in some contexts. Finally, binding the small peptide DSS1 is essential for activity when its binding domain is present, but not when it is absent. Our results reveal functional redundancy within the BRCA2 protein and emphasize the plasticity of this large protein built for optimal HR function in mammalian cells. The occurrence of disease-causing mutations throughout BRCA2 suggests sub-optimal HR from a variety of domain modulations.

The breast tumor suppressor BRCA2 has a major role in DNA repair by homologous recombination (HR). BRCA2 is a large protein with multiple domains that interact with several proteins as well as with DNA, complicating our understanding of how the protein functions in cells. To investigate the mechanism by which BRCA2 functions in HR in cells, we created fusions consisting of various BRCA2 domains and also introduced mutations into these domains to disrupt specific protein and DNA interactions. We find that DNA binding by BRCA2 is critical when a BRCA2 peptide is deficient in binding another breast cancer suppressor, PALB2, but not when the peptide can bind PALB2, suggesting alternative mechanisms of activity. Binding the small peptide DSS1 is also essential for HR only in some contexts, as are activities in the BRCA2 C terminus. Our results reveal redundancy of BRCA2 domains and emphasize plasticity within this large protein built for optimal HR function in mammalian cells. The occurrence of disease-causing mutations throughout BRCA2 suggests sub-optimal HR from a variety of domain modulations.

Unraveling the mechanism of BRCA2 in homologous recombination

BRCA2 is the product of a breast cancer susceptibility gene in humans and the founding member of an emerging family of proteins present throughout the eukaryotic domain that serve in homologous recombination. The function of BRCA2 in recombination is to control RAD51, a protein that catalyzes homologous pairing and DNA strand exchange. By physically interacting with both RAD51 and single-stranded DNA, BRCA2 mediates delivery of RAD51 preferentially to sites of single-stranded DNA (ssDNA) exposed as a result of DNA damage or replication problems. Through its action, BRCA2 helps restore and maintain integrity of the genome. This review highlights recent studies on BRCA2 and its orthologs that have begun to illuminate the molecular mechanisms by which these proteins control homologous recombination.

Mutational analysis of Brh2 reveals requirements for compensating mediator functions

Brh2, a member of the BRCA2 family of proteins, governs homologous recombination in the fungus Ustilago maydis through interaction with Rad51. Brh2 serves at an early step in homologous recombination to mediate Rad51 nucleoprotein filament formation and also has the capability to function at a later step in recombination through its inherent DNA annealing activity. Rec2, a Rad51 paralogue, and Rad52 are additional components of the homologous recombination system, but the absence of either is less critical than Brh2 for operational activity. Here we tested a variety of mutant forms of Brh2 for activity in recombinational repair as measured by DNA repair proficiency. We found that a mutant of Brh2 deleted of the non-canonical DNA-binding domain within the N-terminal region is dependent upon the presence of Rad52 for DNA repair activity. We also determined that a motif first identified in human BRCA2 as important in binding DMC1 also contributes to DNA repair proficiency and cooperates with the BRC element in Rad51 binding.

Dss1 regulates interaction of Brh2 with DNA

Brh2, the BRCA2 homologue in Ustilago maydis, plays a crucial role in homologous recombination by controlling Rad51. In turn, Brh2 is governed by Dss1, an intrinsically disordered protein that forms a tight complex with the C-terminal region of Brh2. This region of the protein associating with Dss1 is highly conserved in sequence and by comparison with mammalian BRCA2 corresponds to a part of the DNA binding domain with characteristic OB folds. The N-terminal region of Brh2 harbors a less-defined but powerful DNA binding site, the activity of which is revealed upon deletion of the C-terminal region. Full-length Brh2 complexed with Dss1 binds DNA slowly, while the N-terminal fragment binds quickly. The DNA binding activity of full-length Brh2 appears to correlate with dissociation of Dss1. Addition of Dss1 to the heterotypic Brh2-Dss1 complex attenuates DNA binding activity, but not by direct competition for the N-terminal DNA binding site. Conversely, the Brh2-Dss1 complex dissociates more quickly when DNA is present. These findings suggest a model in which binding of Brh2 to DNA is subject to allosteric regulation by Dss1.

Brh2 promotes a template-switching reaction enabling recombinational bypass of lesions during DNA synthesis

Accumulating evidence for Rad51-catalyzed DNA strand invasion during double-strand break repair features a 3' single-stranded tail as the preferred substrate for reaction, but paradoxically, the preferred substrate in model reactions in vitro is the 5' end. Here, we examined the Rad51-promoted 5' end invasion reaction in the presence of Brh2, the BRCA2 family protein in Ustilago maydis. Using plasmid DNA and a homologous duplex oligonucleotide with 5' protruding single-stranded tail as substrates, we found that Brh2 can stimulate Rad51 to promote the formation of a four-stranded complement-stabilized D loop. In this structure, the incoming recessed complementary strand of the oligonucleotide has switched partners and can now prime DNA synthesis using the recipient plasmid DNA as template, circumventing a lesion that blocks elongation when the 5' protruding tail serves as template for fill-in synthesis. We propose that template switching promoted by Brh2 provides a mechanism for recombination-mediated bypass of lesions blocking synthesis during DNA replication.

The carboxyl terminus of Brca2 links the disassembly of Rad51 complexes to mitotic entry

Background

The Rad51 recombinase assembles on DNA to execute homologous DNA recombination (HR). This process is essential to repair replication-associated genomic lesions before cells enter mitosis, but how it is started and stopped during the cell cycle remains poorly understood. Rad51 assembly is regulated by the breast cancer suppressor Brca2, via its evolutionarily conserved BRC repeats, and a distinct carboxy (C)-terminal motif whose biological function is uncertain. Using “hit-and-run” gene targeting to insert single-codon substitutions into the avian Brca2 locus, we report here a previously unrecognized role for the C-terminal motif.

Results

We show that the avian C-terminal motif is functionally cognate with its human counterpart and identify point mutations that either abolish or enhance Rad51 binding. When these mutations are introduced into Brca2, we find that they affect neither the assembly of Rad51 into nuclear foci on damaged DNA nor DNA repair by HR. Instead, foci disassemble more rapidly in a point mutant that fails to bind Rad51, associated with faster mitotic entry. Conversely, the slower disassembly of foci in a point mutant that constitutively binds Rad51 correlates with delayed mitosis. Indeed, Rad51 foci do not persist in mitotic cells even after G2 checkpoint suppression, suggesting that their disassembly is a prerequisite for chromosome segregation.

Conclusions

We conclude that Rad51 binding by the C-terminal Brca2 motif is dispensable for the execution of HR but instead links the disassembly of Rad51 complexes to mitotic entry. This mechanism may ensure that HR terminates before chromosome segregation. Our findings assign a biological function for the C-terminal Brca2 motif in a mechanism that coordinates DNA repair with the cell cycle.

Role of Blm and collaborating factors in recombination and survival following replication stress in Ustilago maydis

Inactivation of the structural gene for the RecQ family member, BLM in human, Sgs1 in budding yeast, or Rqh1 in fission yeast leads to inappropriate recombination, chromosome abnormalities, and disturbed replication fork progression. Studies with yeasts have demonstrated that auxiliary gene functions can contribute in overlapping ways with Sgs1 or Rqh1 to circumvent or overcome lesions in DNA caused by certain genotoxic agents. In the combined absence of these functions, recombination-mediated processes lead to severe loss of fitness. Here we performed a genetic study to determine the role of the Ustilago maydis Blm homolog in DNA repair and in alleviating replication stress. We characterized the single mutant as well as double mutants additionally deleted of genes encoding Srs2, Fbh1, Mus81, or Exo1. Unlike yeasts, neither the blm srs2, blm exo1, nor blm mus81 double mutant exhibited extreme loss of fitness. Inactivation of Brh2, the BRCA2 homolog, suppressed toxicity to hydroxyurea caused by loss of Blm function. However, differential suppression by Brh2 derivatives lacking the canonical DNA-binding region suggests that the particular domain structure comprising this DNA-binding region may be instrumental in promoting the observed hydroxyurea toxicity.

DNA-binding Domain within the Brh2 N Terminus Is the Primary Interaction Site for Association with DNA

The C-terminal region of Brh2 (Brh2(CT)), the BRCA2 homolog in Ustilago maydis, is highly conserved and aligns with the DSS1/DNA-binding domain (DBD) of mammalian BRCA2, while the N-terminal region (Brh2(NT)) is poorly conserved and has no obvious functional domain except for the single Rad51-interacting BRC element. Paradoxically, Brh2(NT), but not Brh2(CT), complements the DNA repair and recombination deficiency of the brh2 mutant. We show here that Brh2(NT) exhibits an unexpected DNA binding activity with properties similar to that of the full-length protein. Deletion mapping localized the region responsible for the DNA binding activity to a stretch of residues between the BRC element and the canonical DBD. A heterologous DNA-binding domain from the large subunit of replication protein A substituted for the endogenous binding region within Brh2(NT) in supporting DNA repair. Rad51-promoted strand invasion was stimulated by Brh2(NT), but required the presence of the BRC element. The findings suggest a model in which Brh2(NT) serves as the principal site for association with DNA, while the Brh2(CT) provides a means for regulation.

Sem1 is a functional component of the nuclear pore complex-associated messenger RNA export machinery

The evolutionarily conserved protein Sem1/Dss1 is a subunit of the regulatory particle (RP) of the proteasome, and, in mammalian cells, binds the tumor suppressor protein BRCA2. Here, we describe a new function for yeast Sem1. We show that sem1 mutants are impaired in messenger RNA (mRNA) export and transcription elongation, and induce strong transcription-associated hyper-recombination phenotypes. Importantly, Sem1, independent of the RP, is functionally linked to the mRNA export pathway. Biochemical analyses revealed that, in addition to the RP, Sem1 coenriches with components of two other multisubunit complexes: the nuclear pore complex (NPC)-associated TREX-2 complex that is required for transcription-coupled mRNA export, and the COP9 signalosome, which is involved in deneddylation. Notably, targeting of Thp1, a TREX-2 component, to the NPC is perturbed in a sem1 mutant. These findings reveal an unexpected nonproteasomal function of Sem1 in mRNA export and in prevention of transcription-associated genome instability. Thus, Sem1 is a versatile protein that might stabilize multiple protein complexes involved in diverse pathways.

Suppression of the double-strand-break-repair defect of the Saccharomyces cerevisiae rad57 mutant

The Rad51 paralogs Rad55 and Rad57 form a heterodimer required to mediate the formation and/or stabilization of the Rad51 filament. To further characterize the function of Rad55-Rad57, we used a combination of rad57 partial suppressors to determine whether the DNA repair and recombination defects of the rad57 mutant could be completely suppressed. The combination of all suppressors, elevated temperature, srs2, rad51-I345T, and mating-type (MAT) heterozygosity resulted in almost complete suppression of the rad57 mutant defect in the recruitment of Rad51 to DNA-damaged sites, as well as survival in response to ionizing radiation and camptothecin. In a physical assay to monitor the kinetics of double-strand-break (DSB)-induced gene conversion, the rad57 mutant defect was effectively suppressed by srs2 and MAT heterozygosity, but these same suppressors failed to suppress the spontaneous recombination defect. Thus the Rad55-Rad57 heterodimer appears to have a unique function in spontaneous recombination that is not essential for DSB repair. Furthermore, we investigated the currently unknown mechanism of rad57 suppression by MAT heterozygosity and found that it is independent of DNL4.

Identification of a specific motif of the DSS1 protein required for proteasome interaction and p53 protein degradation

Deleted in Split hand/Split foot 1 (DSS1) was previously identified as a novel 12-O-tetradecanoylphorbol-13-acetate (TPA)-inducible gene with possible involvement in early event of mouse skin carcinogenesis. The mechanisms by which human DSS1 (HsDSS1) exerts its biological effects via regulation of the ubiquitin-proteasome system (UPS) are currently unknown. Here, we demonstrated that HsDSS1 regulates the human proteasome by associating with it in the cytosol and nucleus via the RPN3/S3 subunit of the 19S regulatory particle (RP). Molecular anatomy of HsDSS1 revealed an RPN3/S3-interacting motif (R3IM), located at amino acid residues 15 to 21 of the NH(2) terminus. Importantly, negative charges of the R3IM motif were demonstrated to be required for proteasome interaction and binding to poly-ubiquitinated substrates. Indeed, the R3IM motif of HsDSS1 protein alone was sufficient to replace the ability of intact HsDSS1 protein to pull down proteasome complexes and protein substrates with high-molecular mass ubiquitin conjugates. Interestingly, this interaction is highly conserved throughout evolution from humans to nematodes. Functional study, lowering the levels of the endogenous HsDSS1 using siRNA, indicates that the R3IM/proteasome complex binds and targets p53 for ubiquitin-mediated degradation via gankyrin-MDM2/HDM2 pathway. Most significantly, this work indicates that the R3IM motif of HsDSS1, in conjunction with the complexes of 19S RP and 20S core particle (CP), regulates proteasome interaction through RPN3/S3 molecule, and utilizes a specific subset of poly-ubiquitinated p53 as a substrate.

The homologous recombination system of Ustilago maydis

Homologous recombination is a high fidelity, template-dependent process that is used in repair of damaged DNA, recovery of broken replication forks, and disjunction of homologous chromosomes in meiosis. Much of what is known about recombination genes and mechanisms comes from studies on baker's yeast. Ustilago maydis, a basidiomycete fungus, is distant evolutionarily from baker's yeast and so offers the possibility of gaining insight into recombination from an alternative perspective. Here we have surveyed the genome of U. maydis to determine the composition of its homologous recombination system. Compared to baker's yeast, there are fundamental differences in the function as well as in the repertoire of dedicated components. These include the use of a BRCA2 homolog and its modifier Dss1 rather than Rad52 as a mediator of Rad51, the presence of only a single Rad51 paralog, and the absence of Dmc1 and auxiliary meiotic proteins.

Trypanosoma brucei BRCA2 acts in antigenic variation and has undergone a recent expansion in BRC repeat number that is important during homologous recombination

Antigenic variation in Trypanosoma brucei has selected for the evolution of a massive archive of silent Variant Surface Glycoprotein (VSG) genes, which are activated by recombination into specialized expression sites. Such VSG switching can occur at rates substantially higher than background mutation and is dependent on homologous recombination, a core DNA repair reaction. A key regulator of homologous recombination is BRCA2, a protein that binds RAD51, the enzyme responsible for DNA strand exchange. Here, we show that T. brucei BRCA2 has undergone a recent, striking expansion in the number of BRC repeats, a sequence element that mediates interaction with RAD51. T. brucei BRCA2 mutants are shown to be significantly impaired in antigenic variation and display genome instability. By generating BRCA2 variants with reduced BRC repeat numbers, we show that the BRC expansion is crucial in determining the efficiency of T. brucei homologous recombination and RAD51 localization. Remarkably, however, this appears not to be a major determinant of the activation of at least some VSG genes.

Compensatory role for Rad52 during recombinational repair in Ustilago maydis

A single Rad52-related protein is evident by blast analysis of the Ustilago maydis genome database. Mutants created by disruption of the structural gene exhibited few discernible defects in resistance to UV, ionizing radiation, chemical alkylating or cross-linking agents. No deficiency was noted in spontaneous mutator activity, allelic recombination or meiosis. GFP-Rad51 foci were formed in rad52 cells following DNA damage, but were initially less intense than normal suggesting a possible role for Rad52 in formation of the Rad51 nucleoprotein filament. A search for interacting genes that confer a synthetic fitness phenotype with rad52 after DNA damage by UV irradiation identified the genes for Mph1, Ercc1 and the Rad51 paralogue Rec2. Testing known mutants in recombinational repair revealed an additional interaction with the BRCA2 orthologue Brh2. Suppression of the rec2 mutant's UV sensitivity by overexpressing Brh2 was found to be dependent on Rad52. The results suggest that Rad52 serves in an overlapping, compensatory role with both Rec2 and Brh2 to promote and maintain formation of the Rad51 nucleoprotein filament.

D-loop formation by Brh2 protein of Ustilago maydis

Brh2, the ortholog of the BRCA2 tumor suppressor in Ustilago maydis, works hand in hand with Rad51 to promote repair of DNA by homologous recombination. Previous studies established that Brh2 can stimulate DNA strand exchange by enabling Rad51 nucleoprotein filament formation on replication protein A-coated ssDNA. But, more recently, it was noted that Brh2 has an inherent DNA annealing activity, raising the notion that it might have roles in recombination in addition to or beyond the mediator function. Here, we found that Brh2 can autonomously promote the formation of D-loops in reactions with plasmid DNA and homologous single-stranded oligonucleotides. The reaction differs from that catalyzed by Rad51 in having no requirement for cofactors or preloading phase on ssDNA. D-loop formation was most effective when Brh2 was mixed with plasmid DNA before addition of single-stranded oligomer. D-loop formation catalyzed by Rad51 was also enhanced when Brh2 was premixed with plasmid DNA. Brh2 rendered defective in Rad51 interaction by mutation in the BRC element was still capable of promoting D-loop formation. However, the mutant protein was unable to enhance the Rad51-catalyzed reaction. The results suggest a model in which Brh2 binding to plasmid DNA attracts and helps capture Rad51-coated ssDNA.

Ortholog of BRCA2-interacting protein BCCIP controls morphogenetic responses during DNA replication stress in Ustilago maydis

The BRCA2 tumor suppressor functions in repair of DNA by homologous recombination through regulating the action of Rad51. In turn, BRCA2 appears to be regulated by other interacting proteins. Dss1, a small interacting protein that binds to the C-terminal domain, has a profound effect on activity as deduced from studies on the BRCA2-related protein Brh2 in Ustilago maydis. Evidence accumulating in mammalian systems suggests that BCCIP, another small interacting protein that binds to the C-terminal domain of BRCA2, also serves to regulate homologous recombination activity. Here we were interested in testing the role of the putative U. maydis BCCIP ortholog Bcp1 in DNA repair and recombination. In keeping with the mammalian paradigm, Bcp1 bound to the C-terminal region of Brh2. Mutants deleted of the gene were extremely slow growing, showed a delay passing through S phase and exhibited sensitivity to hydroxyurea, but were otherwise normal in DNA repair and homologous recombination. In the absence of Bcp1 cells were unable to maintain the wild type morphology when challenged by a DNA replication stress. These results suggest that Bcp1 could be involved in coordinating morphogenetic events with DNA processing during replication.

The proteasome is involved in determining differential utilization of double-strand break repair pathways

The DSS1 protein interacts with the breast cancer susceptibility protein BRCA2 that plays an integral role in the repair of DNA double-strand breaks (DSBs). DSS1 has also been shown to interact with components of the 26S proteasome in Saccharomyces cerevisiae and in human tumour cells. This raises the possibility of functional interplay between the DNA repair machinery and the proteasome. We show here that human DSS1 interacts with the RPN3 and RPN7 proteasome subunits and define regions of DSS1 important for the interactions with RPN3, RPN7 and BRCA2. We also show that BRCA2 interacts with RPN3 and RPN7 and that the BRCA2/RPN7 interaction is independent of DSS1. Finally, and most significantly, we demonstrate that the proteolytic activity of the proteasome is a determinant of the choice of DSB repair pathway; inhibition of proteasome proteolytic activity results in an increase in the utilization of potentially mutagenic single-strand annealing at the expense of a reduction in the level of error-free gene conversion. This confirms a functional link between DSB repair and proteasomal activity.

Dss1 interaction with Brh2 as a regulatory mechanism for recombinational repair

Brh2, the BRCA2 ortholog in Ustilago maydis, enables recombinational repair of DNA by controlling Rad51 and is in turn regulated by Dss1. Interplay with Rad51 is conducted via the BRC element located in the N-terminal region of the protein and through an unrelated domain, CRE, at the C terminus. Mutation in either BRC or CRE severely reduces functional activity, but repair deficiency of the brh2 mutant can be complemented by expressing BRC and CRE on different molecules. This intermolecular complementation is dependent upon the presence of Dss1. Brh2 molecules associate through the region overlapping with the Dss1-interacting domain to form at least dimer-sized complexes, which in turn, can be dissociated by Dss1 to monomer. We propose that cooperation between BRC and CRE domains and the Dss1-provoked dissociation of Brh2 complexes are requisite features of Brh2's molecular mechanism.

The roles of BRCA1 and BRCA2 and associated proteins in the maintenance of genomic stability

The BRCA1 and BRCA2 proteins are important in maintaining genomic stability by promoting efficient and precise repair of double-strand breaks. The main role of BRCA2 appears to involve regulating the function of RAD51 in the repair by homologous recombination. BRCA1 has a broader role upstream of BRCA2, participating in various cellular processes in response to DNA damage. The DNA repair defect associated with mutations in BRCA1 or BRCA2 could be exploited to develop new targeted therapeutic approaches for cancer occurring in mutation carriers.

Suppression of the DNA repair defects of BRCA2-deficient cells with heterologous protein fusions

The BRCA2 tumor suppressor plays an important role in the repair of DNA damage by homologous recombination, also termed homology-directed repair (HDR). Human BRCA2 is 3,418 aa and is composed of several domains. The central part of the protein contains multiple copies of a motif that binds the Rad51 recombinase (the BRC repeat), and the C terminus contains domains that have structural similarity to domains in the ssDNA-binding protein replication protein A (RPA). To gain insight into the role of BRCA2 in the repair of DNA damage, we fused a single (BRC3, BRC4) or multiple BRC motifs to the large RPA subunit. Expression of any of these protein fusions in Brca2 mutant cells substantially improved HDR while suppressing mutagenic repair. A fusion containing a Rad52 ssDNA-binding domain also was active in HDR. Mutations that reduced ssDNA or Rad51 binding impaired the ability of the fusion proteins to function in HDR. The high level of spontaneous chromosomal aberrations in Brca2 mutant cells was largely suppressed by the BRC-RPA fusion proteins, supporting the notion that the primary role of BRCA2 in maintaining genomic integrity is in HDR, specifically to deliver Rad51 to ssDNA. The fusion proteins also restored Rad51 focus formation and cellular survival in response to DNA damaging agents. Because as little as 2% of BRCA2 fused to RPA is sufficient to suppress cellular defects found in Brca2-mutant mammalian cells, these results provide insight into the recently discovered diversity of BRCA2 domain structures in different organisms.

Interaction between Arabidopsis Brca2 and its partners Rad51, Dmc1, and Dss1

The Arabidopsis (Arabidopsis thaliana) orthologs of Brca2, a protein whose mutations are involved in breast cancer in humans, were previously shown to be essential at meiosis. In an attempt to better understand the Brca2-interacting properties, we examined four partners of the two isoforms of Brca2 identified in Arabidopsis (AtRad51, AtDmc1, and two AtDss1 isoforms). The two Brca2 and the two Dss1 isoforms are named AtBrca2(IV), AtBrca2(V), AtDss1(I), and AtDss1(V) after their chromosomal localization. We first show that both AtBrca2 proteins can interact with either AtRad51 or AtDmc1 in vitro, and that the N-terminal region of AtBrca2 is responsible for these interactions. More specifically, the BRC motifs (so called because iterated in the Brca2 protein) in Brca2 are involved in these interactions: BRC motif number 2 (BRC2) alone can interact with AtDmc1, whereas BRC motif number 4 (BRC4) recognizes AtRad51. The human Rad51 and Dmc1 proteins themselves can interact with either the complete (HsRad51) or a shorter version of AtBrca2 (HsRad51 or HsDmc1) that comprises all four BRC motifs. We also identified two Arabidopsis isoforms of Dss1, another known partner of Brca2 in other organisms. Although all four Brca2 and Dss1 proteins are much conserved, AtBrca2(IV) interacts with only one of these AtDss1 proteins, whereas AtBrca2(V) interacts with both of them. Finally, we show for the first time that an AtBrca2 protein could bind two different partners at the same time: AtRad51 and AtDss1(I), or AtDmc1 and AtDss1(I).

Rec2 interplay with both Brh2 and Rad51 balances recombinational repair in Ustilago maydis

Rec2 is the single Rad51 paralog in Ustilago maydis. Here, we find that Rec2 is required for radiation-induced Rad51 nuclear focus formation but that Rec2 foci form independently of Rad51 and Brh2. Brh2 foci also form in the absence of Rad51 and Rec2. By coprecipitation from cleared extracts prepared from Escherichia coli cells expressing the proteins, we found that Rec2 interacts physically not only with Rad51 and itself but also with Brh2. Transgenic expression of Brh2 in rec2 mutants can effectively restore radiation resistance, but the frequencies of spontaneous Rad51 focus formation and allelic recombination are elevated. The Dss1-independent Brh2-RPA70 fusion protein is also active in restoring radiation sensitivity of rec2 but is hyperactive to an extreme degree in allelic recombination and in suppressing the meiotic block of rec2. However, the high frequency of chromosome missegregation in meiotic products is an indicator of a corrupted process. The results demonstrate that the importance of Rec2 function is not only in stimulating recombination activity but also in ensuring that recombination is properly controlled.