TopBP1 functions with 53BP1 in the G1 DNA damage checkpoint

TopBP1 controls BLM protein level to maintain genome stability

Human TopBP1 is a key mediator protein involved in DNA replication checkpoint control. In this study, we report a specific interaction between TopBP1 and Bloom syndrome helicase (BLM) that is phosphorylation and cell-cycle dependent. Interestingly, TopBP1 depletion led to decreased BLM protein level and increased sister chromatid exchange (SCE). Moreover, our data indicated that BLM was ubiquitinated by E3 ligase MIB1 and degraded in G1 cells but was stabilized by TopBP1 in S phase cells. Depletion of MIB1 restored BLM protein level and rescued the elevated SCE phenotype in TopBP1-depleted cells. In addition, cells expressing an undegradable BLM mutant showed radiation sensitivity, probably by triggering end resection and inhibiting the nonhomologous end-joining (NHEJ) pathway in G1 phase. Altogether, these data suggest that, although BLM is downregulated in G1 phase in order to promote NHEJ-mediated DNA repair, it is stabilized by TopBP1 in S phase cells in order to suppress SCE and thereby prevent genomic instability.

Phosphorylation-dependent assembly and coordination of the DNA damage checkpoint apparatus by Rad4(TopBP1)

The BRCT-domain protein Rad4(TopBP1) facilitates activation of the DNA damage checkpoint in Schizosaccharomyces pombe by physically coupling the Rad9-Rad1-Hus1 clamp, the Rad3(ATR) -Rad26(ATRIP) kinase complex, and the Crb2(53BP1) mediator. We have now determined crystal structures of the BRCT repeats of Rad4(TopBP1), revealing a distinctive domain architecture, and characterized their phosphorylation-dependent interactions with Rad9 and Crb2(53BP1). We identify a cluster of phosphorylation sites in the N-terminal region of Crb2(53BP1) that mediate interaction with Rad4(TopBP1) and reveal a hierarchical phosphorylation mechanism in which phosphorylation of Crb2(53BP1) residues Thr215 and Thr235 promotes phosphorylation of the noncanonical Thr187 site by scaffolding cyclin-dependent kinase (CDK) recruitment. Finally, we show that the simultaneous interaction of a single Rad4(TopBP1) molecule with both Thr187 phosphorylation sites in a Crb2(53BP1) dimer is essential for establishing the DNA damage checkpoint.

Ribosomal S6 Kinase 2 (RSK2) maintains genomic stability by activating the Atm/p53-dependent DNA damage pathway

Ribosomal S6 Kinase 2 (RSK2) is a member of the p90^RSK family of serine/threonine kinases, which are widely expressed and respond to many growth factors, peptide hormones, and neurotransmitters. Loss-of function mutations in the RPS6KA3 gene, which encodes the RSK2 protein, have been implicated in Coffin-Lowry Syndrome (CLS), an X-linked mental retardation disorder associated with cognitive deficits and behavioral impairments. However, the cellular and molecular mechanisms underlying this neurological disorder are not known. Recent evidence suggests that defective DNA damage signaling might be associated with neurological disorders, but the role of RSK2 in the DNA damage pathway remains to be elucidated. Here, we show that Adriamycin-induced DNA damage leads to the phosphorylation of RSK2 at Ser227 and Thr577 in the chromatin fraction, promotes RSK2 nuclear translocation, and enhances RSK2 and Atm interactions in the nuclear fraction. Furthermore, using RSK2 knockout mouse fibroblasts and RSK2-deficient cells from CLS patients, we demonstrate that ablation of RSK2 impairs the phosphorylation of Atm at Ser1981 and the phosphorylation of p53 at Ser18 (mouse) or Ser15 (human) in response to genotoxic stress. We also show that RSK2 affects p53-mediated downstream cellular events in response to DNA damage, that RSK2 knockout relieves cell cycle arrest at the G2/M phase, and that an increased number of γH2AX foci, which are associated with defects in DNA repair, are present in RSK2-deficient cells. Taken together, our findings demonstrated that RSK2 plays an important role in the DNA damage pathway that maintains genomic stability by mediating cell cycle progression and DNA repair.

ATR kinase activation in G1 phase facilitates the repair of ionizing radiation-induced DNA damage

The kinase ATR is activated by RPA-coated single-stranded DNA generated at aberrant replicative structures and resected double strand breaks. While many hundred candidate ATR substrates have been identified, the essential role of ATR in the replicative stress response has impeded the study of ATR kinase-dependent signalling. Using recently developed selective drugs, we show that ATR inhibition has a significantly more potent effect than ATM inhibition on ionizing radiation (IR)-mediated cell killing. Transient ATR inhibition for a short interval after IR has long-term consequences that include an accumulation of RPA foci and a total abrogation of Chk1 S345 phosphorylation. We show that ATR kinase activity in G1 phase cells is important for survival after IR and that ATR colocalizes with RPA in the absence of detectable RPA S4/8 phosphorylation. Our data reveal that, unexpectedly, ATR kinase inhibitors may be more potent cellular radiosensitizers than ATM kinase inhibitors, and that this is associated with a novel role for ATR in G1 phase cells.

Structural insights into recognition of MDC1 by TopBP1 in DNA replication checkpoint control

Activation of the DNA replication checkpoint by the ATR kinase requires protein interactions mediated by the ATR-activating protein, TopBP1. Accumulation of TopBP1 at stalled replication forks requires the interaction of TopBP1 BRCT5 with the phosphorylated SDT repeats of the adaptor protein MDC1. Here, we present the X-ray crystal structures of the tandem BRCT4/5 domains of TopBP1 free and in complex with a MDC1 consensus pSDpT phosphopeptide. TopBP1 BRCT4/5 adopts a variant BRCT-BRCT packing interface and recognizes its target peptide in a manner distinct from that observed in previous tandem BRCT- peptide structures. The phosphate-binding pocket and positively charged residues in a variant loop in BRCT5 present an extended binding surface for the negatively charged MDC1 phosphopeptide. Mutations in this surface reduce binding affinity and recruitment of TopBP1 to γH2AX foci in cells. These studies reveal a different mode of phosphopeptide binding by BRCT domains in the DNA damage response.

Interaction of polymorphisms in mitotic regulator genes with cigarette smoking and pancreatic cancer risk

Mitotic regulator genes have been associated with several cancers, however little is known about their possible association with pancreatic cancer. Smoking and family history are the strongest risk factors for this highly fatal disease. The main purpose of this study was to determine if polymorphisms of mitotic regulator genes are associated with pancreatic cancer and whether they modify the association between cigarette smoking and pancreatic cancer risk. A population-based case-control study was conducted in Ontario with 455 pathology-confirmed pancreatic cancer cases and 893 controls. Cigarette smoking history was collected using questionnaires and DNA obtained from blood samples. Genotypes were determined by mass-spectrometry. Odds ratio estimates were obtained using multivariate logistic regression. Interactions between genetic variant and smoking were assessed using stratified analyses and the likelihood ratio statistic (significance P < 0.05). Variants of MCPH1, FYN, APC, PRKCA, NIN, TopBP1, RIPK1, and SNW1 were not independently associated with pancreatic cancer risk. A significant interaction was observed between pack-years and MCPH1-2550-C > T (P = 0.02). Compared to never smokers, individuals with 10-27 pack-years and MCPH1-2550-CC genotype were at increased risk for pancreatic cancer (MVOR = 2.49, 95% confidence interval [95% CI]: 1.55, 4.00) as were those with >27 pack-years and MCPH1-2550-TC genotype (MVOR = 2.42, 95% CI: 1.45, 4.05). A significant interaction was observed between smoking status and TopBP1-3257-A > G (P = 0.04) using a dominant model. Current smokers with the TopBP1-3257 A allele were at increased risk for pancreatic cancer (MVOR = 2.55, 95% CI: 1.77, 3.67). MCPH1-2550-C > T and TopBP1-3257-A > G modify the association between smoking and pancreatic cancer. These findings provide insights into the potential molecular mechanisms behind smoking-associated pancreatic cancer.

Oligomerization of TopBP1 is necessary for the localization of TopBP1 to mitotic centrosomes

Human TopBP1 is involved in the DNA damage checkpoint response, chromosome replication, and other functions of cell cycle control. The C-terminal region of TopBP1 (TbpCtr: amino acid residues 1222-1522) is involved in the localization of TopBP1 to the centrosomes during mitosis. Here, we showed that the amino acid residues 741-885 of TopBP1, in addition to TbpCtr, are necessary for the centrosomal localization of TopBP1. Whereas oligomeric tags fused to TbpCtr localized to mitotic centrosomes, monomeric tags fused to TbpCtr did not. Insertion of the amino acid residues 741-885 into the monomeric tag fused to TbpCtr allowed the protein to localize to the mitotic centrosome. These results suggest that the amino acid residues 741-885 are necessary for oligomerization of TopBP1 for centrosomal localization.

WD40-repeat protein WDR18 collaborates with TopBP1 to facilitate DNA damage checkpoint signaling

The genomes of all living organisms are exposed to a wide spectrum of insults. To maintain genomic integrity, eukaryotes have evolved an elaborate surveillance mechanism - DNA damage checkpoint signaling - to detect damaged DNA and to arrest cell cycle progression, allowing time to process and repair DNA damage. TopBP1 plays multiple roles in the regulation of DNA damage checkpoint signaling. However, the molecular mechanism of how TopBP1 regulates ATR-mediated Chk1 phosphorylation is poorly understood. In this communication, we demonstrate (1) that the Chk1 activation domain of TopBP1 is critical in response to several different types of DNA damage; (2) that WD40-repeat protein WDR18 associates with the C-terminus of TopBP1 in vitro and in vivo; (3) that the association between WDR18 and TopBP1 is required for AT70-induced Chk1 phosphorylation; (4) and that WDR18 itself is required for AT70-triggered Chk1 phosphorylation. In addition, WDR18 associates with Chk1 in vitro. The data suggest that WDR18 facilitates ATR-dependent Chk1 phosphorylation via interacting with both C-terminus of TopBP1 and Chk1. Our findings indicate that WDR18 is a bona fide checkpoint protein and that WDR18 works together with TopBP1 to promote DNA damage checkpoint signaling.

Expression of TopBP1 in hereditary breast cancer

TopBP1 protein displays structural as well as functional similarities to BRCA1 and is involved in DNA replication, DNA damage checkpoint response and transcriptional regulation. Aberrant expression of TopBP1 may lead to genomic instability and can have pathological consequences. In this study we aimed to investigate expression of TopBP1 gene at mRNA and protein level in hereditary breast cancer. Real-time quantitative PCR was performed in 127 breast cancer samples. Expression of TopBP1 mRNA in lobular carcinoma was significantly lower compared with ductal carcinoma (p < 0.05). The level of TopBP1 mRNA appeared to be lower in poorly differentiated (III grade) hereditary breast cancer in comparison with moderately (II grade) and well-differentiated cancer (I grade) (p < 0.05 and p < 0.001 respectively). We analyzed TopBP1 protein expression using immunohistochemistry and Western blot techniques. Expression of TopBP1 protein was found to be significantly increased in poorly differentiated breast cancer (III grade) (p < 0.05). The percentage of samples with cytoplasmic apart from nuclear staining increased with increasing histological grade. There was no significant association between level and intracellular localization of TopBP1 protein in hereditary breast cancer and other clinicopathological parameters such as estrogen and progesterone receptors status, appearance of metastasis in the axillary lymph nodes and type of cancer. Our data suggest that decreased level of TopBP1 mRNA and increased level of TopBP1 protein might be associated with progression of hereditary breast cancer.

The Rad4(TopBP1) ATR-activation domain functions in G1/S phase in a chromatin-dependent manner

DNA damage checkpoint activation can be subdivided in two steps: initial activation and signal amplification. The events distinguishing these two phases and their genetic determinants remain obscure. TopBP1, a mediator protein containing multiple BRCT domains, binds to and activates the ATR/ATRIP complex through its ATR-Activation Domain (AAD). We show that Schizosaccharomyces pombe Rad4^TopBP1 AAD–defective strains are DNA damage sensitive during G1/S-phase, but not during G2. Using lacO-LacI tethering, we developed a DNA damage–independent assay for checkpoint activation that is Rad4^TopBP1 AAD–dependent. In this assay, checkpoint activation requires histone H2A phosphorylation, the interaction between TopBP1 and the 9-1-1 complex, and is mediated by the phospho-binding activity of Crb2^53BP1. Consistent with a model where Rad4^TopBP1 AAD–dependent checkpoint activation is ssDNA/RPA–independent and functions to amplify otherwise weak checkpoint signals, we demonstrate that the Rad4^TopBP1 AAD is important for Chk1 phosphorylation when resection is limited in G2 by ablation of the resecting nuclease, Exo1. We also show that the Rad4^TopBP1 AAD acts additively with a Rad9 AAD in G1/S phase but not G2. We propose that AAD–dependent Rad3^ATR checkpoint amplification is particularly important when DNA resection is limiting. In S. pombe, this manifests in G1/S phase and relies on protein–chromatin interactions.

DNA structure–dependent checkpoint activation and the amplification of checkpoint signals are carefully modulated to allow the checkpoint kinases to delay mitosis and regulate DNA metabolism. While much work has gone into understanding how this checkpoint functions, the mechanism by which the checkpoint signal is amplified is less clear. We have characterised a conserved domain in the Schizosaccharomyces pombe TopBP1 homolog, Rad4^TopBP1 (also known as Cut5) that is capable of activating the ATR homolog Rad3^ATR. We demonstrate that this domain is not required for initial checkpoint activation, but functions to amplify the checkpoint signal, likely when the presence of single-stranded DNA is limiting. Our data suggest that the function of the Rad4^TopBP1 ATR-Activation Domain (AAD) is mediated by interactions between checkpoint proteins and phosphorylated histone H2A, which is itself promoted by Rad3^ATR. We propose that the resulting amplification of the checkpoint signal is particularly important in G1-S phase, when resection is limited.

Recognition, signaling, and repair of DNA double-strand breaks produced by ionizing radiation in mammalian cells: the molecular choreography

The faithful maintenance of chromosome continuity in human cells during DNA replication and repair is critical for preventing the conversion of normal diploid cells to an oncogenic state. The evolution of higher eukaryotic cells endowed them with a large genetic investment in the molecular machinery that ensures chromosome stability. In mammalian and other vertebrate cells, the elimination of double-strand breaks with minimal nucleotide sequence change involves the spatiotemporal orchestration of a seemingly endless number of proteins ranging in their action from the nucleotide level to nucleosome organization and chromosome architecture. DNA DSBs trigger a myriad of post-translational modifications that alter catalytic activities and the specificity of protein interactions: phosphorylation, acetylation, methylation, ubiquitylation, and SUMOylation, followed by the reversal of these changes as repair is completed. "Superfluous" protein recruitment to damage sites, functional redundancy, and alternative pathways ensure that DSB repair is extremely efficient, both quantitatively and qualitatively. This review strives to integrate the information about the molecular mechanisms of DSB repair that has emerged over the last two decades with a focus on DSBs produced by the prototype agent ionizing radiation (IR). The exponential growth of molecular studies, heavily driven by RNA knockdown technology, now reveals an outline of how many key protein players in genome stability and cancer biology perform their interwoven tasks, e.g. ATM, ATR, DNA-PK, Chk1, Chk2, PARP1/2/3, 53BP1, BRCA1, BRCA2, BLM, RAD51, and the MRE11-RAD50-NBS1 complex. Thus, the nature of the intricate coordination of repair processes with cell cycle progression is becoming apparent. This review also links molecular abnormalities to cellular pathology as much a possible and provides a framework of temporal relationships.

Eukaryotic DNA damage checkpoint activation in response to double-strand breaks

Double-strand breaks (DSBs) are the most detrimental form of DNA damage. Failure to repair these cytotoxic lesions can result in genome rearrangements conducive to the development of many diseases, including cancer. The DNA damage response (DDR) ensures the rapid detection and repair of DSBs in order to maintain genome integrity. Central to the DDR are the DNA damage checkpoints. When activated by DNA damage, these sophisticated surveillance mechanisms induce transient cell cycle arrests, allowing sufficient time for DNA repair. Since the term "checkpoint" was coined over 20 years ago, our understanding of the molecular mechanisms governing the DNA damage checkpoint has advanced significantly. These pathways are highly conserved from yeast to humans. Thus, significant findings in yeast may be extrapolated to vertebrates, greatly facilitating the molecular dissection of these complex regulatory networks. This review focuses on the cellular response to DSBs in Saccharomyces cerevisiae, providing a comprehensive overview of how these signalling pathways function to orchestrate the cellular response to DNA damage and preserve genome stability in eukaryotic cells.

The helicase domain and C-terminus of human RecQL4 facilitate replication elongation on DNA templates damaged by ionizing radiation

The vertebrate RECQL4 (RECQ4) gene is thought to be the ortholog of budding yeast SLD2. However, RecQL4 contains within its C-terminus a RecQ-like helicase domain, which is absent in Sld2. We established human pre-B lymphocyte Nalm-6 cells, in which the endogenous RECQL4 gene was homozygously targeted such that the entire C-terminus would not be expressed. The RECQL4(ΔC/ΔC) cells behaved like the parental cells during unperturbed DNA replication or after treatment with agents that induce stalling of DNA replication forks, such as hydroxyurea (HU). However, after exposure to ionizing radiation (IR), the RECQL4(ΔC/ΔC) cells exhibited hypersensitivity, inability to complete S phase and prematurely terminated or paused DNA replication forks. Deletion of BLM, a gene that also encodes a RecQ helicase, had the opposite phenotype; an almost wild-type response to IR, but hypersensitivity to HU. Targeting both RECQL4 and BLM resulted in viable cells, which exhibited mostly additive phenotypes compared with those exhibited by the RECQL4(ΔC/ΔC) and the BLM(-/-) cells. We propose that RecQL4 facilitates DNA replication in cells that have been exposed to IR.

53BP1 contributes to a robust genomic stability in human fibroblasts

Faithful repair of damaged DNA is a crucial process in maintaining cell viability and function. A multitude of factors and pathways guides this process and includes repair proteins and cell cycle checkpoint factors. Differences in the maintenance of genomic processes are one feature that may contribute to species-specific differences in lifespan. We predicted that 53BP1, a key transducer of the DNA damage response and cell cycle checkpoint control, is highly involved in maintaining genomic stability and may function differently in cells from different species. We demonstrate a difference in the levels and recruitment of 53BP1 in mouse and human cells following DNA damage. In addition, we show that unresolved DNA damage persists more in mouse cells than in human cells, as evidenced by increased numbers of micronuclei. The difference in micronuclei seems to be related to the levels of 53BP1 present in cells. Finally, we present evidence that unresolved DNA damage correlates with species lifespan. Taken together, these studies suggest a link between recruitment of 53BP1, resolution of DNA damage, and increased species lifespan.

53BP1 suppresses tumor growth and promotes susceptibility to apoptosis of ovarian cancer cells through modulation of the Akt pathway

53BP1 has been extensively studied as a key component of the DNA damage response, but little is known regarding the role of 53BP1 in preventing tumor development. The present study was designed to assess the impact of the modification of 53BP1 gene expression on the biological behavior of ovarian cancer cell lines and to elucidate the cellular pathway(s) triggered by 53BP1 in cancer cells. DNA liposome transfection technology was employed to increase and to knock down the expression of 53BP1 in A2780 and HO-8910PM cells, respectively. Viability, clonogenicity and cell cycle profiles were evaluated. Cell apoptosis was analyzed using flow cytometric assay. The expression of proteins related to apoptosis and cell signal transduction was assessed using western blotting. Increased expression of 53BP1 decreased the viability and the clonogenicity, and induced G₂/M arrest and apoptosis of the treated cells. The protein expression of Bax, P21 and caspase-3 was upregulated, while the levels of Bcl-2 and p-Akt were reduced to a statistically significant level. In contrast, deregulation of 53BP1 significantly increased proliferative ability. Collectively, our data suggest that 53BP1 is involved in several important steps in controlling cell proliferation and growth and preventing tumor development.

More than just a focus: The chromatin response to DNA damage and its role in genome integrity maintenance

Following the discovery in 1998 of γ-H2AX, the first histone modification induced by DNA damage, interest in the changes to chromatin induced by DNA damage has exploded, and a vast amount of information has been generated. However, there has been a discrepancy between our rapidly advancing knowledge of how chromatin responds to DNA damage and the understanding of why cells mobilize large segments of chromatin to protect the genome against destabilizing effects posed by tiny DNA lesions. Recent research has provided insights into these issues and suggests that chromatin responses induced by DNA damage are not simply the accumulation of 'nuclear foci' but are mechanisms required to guard genome integrity.

TopBP1 mediates mutant p53 gain of function through NF-Y and p63/p73

Nearly half of human cancers harbor p53 mutations, which can promote cancerous growth, metastasis, and resistance to therapy. The gain of function of mutant p53 is partly mediated by its ability to form a complex with NF-Y or p63/p73. Here, we demonstrate that TopBP1 mediates these activities in cancer, and we provide both in vitro and in vivo evidence to support its role. We show that TopBP1 interacts with p53 hot spot mutants and NF-YA and promotes mutant p53 and p300 recruitment to NF-Y target gene promoters. TopBP1 also facilitates mutant p53 interaction with and inhibition of the transcriptional activities of p63/p73. Depletion of TopBP1 in mutant p53 cancer cells leads to downregulation of NF-Y target genes cyclin A and Cdk1 and upregulation of p63/p73 target genes such as Bax and Noxa. Mutant p53-mediated resistance to chemotherapeutic agents depends on TopBP1. The growth-promoting activity of mutant p53 in a xenograft model also requires TopBP1. Thus, TopBP1 mediates mutant p53 gain of function in cancer. Since TopBP1 is often overexpressed in cancer cells and is recruited to cooperate with mutant p53 for tumor progression, TopBP1/mutant p53 interaction may be a new therapeutic target in cancer.

ATR signalling: more than meeting at the fork

Preservation of genome integrity via the DNA-damage response is critical to prevent disease. ATR (ataxia telangiectasia mutated- and Rad3-related) is essential for life and functions as a master regulator of the DNA-damage response, especially during DNA replication. ATR controls and co-ordinates DNA replication origin firing, replication fork stability, cell cycle checkpoints and DNA repair. Since its identification 15 years ago, a model of ATR activation and signalling has emerged that involves localization to sites of DNA damage and activation through protein-protein interactions. Recent research has added an increasingly detailed understanding of the canonical ATR pathway, and an appreciation that the canonical model does not fully capture the complexity of ATR regulation. In the present article, we review the ATR signalling process, focusing on mechanistic findings garnered from the identification of new ATR-interacting proteins and substrates. We discuss how to incorporate these new insights into a model of ATR regulation and point out the significant gaps in our understanding of this essential genome-maintenance pathway.

BRCT domains: easy as one, two, three

BRCA1 C-terminal (BRCT) domains are integral signaling modules in the DNA damage response (DDR). Aside from their established roles as phospho-peptide binding modules, BRCT domains have been implicated in phosphorylation-independent protein interactions, DNA binding and poly(ADP-ribose) (PAR) binding. These numerous functions can be attributed to the diversity in BRCT domain structure and architecture, where domains can exist as isolated single domains or assemble into higher order homo- or hetero- domain complexes. In this review, we incorporate recent structural and biochemical studies to demonstrate how structural features allow single and tandem BRCT domains to attain a high degree of functional diversity.

MDC1 collaborates with TopBP1 in DNA replication checkpoint control

The DNA damage checkpoint protein MDC1 also interacts with TopBP1 to promote DNA replication checkpoint control.

Human TopBP1 is a major player in the control of the DNA replication checkpoint. In this study, we identified MDC1, a key checkpoint protein involved in the cellular response to DNA double-strand breaks, as a TopBP1-associated protein. The specific TopBP1–MDC1 interaction is mediated by the fifth BRCT domain of TopBP1 and the Ser-Asp-Thr (SDT) repeats of MDC1. In addition, we demonstrated that TopBP1 accumulation at stalled replication forks is promoted by the H2AX/MDC1 signaling cascade. Moreover, MDC1 is important for ATR-dependent Chk1 activation in response to replication stress. Collectively, our data suggest that MDC1 facilitates several important steps in both cellular DNA damage response and the DNA replication checkpoint.

Molecular basis of BACH1/FANCJ recognition by TopBP1 in DNA replication checkpoint control

The diverse roles of TopBP1 in DNA replication and checkpoint signaling are associated with the scaffolding ability of TopBP1 to initiate various protein-protein interactions. The recognition of the BACH1/FANCJ helicase by TopBP1 is critical for the activation of the DNA replication checkpoint at stalled replication forks and is facilitated by the C-terminal tandem BRCT7/8 domains of TopBP1 and a phosphorylated Thr(1133) binding motif in BACH1. Here we provide the structural basis for this interaction through analysis of the x-ray crystal structures of TopBP1 BRCT7/8 both free and in complex with a BACH1 phospho-peptide. In contrast to canonical BRCT-phospho-peptide recognition, TopBP1 BRCT7/8 undergoes a dramatic conformational change upon BACH1 binding such that the two BRCT repeats pivot about the central BRCT-BRCT interface to provide an extensive and deep peptide-binding cleft. Additionally, we provide the first structural mechanism for Thr(P) recognition among BRCT domains. Together with systematic mutagenesis studies, we highlight the role of key contacts in governing the unique specificity of the TopBP1-BACH1 interaction.