Thermodynamics of human DNA ligase I trimerization and association with DNA polymerase beta

Impact of DNA Ligase 1 Genotypes on Childhood Acute Lymphocytic Leukemia

BACKGROUND/AIM

Genetic polymorphisms in DNA repair mechanisms can modulate overall DNA repair capacity, potentially influencing individual susceptibility to cancer. This study investigated the relationship between polymorphic variations in DNA ligase 1 and the risk of childhood acute lymphocytic leukemia (cALL).

MATERIALS AND METHODS

The genotypes of DNA ligase 1 rs20579 were determined using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis. The study assessed the potential association between DNA ligase 1 rs20579 genotypes and cALL risk in a Taiwanese cohort, consisting of 266 cALL cases and an equal number of age- and sex-matched controls.

RESULTS

The distribution of GG, AG, and AA genotypes for DNA ligase 1 rs20579 was 78.6%, 19.5%, and 1.9% among controls, and 76.0%, 21.4%, and 2.6% among cALL cases, respectively (p for trend=0.7111). No significant difference was observed in the distribution of AG and AA genotypes between the two groups (p=0.6340 and 0.7381, respectively). Allelic frequency analysis revealed that carriers of the variant A allele of DNA ligase 1 rs20579 had a non-significant increase in cALL risk compared to those with the wild-type G allele [odds ratio (OR)=1.17, 95% confidence interval (CI)=0.81-1.68, p=0.4583]. While no significant genotype distribution difference was noted among males (p=0.4635), females carrying the AG and AA genotypes exhibited a significantly increased risk of cALL (p=0.0328).

CONCLUSION

In the Taiwanese population, the variant A allele of DNA ligase 1 rs20579 may serve as a potential diagnostic marker for elevated cALL risk in young females.

Probing the mechanism of nick searching by LIG1 at the single-molecule level

DNA ligase 1 (LIG1) joins Okazaki fragments during the nuclear replication and completes DNA repair pathways by joining 3'-OH and 5'-PO4 ends of nick at the final step. Yet, the mechanism of how LIG1 searches for a nick at single-molecule level is unknown. Here, we combine single-molecule fluorescence microscopy approaches, C-Trap and total internal reflection fluorescence (TIRF), to investigate the dynamics of LIG1-nick DNA binding. Our C-Trap data reveal that DNA binding by LIG1 full-length is enriched near the nick sites and the protein exhibits diffusive behavior to form a long-lived ligase/nick complex after binding to a non-nick region. However, LIG1 C-terminal mutant, containing the catalytic core and DNA-binding domain, predominantly binds throughout DNA non-specifically to the regions lacking nick site for shorter time. These results are further supported by TIRF data for LIG1 binding to DNA with a single nick site and demonstrate that a fraction of LIG1 full-length binds significantly longer period compared to the C-terminal mutant. Overall comparison of DNA binding modes provides a mechanistic model where the N-terminal domain promotes 1D diffusion and the enrichment of LIG1 binding at nick sites with longer binding lifetime, thereby facilitating an efficient nick search process.

Impact of DNA ligase 1 and IIIα interactions with APE1 and polβ on the efficiency of base excision repair pathway at the downstream steps

Base excision repair (BER) requires a tight coordination between the repair enzymes through protein-protein interactions and involves gap filling by DNA polymerase (pol) β and subsequent nick sealing by DNA ligase (LIG) 1 or LIGIIIα at the downstream steps. Apurinic/apyrimidinic-endonuclease 1 (APE1), by its exonuclease activity, proofreads 3' mismatches incorporated by polβ during BER. We previously reported that the interruptions in the functional interplay between polβ and the BER ligases result in faulty repair events. Yet, how the protein interactions of LIG1 and LIGIIIα could affect the repair pathway coordination during nick sealing at the final steps remains unknown. Here, we demonstrate that LIGIIIα interacts more tightly with polβ and APE1 than LIG1, and the N-terminal noncatalytic region of LIG1 as well as the catalytic core and BRCT domain of LIGIIIα mediate interactions with both proteins. Our results demonstrated less efficient nick sealing of polβ nucleotide insertion products in the absence of LIGIIIα zinc-finger domain and LIG1 N-terminal region. Furthermore, we showed a coordination between APE1 and LIG1/LIGIIIα during the removal of 3' mismatches from the nick repair intermediate on which both BER ligases can seal noncanonical ends or gap repair intermediate leading to products of single deletion mutagenesis. Overall results demonstrate the importance of functional coordination from gap filling by polβ coupled to nick sealing by LIG1/LIGIIIα in the presence of proofreading by APE1, which is mainly governed by protein-protein interactions and protein-DNA intermediate communications, to maintain repair efficiency at the downstream steps of the BER pathway.

Uncovering nick DNA binding by LIG1 at the single-molecule level

DNA ligases repair the strand breaks are made continually and naturally throughout the genome, if left unrepaired and allowed to persist, they can lead to genome instability in the forms of lethal double-strand (ds) breaks, deletions, and duplications. DNA ligase 1 (LIG1) joins Okazaki fragments during the replication machinery and seals nicks at the end of most DNA repair pathways. Yet, how LIG1 recognizes its target substrate is entirely missing. Here, we uncover the dynamics of nick DNA binding by LIG1 at the single-molecule level. Our findings reveal that LIG1 binds to dsDNA both specifically and non-specifically and exhibits diffusive behavior to form a stable complex at the nick. Furthermore, by comparing with the LIG1 C-terminal protein, we demonstrate that the N-terminal non-catalytic region promotes binding enriched at nick sites and facilitates an efficient nick search process by promoting 1D diffusion along the DNA. Our findings provide a novel single-molecule insight into the nick binding by LIG1, which is critical to repair broken phosphodiester bonds in the DNA backbone to maintain genome integrity.

Fluorescently labeled human apurinic/apyrimidinic endonuclease APE1 reveals effects of DNA polymerase β on the APE1-DNA interaction

The base excision repair (BER) pathway involves sequential action of DNA glycosylases and apurinic/apyrimidinic (AP) endonucleases to incise damaged DNA and prepare DNA termini for incorporation of a correct nucleotide by DNA polymerases. It has been suggested that the enzymatic steps in BER include recognition of a product-enzyme complex by the next enzyme in the pathway, resulting in the "passing-the-baton" model of transfer of DNA intermediates between enzymes. To verify this model, in this work, we aimed to create a suitable experimental system. We prepared APE1 site-specifically labeled with a fluorescent reporter that is sensitive to stages of APE1-DNA binding, of formation of the catalytic complex, and of subsequent dissociation of the enzyme-product complex. Interactions of the labeled APE1 with various model DNA substrates (containing an abasic site) of varied lengths revealed that the enzyme remains mostly in complex with the DNA product. By means of the fluorescently labeled APE1 in combination with a stopped-flow fluorescence assay, it was found that Polβ stimulates both i) APE1 binding to an abasic-site-containing DNA duplex with the formation of a catalytically competent complex and ii) the dissociation of APE1 from its product. These findings confirm DNA-mediated coordination of APE1 and Polβ activities and suggest that Polβ is the key trigger of the DNA transfer between the enzymes participating in initial steps of BER.

Human DNA ligases in replication and repair

To ensure genome integrity, the joining of breaks in the phosphodiester backbone of duplex DNA is required during DNA replication and to complete the repair of almost all types of DNA damage. In human cells, this task is accomplished by DNA ligases encoded by three genes, LIG1, LIG3 and LIG4. Mutations in LIG1 and LIG4 have been identified as the causative factor in two inherited immunodeficiency syndromes. Moreover, there is emerging evidence that DNA ligases may be good targets for the development of novel anti-cancer agents. In this graphical review, we provide an overview of the roles of the DNA ligases encoded by the three human LIG genes in DNA replication and repair.

Effect of Human XRCC1 Protein Oxidation on the Functional Activity of Its Complexes with the Key Enzymes of DNA Base Excision Repair

Base excision repair (BER) ensures correction of most abundant DNA lesions in mammals. The efficiency of this multistep DNA repair process that can occur via different pathways depends on the coordinated action of enzymes catalyzing its individual steps. The scaffold XRCC1 (X-ray repair cross-complementing protein 1) protein plays an important coordinating role in the repair of damaged bases and apurinic/apyrimidinic (AP) sites via short-patch (SP) BER pathway, as well as in the repair of single-strand DNA breaks. In this study, we demonstrated for the first time in vitro formation of the ternary XRCC1 complex with the key enzymes of SP BER - DNA polymerase β (Polβ) and DNA ligase IIIα (LigIIIα) - using the fluorescence-based technique. It was found that Polβ directly interacts with LigIIIα, but their complex is less stable than the XRCC1-Polβ and XRCC1-LigIIIα complexes. The effect of XRCC1 oxidation and composition of the multiprotein complex on the efficiency of DNA synthesis and DNA ligation during DNA repair has been explored. We found that formation of the disulfide bond between Cys12 and Cys20 residues as a result of XRCC1 oxidation (previously shown to modulate the protein affinity for Polβ), affects the yield of the final product of SP BER and of non-ligated DNA intermediates (substrates of long-patch BER). The effect of XRCC1 oxidation on the final product yield depended on the presence of AP endonuclease 1. Together with the data from our previous work, the results of this study suggest an important role of XRCC1 oxidation in the fine regulation of formation of BER complexes and their functional activity.

Pol β gap filling, DNA ligation and substrate-product channeling during base excision repair opposite oxidized 5-methylcytosine modifications

DNA methylation on cytosine in CpG islands generates 5-methylcytosine (5mC), and further modification of 5mC can result in the oxidized variants 5-hydroxymethyl (5hmC), 5-formyl (5fC), and 5-carboxy (5caC). Base excision repair (BER) is crucial for both genome maintenance and active DNA demethylation of modified cytosine products and involves substrate-product channeling from nucleotide insertion by DNA polymerase (pol) β to the subsequent ligation step. Here, we report that, in contrast to the pol β mismatch insertion products (dCTP, dATP, and dTTP), the nicked products after pol β dGTP insertion can be ligated by DNA ligase I or DNA ligase III/XRCC1 complex when a 5mC oxidation modification is present opposite in the template position in vitro. A Pol β K280A mutation, which perturbates the stabilization of these base modifications within the active site, hinders the BER ligases. Moreover, the nicked repair intermediates that mimic pol β mismatch insertion products, i.e., with 3'-preinserted dGMP or dTMP opposite templating 5hmC, 5fC or 5caC, can be efficiently ligated, whereas preinserted 3'-dAMP or dCMP mismatches result in failed ligation reactions. These findings herein contribute to our understanding of the insertion tendencies of pol β opposite different cytosine base forms, the ligation properties of DNA ligase I and DNA ligase III/XRCC1 complex in the context of gapped and nicked damage-containing repair intermediates, and the efficiency and fidelity of substrate channeling during the final steps of BER in situations involving oxidative 5mC base modifications in the template strand.

The ligation of pol β mismatch insertion products governs the formation of promutagenic base excision DNA repair intermediates

DNA ligase I and DNA ligase III/XRCC1 complex catalyze the ultimate ligation step following DNA polymerase (pol) β nucleotide insertion during base excision repair (BER). Pol β Asn279 and Arg283 are the critical active site residues for the differentiation of an incoming nucleotide and a template base and the N-terminal domain of DNA ligase I mediates its interaction with pol β. Here, we show inefficient ligation of pol β insertion products with mismatched or damaged nucleotides, with the exception of a Watson–Crick-like dGTP insertion opposite T, using BER DNA ligases in vitro. Moreover, pol β N279A and R283A mutants deter the ligation of the promutagenic repair intermediates and the presence of N-terminal domain of DNA ligase I in a coupled reaction governs the channeling of the pol β insertion products. Our results demonstrate that the BER DNA ligases are compromised by subtle changes in all 12 possible noncanonical base pairs at the 3′-end of the nicked repair intermediate. These findings contribute to understanding of how the identity of the mismatch affects the substrate channeling of the repair pathway and the mechanism underlying the coordination between pol β and DNA ligase at the final ligation step to maintain the BER efficiency.

Interplay between DNA Polymerases and DNA Ligases: Influence on Substrate Channeling and the Fidelity of DNA Ligation

DNA ligases are a highly conserved group of nucleic acid enzymes that play an essential role in DNA repair, replication, and recombination. This review focuses on functional interaction between DNA polymerases and DNA ligases in the repair of single- and double-strand DNA breaks, and discusses the notion that the substrate channeling during DNA polymerase-mediated nucleotide insertion coupled to DNA ligation could be a mechanism to minimize the release of potentially mutagenic repair intermediates. Evidence suggesting that DNA ligases are essential for cell viability includes the fact that defects or insufficiency in DNA ligase are casually linked to genome instability. In the future, it may be possible to develop small molecule inhibitors of mammalian DNA ligases and/or their functional protein partners that potentiate the effects of chemotherapeutic compounds and improve cancer treatment outcomes.

Dynamic light scattering study of base excision DNA repair proteins and their complexes

Base excision repair (BER) involves many enzymes acting in a coordinated fashion at the most common types of DNA damage. The coordination is facilitated by interactions between the enzymes and accessory proteins, X-ray repair cross-complementing protein 1 (XRCC1) and poly(ADP-ribose) polymerase 1 (PARP1). Here we use dynamic light scattering (DLS) technique to determine the hydrodynamic sizes of several BER enzymes and proteins, DNA polymerase β (Polβ), apurinic/apyrimidinic endonuclease 1 (APE1), tyrosyl-DNA phosphodiesterase 1 (TDP1), XRCC1 and PARP1, present alone or in the equimolar mixtures with each other. From the DLS data combined with glutaraldehyde cross-linking experiments and previous quantitative binding data the oligomeric states of BER proteins and their complexes are estimated. All the proteins have been proposed to form homodimers upon their self-association. The most probable oligomerization state of the binary complexes formed by PARP1 with various proteins is a heterotetramer. The oligomerization state of the binary complexes formed by XRCC1 varies from heterodimer to heterotetramer, depending on the partner. The DLS technique is applied for the first time to measure the hydrodynamic sizes of PARP1 molecules covalently bound with poly(ADP-ribose) (PAR) synthesized upon the automodification reaction. PARP1 has been detected to form huge conglomerates stabilized by Mg²⁺ coordinated bonds with PAR polymers.

DNA polymerase β contains a functional nuclear localization signal at its N-terminus

DNA polymerase β (pol β) requires nuclear localization to fulfil its DNA repair function. Although its small size has been interpreted to imply the absence of a need for active nuclear import, sequence and structural analysis suggests that a monopartite nuclear localization signal (NLS) may reside in the N-terminal lyase domain. Binding of this domain to Importin α1 (Impα1) was confirmed by gel filtration and NMR studies. Affinity was quantified by fluorescence polarization analysis of a fluorescein-tagged peptide corresponding to pol β residues 2–13. These studies indicate high affinity binding, characterized by a low micromolar K_d, that is selective for the murine Importin α1 (mImpα1) minor site, with the K_d strengthening to ∼140 nM for the full lyase domain (residues 2–87). A further reduction in K_d obtains in binding studies with human Importin α5 (hImpα5), which in some cases has been demonstrated to bind small domains connected to the NLS. The role of this NLS was confirmed by fluorescent imaging of wild-type and NLS-mutated pol β(R4S,K5S) in mouse embryonic fibroblasts lacking endogenous pol β. Together these data demonstrate that pol β contains a specific NLS sequence in the N-terminal lyase domain that promotes transport of the protein independent of its interaction partners. Active nuclear uptake allows development of a nuclear/cytosolic concentration gradient against a background of passive diffusion.

Proliferating cell nuclear antigen restores the enzymatic activity of a DNA ligase I deficient in DNA binding

Proliferating cell nuclear antigen (PCNA) coordinates multienzymatic reactions by interacting with a variety of protein partners. Family I DNA ligases are multidomain proteins involved in sealing of DNA nicks during Okazaki fragment maturation and DNA repair. The interaction of DNA ligases with the interdomain connector loop (IDCL) of PCNA through its PCNA‐interacting peptide (PIP box) is well studied but the role of the interacting surface between both proteins is not well characterized. In this work, we used a minimal DNA ligase I and two N‐terminal deletions to establish that DNA binding and nick‐sealing stimulation of DNA ligase I by PCNA are not solely dependent on the PIP box–IDCL interaction. We found that a truncated DNA ligase I with a deleted PIP box is stimulated by PCNA. Furthermore, the activity of a DNA ligase defective in DNA binding is rescued upon PCNA addition. As the rate constants for single‐turnover ligation for the full‐length and truncated DNA ligases are not affected by PCNA, our data suggest that PCNA stimulation is achieved by increasing the affinity for nicked DNA substrate and not by increasing catalytic efficiency. Surprisingly C‐terminal mutants of PCNA are not able to stimulate nick‐sealing activity of Entamoeba histolytica DNA ligase I. Our data support the notion that the C‐terminal region of PCNA may be involved in promoting an allosteric transition in E. histolytica DNA ligase I from a spread‐shaped to a ring‐shaped structure. This study suggests that the ring‐shaped PCNA is a binding platform able to stabilize coevolved protein–protein interactions, in this case an interaction with DNA ligase I.

Oxidized nucleotide insertion by pol β confounds ligation during base excision repair

Oxidative stress in cells can lead to accumulation of reactive oxygen species and oxidation of DNA precursors. Oxidized purine nucleotides can be inserted into DNA during replication and repair. The main pathway for correcting oxidized bases in DNA is base excision repair (BER), and in vertebrates DNA polymerase β (pol β) provides gap filling and tailoring functions. Here we report that the DNA ligation step of BER is compromised after pol β insertion of oxidized purine nucleotides into the BER intermediate in vitro. These results suggest the possibility that BER mediated toxic strand breaks are produced in cells under oxidative stress conditions. We observe enhanced cytotoxicity in oxidizing-agent treated pol β expressing mouse fibroblasts, suggesting formation of DNA strand breaks under these treatment conditions. Increased cytotoxicity following MTH1 knockout or treatment with MTH1 inhibitor suggests the oxidation of precursor nucleotides.

Oxidative stress in cells leads to the oxidations of DNA precursors. Here the authors show that these oxidized precursors can be incorporated in vivo during base excision repair, leading to DNA breaks and cytotoxicity.

Quantitative characterization of protein-protein complexes involved in base excision DNA repair

Base Excision Repair (BER) efficiently corrects the most common types of DNA damage in mammalian cells. Step-by-step coordination of BER is facilitated by multiple interactions between enzymes and accessory proteins involved. Here we characterize quantitatively a number of complexes formed by DNA polymerase β (Polβ), apurinic/apyrimidinic endonuclease 1 (APE1), poly(ADP-ribose) polymerase 1 (PARP1), X-ray repair cross-complementing protein 1 (XRCC1) and tyrosyl-DNA phosphodiesterase 1 (TDP1), using fluorescence- and light scattering-based techniques. Direct physical interactions between the APE1-Polβ, APE1-TDP1, APE1-PARP1 and Polβ-TDP1 pairs have been detected and characterized for the first time. The combined results provide strong evidence that the most stable complex is formed between XRCC1 and Polβ. Model DNA intermediates of BER are shown to induce significant rearrangement of the Polβ complexes with XRCC1 and PARP1, while having no detectable influence on the protein–protein binding affinities. The strength of APE1 interaction with Polβ, XRCC1 and PARP1 is revealed to be modulated by BER intermediates to different extents, depending on the type of DNA damage. The affinity of APE1 for Polβ is higher in the complex with abasic site-containing DNA than after the APE1-catalyzed incision. Our findings advance understanding of the molecular mechanisms underlying coordination and regulation of the BER process.

Base excision repair in a glucocorticoid response element: effect of glucocorticoid receptor binding

DNA repair takes place in the context of chromatin. Previous studies showed that histones impair base excision repair (BER) of modified bases at both the excision and synthesis steps. We examined BER of uracil in a glucocorticoid response element (GRE) complexed with the glucocorticoid receptor DNA binding domain (GR-DBD). Five substrates were designed, each containing a unique C→U substitution within the mouse mammary tumor virus promoter, one located within each GRE half-site and the others located outside the GRE. To examine distinct steps of BER, DNA cleavage by uracil-DNA glycosylase and Ape1 endonuclease was used to assess initiation, dCTP incorporation by DNA polymerase (pol) β was used to measure repair synthesis, and DNA ligase I was used to seal the nick. For uracil sites within the GRE, there was a reduced rate of uracil-DNA glycosylase/Ape1 activity following GR-DBD binding. Cleavage in the right half-site, with higher GR-DBD binding affinity, was reduced ∼5-fold, whereas cleavage in the left half-site was reduced ∼3.8-fold. Conversely, uracil-directed cleavage outside the GRE was unaffected by GR-DBD binding. Surprisingly, there was no reduction in the rate of pol β synthesis or DNA ligase activity on any of the fragments bound to GR-DBD. Indeed, we observed a small increase (∼1.5–2.2-fold) in the rate of pol β synthesis at uracil residues in both the GRE and one site six nucleotides downstream. These results highlight the potential for both positive and negative impacts of DNA-transcription factor binding on the rate of BER.

Coordination of steps in single-nucleotide base excision repair mediated by apurinic/apyrimidinic endonuclease 1 and DNA polymerase beta

The individual steps in single-nucleotide base excision repair (SN-BER) are coordinated to enable efficient repair without accumulation of cytotoxic DNA intermediates. The DNA transactions and various proteins involved in SN-BER of abasic sites are well known in mammalian systems. Yet, despite a wealth of information on SN-BER, the mechanism of step-by-step coordination is poorly understood. In this study we conducted experiments toward understanding step-by-step coordination during BER by comparing DNA binding specificities of two major human SN-BER enzymes, apurinic/aprymidinic endonuclease 1 (APE) and DNA polymerase beta (Pol beta). It is known that these enzymes do not form a stable complex in solution. For each enzyme, we found that DNA binding specificity appeared sufficient to explain the sequential processing of BER intermediates. In addition, however, we identified at higher enzyme concentrations a ternary complex of APE.Pol beta.DNA that formed specifically at BER intermediates containing a 5'-deoxyribose phosphate group. Formation of this ternary complex was associated with slightly stronger Pol beta gap-filling and much stronger 5'-deoxyribose phosphate lyase activities than was observed with the Pol beta.DNA binary complex. These results indicate that step-by-step coordination in SN-BER can rely on DNA binding specificity inherent in APE and Pol beta, although coordination also may be facilitated by APE.Pol beta.DNA ternary complex formation with appropriate enzyme expression levels or enzyme recruitment to sites of repair.

Differential recruitment of DNA Ligase I and III to DNA repair sites

DNA ligation is an essential step in DNA replication, repair and recombination. Mammalian cells contain three DNA Ligases that are not interchangeable although they use the same catalytic reaction mechanism. To compare the recruitment of the three eukaryotic DNA Ligases to repair sites in vivo we introduced DNA lesions in human cells by laser microirradiation. Time lapse microscopy of fluorescently tagged proteins showed that DNA Ligase III accumulated at microirradiated sites before DNA Ligase I, whereas we could detect only a faint accumulation of DNA Ligase IV. Recruitment of DNA Ligase I and III to repair sites was cell cycle independent. Mutational analysis and binding studies revealed that DNA Ligase I was recruited to DNA repair sites by interaction with PCNA while DNA Ligase III was recruited via its BRCT domain mediated interaction with XRCC1. Selective recruitment of specialized DNA Ligases may have evolved to accommodate the particular requirements of different repair pathways and may thus enhance efficiency of DNA repair.

Human DNA ligases I, III, and IV-purification and new specific assays for these enzymes

The joining of DNA strand breaks by DNA ligases is required to seal Okazaki fragments during DNA replication and to complete almost all DNA repair pathways. In human cells, there are multiple species of DNA ligase encoded by the LIG1, LIG3, and LIG4 genes. Here we describe protocols to overexpress and purify recombinant DNA ligase I, DNA ligase IIIbeta, and DNA ligase IV/XRCC4 and the assays used to purify and distinguish between these enzymes. In addition, we describe a fluorescence-based ligation assay that can be used for high throughput screening of chemical libraries.

Human DNA ligase I completely encircles and partially unwinds nicked DNA

The end-joining reaction catalysed by DNA ligases is required by all organisms and serves as the ultimate step of DNA replication, repair and recombination processes. One of three well characterized mammalian DNA ligases, DNA ligase I, joins Okazaki fragments during DNA replication. Here we report the crystal structure of human DNA ligase I (residues 233 to 919) in complex with a nicked, 5' adenylated DNA intermediate. The structure shows that the enzyme redirects the path of the double helix to expose the nick termini for the strand-joining reaction. It also reveals a unique feature of mammalian ligases: a DNA-binding domain that allows ligase I to encircle its DNA substrate, stabilizes the DNA in a distorted structure, and positions the catalytic core on the nick. Similarities in the toroidal shape and dimensions of DNA ligase I and the proliferating cell nuclear antigen sliding clamp are suggestive of an extensive protein-protein interface that may coordinate the joining of Okazaki fragments.

A Conserved Interaction between the Replicative Clamp Loader and DNA Ligase in Eukaryotes

The recruitment of DNA ligase I to replication foci and the efficient joining of Okazaki fragments is dependent on the interaction between DNA ligase I and proliferating cell nuclear antigen (PCNA). Although the PCNA sliding clamp tethers DNA ligase I to nicked duplex DNA circles, the interaction does not enhance DNA joining. This suggests that other factors may be involved in the joining of Okazaki fragments. In this study, we describe an association between replication factor C (RFC), the clamp loader, and DNA ligase I in human cell extracts. Subsequently, we demonstrate that there is a direct physical interaction between these proteins that involves both the N- and C-terminal domains of DNA ligase I, the N terminus of the large RFC subunit p140, and the p36 and p38 subunits of RFC. Although RFC inhibited DNA joining by DNA ligase I, the addition of PCNA alleviated inhibition by RFC. Notably, the effect of PCNA on ligation was dependent on the PCNA-binding site of DNA ligase I. Together, these results provide a molecular explanation for the key in vivo role of the DNA ligase I/PCNA interaction and suggest that the joining of Okazaki fragments is coordinated by pairwise interactions among RFC, PCNA, and DNA ligase I.

Exchangeability of mammalian DNA ligases between base excision repair pathways

In mammalian cells, DNA ligase IIIalpha and DNA ligase I participate in the short- and long-patch base excision repair pathways, respectively. Using an in vitro repair assay employing DNA ligase-depleted cell extracts and DNA substrates containing a single lesion repaired either through short-patch (regular abasic site) or long-patch (reduced abasic site) base excision repair pathways, we addressed the question whether DNA ligases are specific to each pathway or if they are exchangeable. We find that immunodepletion of DNA ligase I did not affect the short-patch repair pathway but blocked long-patch repair, suggesting that DNA ligase IIIalpha is not able to substitute DNA ligase I during long-patch repair. In contrast, immunodepletion of DNA ligase IIIalpha did not significantly affect either pathway. Moreover, repair of normal abasic sites in wild-type and X-ray cross-complementing gene 1 (XRCC1)-DNA ligase IIIalpha-immunodepleted cell extracts involved similar proportions of short- and long-patch repair events. This suggests that DNA ligase I was able to efficiently substitute the XRCC1-DNA ligase IIIalpha complex during short-patch repair.

Repair of U/G and U/A in DNA by UNG2-associated repair complexes takes place predominantly by short-patch repair both in proliferating and growth-arrested cells

Nuclear uracil-DNA glycosylase UNG2 has an established role in repair of U/A pairs resulting from misincorporation of dUMP during replication. In antigen-stimulated B-lymphocytes UNG2 removes uracil from U/G mispairs as part of somatic hypermutation and class switch recombination processes. Using antibodies specific for the N-terminal non-catalytic domain of UNG2, we isolated UNG2-associated repair complexes (UNG2-ARC) that carry out short-patch and long-patch base excision repair (BER). These complexes contain proteins required for both types of BER, including UNG2, APE1, POLbeta, POLdelta, XRCC1, PCNA and DNA ligase, the latter detected as activity. Short-patch repair was the predominant mechanism both in extracts and UNG2-ARC from proliferating and less BER-proficient growth-arrested cells. Repair of U/G mispairs and U/A pairs was completely inhibited by neutralizing UNG-antibodies, but whereas added recombinant SMUG1 could partially restore repair of U/G mispairs, it was unable to restore repair of U/A pairs in UNG2-ARC. Neutralizing antibodies to APE1 and POLbeta, and depletion of XRCC1 strongly reduced short-patch BER, and a fraction of long-patch repair was POLbeta dependent. In conclusion, UNG2 is present in preassembled complexes proficient in BER. Furthermore, UNG2 is the major enzyme initiating BER of deaminated cytosine (U/G), and possibly the sole enzyme initiating BER of misincorporated uracil (U/A).

Gene structure, purification and characterization of DNA polymerase beta from Xiphophorus maculatus

Cloning of the Xiphophorus maculatus Polbeta gene and overexpression of the recombinant Polbeta protein has been performed. The organization of the XiphPolbeta introns and exons, including intron-exon boundaries, have been assigned and were found to be similar to that for human Polbeta with identical exon sizes except for exon XII coding for an additional two amino acid residues in Xiphophorus. The cDNA sequence encoding the 337-amino acid X. maculatus DNA polymerase beta (Polbeta) protein was subcloned into the Escherichia coli expression plasmid pET. Induction of transformed E. coli cells resulted in the high-level expression of soluble recombinant Polbeta, which catalyzed DNA synthesis on template-primer substrates. The steady-state Michaelis constants (Km) and catalytic efficiencies (kcat/Km) of the recombinant XiphPolbeta for nucleotide insertion opposite single-nucleotide gap DNA substrates were measured and compared with previously published values for recombinant human Polbeta. Steady-state in vitro Km and kcat/Km values for correct nucleotide insertion by XiphPolbeta and human Polbeta were similar, although the recombinant Xiphophorus protein exhibited 2.5-7-fold higher catalytic efficiencies for dGTP and dCTP insertion versus human Polbeta. In contrast, the recombinant XiphPolbeta displayed significantly lower fidelities than human Polbeta for dNTP insertion opposite a single-nucleotide gap at 37 degrees C.

Escherichia coli uracil- and ethenocytosine-initiated base excision DNA repair: rate-limiting step and patch size distribution

The rate, extent, and DNA synthesis patch size of base excision repair (BER) were measured using Escherichia coli GM31 cell-free extracts and a pGEM (form I) DNA substrate containing a site-specific uracil or ethenocytosine target. The rate of complete BER was stimulated (approximately 3-fold) by adding exogenous E. coli DNA ligase to the cell-free extract, whereas addition of E. coli Ung, Nfo, Fpg, or Pol I did not stimulate BER. Hence, DNA ligation was identified as the rate-limiting step in the E. coli BER pathway. The addition of exogenous DNA polymerase I caused modest inhibition of BER, which was overcome by concomitant addition of DNA ligase. Repair patch size determinations were performed to assess the distribution of DNA synthesis associated with both uracil- and ethenocytosine-initiated BER. During the early phase (0-5 min) of the BER reaction, the large majority of repair events resulted from short patch (1-nucleotide) DNA synthesis. However, during the late phase (>10 min) both short and long (2-20 nucleotide) patches were observed, with long patch BER progressively dominating the repair process. In addition, the patch size distribution was influenced by the ratio of DNA polymerase I to DNA ligase activity in the reaction. A novel mode of BER was identified that involved DNA synthesis tracts of >205 nucleotides in length and termed very-long patch BER. This BER process was dependent upon DNA polymerase I since very-long patch BER was inhibited by DNA polymerase I antibody and addition of excess DNA polymerase I reversed this inhibition.

Long-patch base excision repair of apurinic/apyrimidinic site DNA is decreased in mouse embryonic fibroblast cell lines treated with plumbagin: involvement of cyclin-dependent kinase inhibitor p21Waf-1/Cip-1

Molecular interactions among cell cycle and DNA repair proteins have been described, but the impact of many of these interactions on cell cycle control and DNA repair remains unclear. The cyclin-dependent kinase inhibitor, p21, is known to be involved in DNA damage-induced cell cycle arrest and blocking DNA replication and repair. Participation of p21 has been implicated in nucleotide excision repair. However, the role of p21 in the base excision repair (BER) pathway has not been thoroughly studied. In the present investigation, we treated isogenic mouse embryonic fibroblast (MEF) cell lines containing wild-type (MEF-polbeta) or DNA polymerase beta (polbeta) gene-knockout (MEFpolbetaKO) with oxidative DNA-damaging agent, plumbagin, and examined its effect on p21 levels and BER activity. Plumbagin treatment caused a S-G(2)/M phase arrest and cell death of both MEF cell lines, induced p21 levels, and decreased p21-mediated long-patch (LP) BER by blocking DNA ligase activity in the polbeta-dependent pathway and by blocking both FEN1 and DNA ligase activity in polbeta-independent pathway. These findings suggest that plumbagin induced p21 levels play a regulatory role in cell cycle arrest, apoptosis, and polbeta-dependent and -independent LP-BER pathways in MEF cells.

Direct interaction between mammalian DNA polymerase beta and proliferating cell nuclear antigen

Proliferating cell nuclear antigen (PCNA) plays an essential role in nucleic acid metabolism as a component of the DNA replication and DNA repair machinery. As such, PCNA interacts with many proteins that have a sequence motif termed the PCNA interacting motif (PIM) and also with proteins lacking a PIM. Three regions in human and rat DNA polymerases beta (beta-pol) that resemble the consensus PIM were identified, and we show here that beta-polymerase and PCNA can form a complex both in vitro and in vivo. Immunoprecipitation experiments, yeast two-hybrid analysis, and overlay binding assays were used to examine the interaction between the two proteins. Competition experiments with synthetic PIM-containing peptides suggested the importance of a PIM in the interaction, and studies of a beta-polymerase PIM mutant, H222A/F223A, demonstrated that this alteration blocked the interaction with PCNA. The results indicate that at least one of the PIM-like sequences in beta-polymerase appears to be a functional PIM and was required in the interaction between beta-polymerase and PCNA.

Can DNA repair cause enhanced cell killing following treatment with ionizing radiation?

Production of DNA damage is the basis of cancer treatments, such as chemotherapy and radiotherapy. The limitation of the treatment dose tends to be how well the normal cells within the body can tolerate the therapy. Although it is possible, to some extent, to localize the treatment area during radiotherapy by targeting the beam of ionizing radiation, chemotherapy usually involves a whole body treatment. In order to improve the effectiveness of treatments, it is important to understand how cells repair the DNA damage. This review will attempt to explain how DNA repair, which would be expected to always enhance cell survival, actually may result in increased cell killing following certain types of cancer treatments, such as ionizing radiation and bleomycin sulfate. Work is underway in many laboratories to unravel how the repair systems handle specific types of DNA damage. Such information will pave the way in designing adjuvant therapies that alter a tumor cell's DNA repair capacity and increase tumor cell killing.

Deregulated DNA polymerase beta strengthens ionizing radiation-induced nucleotidic and chromosomal instabilities

DNA polymerase beta (Pol beta) is an error-prone enzyme which has been found to be overexpressed in several human tumors. By using a couple of recombinant CHO cells differing only from the exogenous expression of Pol beta, we showed here that cells overexpressing Pol beta are much more sensitive to IR treatments by increasing apoptosis. We also found that the surviving cells displayed an hypermutator phenotype which could be explained by different pathways involving Pol beta, such as (i) an increased capacity to incorporate into DNA the mutagenic dGTP analog, 8-oxo-dGTP, one of the most abundant purine-derived nucleotides exposed to gamma-irradiation, (ii) the induction of IR-induced DNA breaks and (iii) accumulation of chromosome aberrations induced by radiation. Alteration of Pol beta expression in irradiated cells thus appears to strengthen both cell death and genetic changes associated with a malignant phenotype. These data provide new insights into the cellular response to radiations and the associated carcinogenic consequences.

Completion of base excision repair by mammalian DNA ligases

Three mammalian genes encoding DNA ligases--LIG1, LIG3, and LIG4--have been identified. Genetic, biochemical, and cell biology studies indicate that the products of each of these genes play a unique role in mammalian DNA metabolism. Interestingly, cell lines deficient in either DNA ligase I (46BR.1G1) or DNA ligase III (EM9) are sensitive to simple alkylating agents. One interpretation of these observations is that DNA ligases I and III participate in functionally distinct base excision repair (BER) subpathways. In support of this idea, extracts from both DNA ligase-deficient cell lines are defective in catalyzing BER in vitro and both DNA ligases interact with other BER proteins. DNA ligase I interacts directly with proliferating cell nuclear antigen (PCNA) and DNA polymerase beta (Pol beta), linking this enzyme with both short-patch and long-patch BER. In somatic cells, DNA ligase III alpha forms a stable complex with the DNA repair protein Xrcc1. Although Xrcc1 has no catalytic activity, it also interacts with Pol beta and poly(ADP-ribose) polymerase (PARP), linking DNA ligase III alpha with BER and single-strand break repair, respectively. Biochemical studies suggest that the majority of short-patch base excision repair events are completed by the DNA ligase III alpha/Xrcc1 complex. Although there is compelling evidence for the participation of PARP in the repair of DNA single-strand breaks, the role of PARP in BER has not been established.

Fidelity of uracil-initiated base excision DNA repair in DNA polymerase beta-proficient and -deficient mouse embryonic fibroblast cell extracts

Uracil-initiated base excision DNA repair was conducted using homozygous mouse embryonic fibroblast DNA polymerase beta (+/+) and (-/-) cells to determine the error frequency and mutational specificity associated with the completed repair process. Form I DNA substrates were constructed with site-specific uracil residues at U.A, U.G, and U.T targets contained within the lacZalpha gene of M13mp2 DNA. Efficient repair was observed in both DNA polymerase beta (+/+) and (-/-) cell-free extracts. Repair was largely dependent on uracil-DNA glycosylase activity because addition of the PBS-2 uracil-DNA glycosylase inhibitor (Ugi) protein reduced ( approximately 88%) the initial rate of repair in both types of cell-free extracts. In each case, the DNA repair patch size was primarily distributed between 1 and 8 nucleotides in length with 1 nucleotide repair patch constituting approximately 20% of the repair events. Addition of p21 peptide or protein to DNA polymerase beta (+/+) cell-free extracts increased the frequency of short-patch (1 nucleotide) repair by approximately 2-fold. The base substitution reversion frequency associated with uracil-DNA repair of M13mp2op14 (U.T) DNA was determined to be 5.7-7.2 x 10(-4) when using DNA polymerase beta (+/+) and (-/-) cell-free extracts. In these two cases, the error frequency was very similar, but the mutational spectrum was noticeably different. The presence or absence of Ugi did not dramatically influence either the error rate or mutational specificity. In contrast, the combination of Ugi and p21 protein promoted an increase in the mutation frequency associated with repair of M13mp2 (U.G) DNA. Examination of the mutational spectra generated by a forward mutation assay revealed that errors in DNA repair synthesis occurred predominantly at the position of the U.G target and frequently involved a 1-base deletion or incorporation of dTMP.

Nuclear DNA polymerase beta from Leishmania infantum. Cloning, molecular analysis and developmental regulation

We have identified a novel polymerase beta (Pol beta)-like enzyme from Leishmania infantum, a parasite protozoon causing disease in humans. This protein, named Li Pol beta, shows a nuclear localization that contrasts with the mitochondrial localization of Pol beta from Crithidia fasciculata, a closely related parasite, the only polymerase beta described so far in Trypanosomatidae. Li Pol beta, that belongs to the DNA polymerase X family, displays an evolutionarily conserved Pol beta-type DNA polymerase core, in which most of the key residues involved in DNA binding, nucleotide binding, dRPase and polymerization catalysis are conserved. In agreement with this, Li Pol beta, overproduced in Escherichia coli, displayed intrinsic DNA polymerase activity. Cell synchronization experiments showed a correlation between both Li Pol beta mRNA and protein levels along the parasite cell cycle. Analysis of these parameters at the different growth phases of the parasite, from the proliferative (non-infective) logarithmic phase to the non-dividing (highly infectious) stationary phase, showed high levels of Li Pol beta at the infective phase of the parasite. The data suggest a role of Li Pol beta in base excision repair in L.infantum, a parasite usually affected by oxygen stress environments into the macrophage host cells.

The major human abasic endonuclease: formation, consequences and repair of abasic lesions in DNA

DNA continuously suffers the loss of its constituent bases, and thereby, a loss of potentially vital genetic information. Sites of missing bases--termed abasic or apurinic/apyrimidinic (AP) sites--form spontaneously, through damage-induced hydrolytic base release, or by enzyme-catalyzed removal of modified or mismatched bases during base excision repair (BER). In this review, we discuss the structural and biological consequences of abasic lesions in DNA, as well as the multiple repair pathways for such damage, while emphasizing the mechanistic operation of the multi-functional human abasic endonuclease APE1 (or REF-1) and its potential relationship to disease.

Mammalian DNA single-strand break repair: an X-ra(y)ted affair

The genetic stability of living cells is continuously threatened by the presence of endogenous reactive oxygen species and other genotoxic molecules. Of particular threat are the thousands of DNA single-strand breaks that arise in each cell, each day, both directly from disintegration of damaged sugars and indirectly from the excision repair of damaged bases. If un-repaired, single-strand breaks can be converted into double-strand breaks during DNA replication, potentially resulting in chromosomal rearrangement and genetic deletion. Consequently, cells have adopted multiple pathways to ensure the rapid and efficient removal of single-strand breaks. A general feature of these pathways appears to be the extensive employment of protein-protein interactions to stimulate both the individual component steps and the overall repair reaction. Our current understanding of DNA single-strand break repair is discussed, and testable models for the architectural coordination of this important process are presented.

Complexities of the DNA base excision repair pathway for repair of oxidative DNA damage

Oxidative damage represents the most significant insult to organisms because of continuous production of the reactive oxygen species (ROS) in vivo. Oxidative damage in DNA, a critical target of ROS, is repaired primarily via the base excision repair (BER) pathway which appears to be the simplest among the three excision repair pathways. However, it is now evident that although BER can be carried with four or five enzymes in vitro, a large number of proteins, including some required for nucleotide excision repair (NER), are needed for in vivo repair of oxidative damage. Furthermore, BER in transcribed vs. nontranscribed DNA regions requires distinct sets of proteins, as in the case of NER. We propose an additional complexity in repair of replicating vs. nonreplicating DNA. Unlike DNA bulky adducts, the oxidized base lesions could be incorporated in the nascent DNA strand, repair of which may share components of the mismatch repair process. Distinct enzyme specificities are thus warranted for repair of lesions in the parental vs. nascent DNA strand. Repair synthesis may be carried out by DNA polymerase beta or replicative polymerases delta and epsilon. Thus, multiple subpathways are needed for repairing oxidative DNA damage, and the pathway decision may require coordination of the successive steps in repair. Such coordination includes transfer of the product of a DNA glycosylase to AP-endonuclease, the next enzyme in the pathway. Interactions among proteins in the pathway may also reflect such coordination, characterization of which should help elucidate these subpathways and their in vivo regulation.

Human DNA repair genes

DNA repair systems are essential for the maintenance of genome integrity. Consequently, the disregulation of repair genes can be expected to be associated with significant, detrimental health effects, which can include an increased prevalence of birth defects, an enhancement of cancer risk, and an accelerated rate of aging. Although original insights into DNA repair and the genes responsible were largely derived from studies in bacteria and yeast, well over 125 genes directly involved in DNA repair have now been identified in humans, and their cDNA sequence established. These genes function in a diverse set of pathways that involve the recognition and removal of DNA lesions, tolerance to DNA damage, and protection from errors of incorporation made during DNA replication or DNA repair. Additional genes indirectly affect DNA repair, by regulating the cell cycle, ostensibly to provide an opportunity for repair or to direct the cell to apoptosis. For about 70 of the DNA repair genes listed in Table I, both the genomic DNA sequence and the cDNA sequence and chromosomal location have been elucidated. In 45 cases single-nucleotide polymorphisms have been identified and, in some cases, genetic variants have been associated with specific disorders. With the accelerating rate of gene discovery, the number of identified DNA repair genes and sequence variants is quickly rising. This report tabulates the current status of what is known about these genes. The report is limited to genes whose function is directly related to DNA repair.

Nucleotide excision repair DNA synthesis by excess DNA polymerase beta: a potential source of genetic instability in cancer cells

The nucleotide excision repair pathway contributes to genetic stability by removing a wide range of DNA damage through an error-free reaction. When the lesion is located, the altered strand is incised on both sides of the lesion and a damaged oligonucleotide excised. A repair patch is then synthesized and the repaired strand is ligated. It is assumed that only DNA polymerases delta and/or epsilon participate to the repair DNA synthesis step. Using UV and cisplatin-modified DNA templates, we measured in vitro that extracts from cells overexpressing the error-prone DNA polymerase beta exhibited a five- to sixfold increase of the ultimate DNA synthesis activity compared with control extracts and demonstrated the specific involvement of Pol beta in this step. By using a 28 nt gapped, double-stranded DNA substrate mimicking the product of the incision step, we showed that Pol beta is able to catalyze strand displacement downstream of the gap. We discuss these data within the scope of a hypothesis previously presented proposing that excess error-prone Pol beta in cancer cells could perturb the well-defined specific functions of DNA polymerases during error-free DNA transactions.

DNA ligases in the repair and replication of DNA

DNA ligases are critical enzymes of DNA metabolism. The reaction they catalyse (the joining of nicked DNA) is required in DNA replication and in DNA repair pathways that require the re-synthesis of DNA. Most organisms express DNA ligases powered by ATP, but eubacteria appear to be unique in having ligases driven by NAD(+). Interestingly, despite protein sequence and biochemical differences between the two classes of ligase, the structure of the adenylation domain is remarkably similar. Higher organisms express a variety of different ligases, which appear to be targetted to specific functions. DNA ligase I is required for Okazaki fragment joining and some repair pathways; DNA ligase II appears to be a degradation product of ligase III; DNA ligase III has several isoforms, which are involved in repair and recombination and DNA ligase IV is necessary for V(D)J recombination and non-homologous end-joining. Sequence and structural analysis of DNA ligases has shown that these enzymes are built around a common catalytic core, which is likely to be similar in three-dimensional structure to that of T7-bacteriophage ligase. The differences between the various ligases are likely to be mediated by regions outside of this common core, the structures of which are not known. Therefore, the determination of these structures, along with the structures of ligases bound to substrate DNAs and partner proteins ought to be seen as a priority.

Structural design of a eukaryotic DNA repair polymerase: DNA polymerase beta

DNA polymerase beta, the smallest eukaryotic DNA polymerase, is designed to synthesize DNA in short DNA gaps during DNA repair. It is composed of two specialized domains that contribute essential enzymatic activities to base excision repair (BER). Its amino-terminal domain possesses a lyase activity necessary to remove the 5'-deoxyribose phosphate (dRP) intermediate generated during BER. Removal of the dRP moiety is often the rate-limiting step during BER. Failure to remove this group may initiate alternate BER pathways. The larger polymerase domain has nucleotidyl transferase activity. This domain has a modular organization with sub-domains that bind duplex DNA, catalytic metals, and the correct nucleoside triphosphate in a template-dependent manner. X-ray crystal structures of DNA polymerase beta, with and without bound substrates, has inferred that domain, sub-domain, and substrate conformational changes occur upon ligand binding. Many of these conformational changes are distinct from those observed in structures of other DNA polymerases. This review will examine the structural aspects of DNA polymerase beta that facilitate its role in BER.

Interactions of the DNA ligase IV-XRCC4 complex with DNA ends and the DNA-dependent protein kinase

The DNA-dependent protein kinase (DNA-PK), consisting of Ku and the DNA-PK catalytic subunit (DNA-PKcs), and the DNA ligase IV-XRCC4 complex function together in the repair of DNA double-strand breaks by non-homologous end joining. These protein complexes are also required for the completion of V(D)J recombination events in immune cells. Here we demonstrate that the DNA ligase IV-XRCC4 complex binds specifically to the ends of duplex DNA molecules and can act as a bridging factor, linking together duplex DNA molecules with complementary but non-ligatable ends. Although the DNA end-binding protein Ku inhibited DNA joining by DNA ligase IV-XRCC4, it did not prevent this complex from binding to DNA. Instead, DNA ligase IV-XRCC4 and Ku bound simultaneously to the ends of duplex DNA molecules. DNA ligase IV-XRCC4 and DNA-PKcs also formed complexes at the ends of DNA molecules, but DNA-PKcs did not inhibit ligation. Interestingly, DNA-PKcs stimulated intermolecular ligation by DNA ligase IV-XRCC4. In the presence of DNA-PK, the majority of the joining events catalyzed by DNA ligase IV-XRCC4 were intermolecular because Ku inhibited intramolecular ligation, but DNA-PKcs still stimulated intramolecular ligation. We suggest that DNA-PKcs-containing complexes formed at DNA ends enhance the association of DNA ends via protein-protein interactions, thereby stimulating intermolecular ligation.

Requirement for human AP endonuclease 1 for repair of 3'-blocking damage at DNA single-strand breaks induced by reactive oxygen species

The major mammalian apurinic/apyrimidinic (AP) endonuclease (APE1) plays a central role in the DNA base excision repair pathway (BER) in two distinct ways. As an AP endonuclease, it initiates repair of AP sites in DNA produced either spontaneously or after removal of uracil and alkylated bases in DNA by monofunctional DNA glycosylases. Alternatively, by acting as a 3'-phosphoesterase, it initiates repair of DNA strand breaks with 3'-blocking damage, which are produced either directly by reactive oxygen species (ROS) or indirectly through the AP lyase reaction of damage-specific DNA glycosylases. The endonuclease activity of APE1, however, is much more efficient than its DNA 3'-phosphoesterase activity. Using whole extracts from human HeLa and lymphoblastoid TK6 cells, we have investigated whether these two activities differentially affect BER efficiency. The repair of ROS-induced DNA strand breaks was significantly stimulated by supplementing the reaction with purified APE1. This enhancement was linearly dependent on the amount of APE1 added, while addition of other BER enzymes, such as DNA ligase I and FEN1, had no effect. Moreover, depletion of endogenous APE1 from the extract significantly reduced the repair activity, suggesting that APE1 is essential for repairing such DNA damage and is limiting in extracts of human cells. In contrast, when uracil-containing DNA was used as the substrate, the efficiency of repair was not affected by exogenous APE1, presumably because the AP endonuclease activity was not limiting. These results indicate that the cellular level of APE1 may differentially affect repair efficiency for DNA strand breaks but not for uracil and AP sites in DNA.

Initiation of base excision repair: glycosylase mechanisms and structures

The base excision repair pathway is an organism's primary defense against mutations induced by oxidative, alkylating, and other DNA-damaging agents. This pathway is initiated by DNA glycosylases that excise the damaged base by cleavage of the glycosidic bond between the base and the DNA sugar-phosphate backbone. A subset of glycosylases has an associated apurinic/apyrimidinic (AP) lyase activity that further processes the AP site to generate cleavage of the DNA phosphate backbone. Chemical mechanisms that are supported by biochemical and structural data have been proposed for several glycosylases and glycosylase/AP lyases. This review focuses on the chemical mechanisms of catalysis in the context of recent structural information, with emphasis on the catalytic residues and the active site conformations of several cocrystal structures of glycosylases with their substrate DNAs. Common structural motifs for DNA binding and damage specificity as well as conservation of acidic residues and amino groups for catalysis are discussed.

Characterization of an ATP-dependent DNA ligase from the thermophilic archaeon Methanobacterium thermoautotrophicum

We report the production, purification and characterization of a DNA ligase encoded by the thermophilic archaeon Methanobacterium thermoautotrophicum. The 561 amino acid MTH: ligase catalyzed strand-joining on a singly nicked DNA in the presence of a divalent cation (magnesium, manganese or cobalt) and ATP (K(m) 1.1 microM). dATP can substitute for ATP, but CTP, GTP, UTP and NAD(+) cannot. MTH: ligase activity is thermophilic in vitro, with optimal nick-joining at 60 degrees C. Mutational analysis of the conserved active site motif I (KxDG) illuminated essential roles for Lys251 and Asp253 at different steps of the ligation reaction. Mutant K251A is unable to form the covalent ligase-adenylate intermediate (step 1) and hence cannot seal a 3'-OH/5'-PO(4) nick. Yet, K251A catalyzes phosphodiester bond formation at a pre-adenylated nick (step 3). Mutant D253A is active in ligase-adenylate formation, but defective in activating the nick via formation of the DNA-adenylate intermediate (step 2). D253A is also impaired in phosphodiester bond formation at a pre-adenylated nick. A profound step 3 arrest, with accumulation of high levels of DNA-adenylate, could be elicited for the wild-type MTH: ligase by inclusion of calcium as the divalent cation cofactor. MTH: ligase sediments as a monomer in a glycerol gradient. Structure probing by limited proteolysis suggested that MTH: ligase is a tightly folded protein punctuated by a surface-accessible loop between nucleotidyl transferase motifs III and IIIa.

Domain specific interaction in the XRCC1-DNA polymerase beta complex

XRCC1 (X-ray cross-complementing group 1) is a DNA repair protein that forms complexes with DNA polymerase beta (beta-Pol), DNA ligase III and poly-ADP-ribose polymerase in the repair of DNA single strand breaks. The domains in XRCC1 have been determined, and characterization of the domain-domain interaction in the XRCC1-beta-Pol complex has provided information on the specificity and mechanism of binding. The domain structure of XRCC1, determined using limited proteolysis, was found to include an N-terminal domain (NTD), a central BRCT-I (breast cancer susceptibility protein-1) domain and a C-terminal BRCT-II domain. The BRCT-I-linker-BRCT-II C-terminal fragment and the linker-BRCT-II C-terminal fragment were relatively stable to proteolysis suggestive of a non-random conformation of the linker. A predicted inner domain was found not to be stable to proteolysis. Using cross-linking experiments, XRCC1 was found to bind intact beta-Pol and the beta-Pol 31 kDa domain. The XRCC1-NTD(1-183)(residues 1-183) was found to bind beta-Pol, the beta-Pol 31 kDa domain and the beta-Pol C-terminal palm-thumb (residues 140-335), and the interaction was further localized to XRCC1-NTD(1-157)(residues 1-157). The XRCC1-NTD(1-183)-beta-Pol 31 kDa domain complex was stable at high salt (1 M NaCl) indicative of a hydrophobic contribution. Using a yeast two-hybrid screen, polypeptides expressed from two XRCC1 constructs, which included residues 36-355 and residues 1-159, were found to interact with beta-Pol, the beta-Pol 31 kDa domain, and the beta-Pol C-terminal thumb-only domain polypeptides expressed from the respective beta-Pol constructs. Neither the XRCC1-NTD(1-159), nor the XRCC1(36-355)polypeptide was found to interact with a beta-Pol thumbless polypeptide. A third XRCC1 polypeptide (residues 75-212) showed no interaction with beta-Pol. In quantitative gel filtration and analytical ultracentrifugation experiments, the XRCC1-NTD(1-183)was found to bind beta-Pol and its 31 kDa domain in a 1:1 complex with high affinity (K(d) of 0.4-2.4 microM). The combined results indicate a thumb-domain specific 1:1 interaction between the XRCC1-NTD(1-159)and beta-Pol that is of an affinity comparable to other binding interactions involving beta-Pol.

Mapping the protein-DNA interface and the metal-binding site of the major human apurinic/apyrimidinic endonuclease

Apurinic/apyrimidinic (AP) endonuclease Ape1 is a key enzyme in the mammalian base excision repair pathway that corrects AP sites in the genome. Ape1 cleaves the phosphodiester bond immediately 5' to AP sites through a hydrolytic reaction involving a divalent metal co-factor. Here, site-directed mutagenesis, chemical footprinting techniques, and molecular dynamics simulations were employed to gain insights into how Ape1 interacts with its metal cation and AP DNA. It was found that Ape1 binds predominantly to the minor groove of AP DNA, and that residues R156 and Y128 contribute to protein-DNA complex stability. Furthermore, the Ape1-AP DNA footprint does not change along its reaction pathway upon active-site coordination of Mg(2+) or in the presence of DNA polymerase beta (polbeta), an interactive protein partner in AP site repair. The DNA region immediately 5' to the abasic residue was determined to be in close proximity to the Ape1 metal-binding site. Experimental evidence is provided that amino acid residues E96, D70, and D308 of Ape1 are involved in metal coordination. Molecular dynamics simulations, starting from the active site of the Ape1 crystal structure, suggest that D70 and E96 bind directly to the metal, while D308 coordinates the cation through the first hydration shell. These studies define the Ape1-AP DNA interface, determine the effect of polbeta on the Ape1-DNA interaction, and reveal new insights into the Ape1 active site and overall protein dynamics.

Backbone dynamics and refined solution structure of the N-terminal domain of DNA polymerase beta. Correlation with DNA binding and dRP lyase activity

Mammalian DNA polymerase beta functions in the base excision DNA repair pathway filling in short patches (1-5 nt) in damaged DNA and removing deoxyribose 5'-phosphate from the 5'-side of damaged DNA. The backbone dynamics and the refined solution structure of the N-terminal domain of beta-Pol have been characterized in order to establish the potential contribution(s) of backbone motion to the DNA binding and deoxyribose 5'-phosphate lyase function of this domain. The N-terminal domain is formed from four helices packed as two antiparallel pairs with a 60 degrees crossing between the pairs. The RMSD of the NMR conformers (residues 13-80) is 0.37 A for the backbone heavy atoms and 0.78 A for all heavy atoms. NMR characterization of the binding site(s) for a ssDNA-5mer, ssDNA-8mer, ssDNA-9mer, and dsDNA-12mer shows a consensus surface for the binding of these various DNA oligomers, that surrounds and includes the deoxyribose 5'-phosphate lyase active site region. Connection segments between helices 1 and 2 and between helices 3 and 4 each contribute to DNA binding. Helix-3-turn-helix-4 forms a helix-hairpin-helix motif. The highly conserved hairpin sequence (LPGVG) displays a significant degree of picosecond time-scale motion within the backbone, that is possibly important for DNA binding at the phosphodiester backbone. An Omega-loop connecting helices 1 and 2 and helix-2 itself display significant exchange contributions (R(ex)) at the backbone amides due to apparent conformational type motion on a millisecond time-scale. This motion is likely important in allowing the Omega-loop and helix-2 to shift toward, and productively interact with, gapped DNA. The deoxyribose 5'-phosphate lyase catalytic residues that include K72 which forms the Schiff's base, Y39 which is postulated to promote proton transfer to the aldehyde, and K35 which assists in phosphate elimination, show highly restricted backbone motion. H34, which apparently participates in detection of the abasic site hole and assists in the opening of the hemiacetal, shows conformational exchange.