Human Tdp1 cleaves a broad spectrum of substrates, including phosphoamide linkages

Human tyrosyl-DNA phosphodiesterase (Tdp1) hydrolyzes the phosphodiester bond between a DNA 3' end and a tyrosyl moiety. In eukaryotic cells, this type of linkage is found in stalled topoisomerase I-DNA covalent complexes, and Tdp1 has been implicated in the repair of such complexes in vivo. We confirm here that the Tdp1 catalytic cycle involves a covalent reaction intermediate in which a histidine residue is connected to a DNA 3'-phosphate through a phosphoamide linkage. Most surprisingly, this linkage can be hydrolyzed by Tdp1, and unlike a topoisomerase I-DNA complex, which requires modification to be an efficient substrate for Tdp1, the native form of Tdp1 can be removed from the DNA. The spinocerebellar ataxia with axonal neuropathy neurodegenerative disease is caused by the H493R mutant form of Tdp1, which shows reduced enzymatic activity and accumulates the Tdp1-DNA covalent intermediate. The ability of wild type Tdp1 to remove the stalled mutant protein from the DNA likely explains the recessive nature of spinocerebellar ataxia with axonal neuropathy. In addition to its activity on phosphotyrosine and phosphohistidine substrates, Tdp1 also possesses a limited DNA and RNA 3'-exonuclease activity in which a single nucleoside is removed from the 3'-hydroxyl end of the substrate. Furthermore, Tdp1 also removes a 3' abasic site and an artificial 3'-biotin adduct from the DNA. In combination with earlier data showing that Tdp1 can use 3'-phosphoglycolate as a substrate, these data suggest that Tdp1 may function to remove a variety of 3' adducts from DNA during DNA repair.     

The crystal structure of human tyrosyl-DNA phosphodiesterase, Tdp1

Tyrosyl-DNA phosphodiesterase (Tdp1) catalyzes the hydrolysis of a phosphodiester bond between a tyrosine residue and a DNA 3' phosphate. The enzyme appears to be responsible for repairing the unique protein-DNA linkage that occurs when eukaryotic topoisomerase I becomes stalled on the DNA in the cell. The 1.69 A crystal structure reveals that human Tdp1 is a monomer composed of two similar domains that are related by a pseudo-2-fold axis of symmetry. Each domain contributes conserved histidine, lysine, and asparagine residues to form a single active site. The structure of Tdp1 confirms that the protein has many similarities to the members of the phospholipase D (PLD) superfamily and indicates a similar catalytic mechanism. The structure also suggests how the unusual protein-DNA substrate binds and provides insights about the nature of the substrate in vivo.     

Novel high-throughput electrochemiluminescent assay for identification of human tyrosyl-DNA phosphodiesterase (Tdp1) inhibitors and characterization of furamidine (NSC 305831) as an inhibitor of Tdp1

By enzymatically hydrolyzing the terminal phosphodiester bond at the 3′-ends of DNA breaks, tyrosyl-DNA phosphodiesterase (Tdp1) repairs topoisomerase-DNA covalent complexes and processes the DNA ends for DNA repair. To identify novel Tdp1 inhibitors, we developed a high-throughput assay that uses electrochemiluminescent (ECL) substrates. Subsequent to screening of 1981 compounds from the ‘diversity set’ of the NCI-Developmental Therapeutics Program, here we report that furamidine inhibits Tdp1at low micromolar concentrations. Inhibition of Tdp1 by furamidine is effective both with single- and double-stranded substrates but is slightly stronger with the duplex DNA. Surface plasmon resonance studies show that furamidine binds both single- and double-stranded DNA, though more weakly with the single-stranded substrate DNA. Thus, the inhibition of Tdp1 activity could in part be due to the binding of furamidine to DNA. However, the inhibition of Tdp1 by furamidine is independent of the substrate DNA sequence. The kinetics of Tdp1 inhibition by furamidine was influenced by the drug to enzyme ratio and duration of the reaction. Comparison with related dications shows that furamidine inhibits Tdp1 more effectively than berenil, while pentamidine was inactive. Thus, furamidine represents the most potent Tdp1 inhibitor reported to date.     

A tyrosyl DNA phosphodiesterase 1 from kinetoplastid parasite Leishmania donovani (LdTdp1) capable of removing topo I–DNA covalent complexes

Tyrosyl DNA phosphodiesterase 1 (Tdp1) is a member of phospholipase D superfamily, which cleaves a broad range of 3′‐DNA adducts, the best characterized of which is the phosphodiester bond formed between DNA and topoisomerase IB. This study describes cloning and functional characterization of the enzyme, termed as LdTdp1 in the kinetoplastid parasite Leishmania donovani. Sequence analysis confirmed conservation of the active site motifs typical for all Tdp1 proteins. LdTdp1 activity was detected in the parasite nucleus as well as in the kinetoplast. The enzyme harbours a nuclear localization signal at its C‐terminus. Overexpression of the active enzyme protected the parasites against topoisomerase IB inhibitor camptothecin (CPT) and oxidative agent H₂O₂‐mediated cytotoxicity and its downregulation rendered the parasites hypersensitive to CPT. Trapping of mutant LdTdp1 on DNA takes place following CPT treatment in L. donovani cells. The expression level and associated activity of LdTdp1 were found to be higher in CPT‐resistant L. donovani parasites. Altogether, this is the first report of Tdp1 from the kinetoplastid parasite L. donovani, which actively participates in topoisomerase I‐mediated DNA damage repair process and thereby counteracts the cytotoxic effect of topoisomerase I inhibitors.     

Substrate specificity of tyrosyl-DNA phosphodiesterase I (Tdp1)

Tyrosyl-DNA phosphodiesterase I (Tdp1) hydrolyzes 3'-phosphotyrosyl bonds to generate 3'-phosphate DNA and tyrosine in vitro. Tdp1 is involved in the repair of DNA lesions created by topoisomerase I, although the in vivo substrate is not known. Here we study the kinetic and binding properties of human Tdp1 (hTdp1) to identify appropriate 3'-phosphotyrosyl DNA substrates. Genetic studies argue that Tdp1 is involved in double and single strand break repair pathways; however, x-ray crystal structures suggest that Tdp1 can only bind single strand DNA. Separate kinetic and binding experiments show that hTdp1 has a preference for single-stranded and blunt-ended duplex substrates over nicked and tailed duplex substrate conformations. Based on these results, we present a new model to explain Tdp1/DNA binding properties. These results suggest that Tdp1 only acts upon double strand breaks in vivo, and the roles of Tdp1 in yeast and mammalian cells are discussed.     

The mechanism of human tyrosyl-DNA phosphodiesterase 1 in the cleavage of AP site and its synthetic analogs

The mechanism of hydrolysis of the apurinic/apyrimidinic (AP) site and its synthetic analogs by using tyrosyl-DNA phosphodiesterase 1 (Tdp1) was analyzed. Tdp1 catalyzes the cleavage of AP site and the synthetic analog of the AP site, 3-hydroxy-2(hydroxymethyl)-tetrahydrofuran (THF), in DNA by hydrolysis of the phosphodiester bond between the substituent and 5′ adjacent phosphate. The product of Tdp1 cleavage in the case of the AP site is unstable and is hydrolyzed with the formation of 3′- and 5′-margin phosphates. The following repair demands the ordered action of polynucleotide kinase phosphorylase, with XRCC1, DNA polymerase β, and DNA ligase. In the case of THF, Tdp1 generates break with the 5′-THF and the 3′-phosphate termini. Tdp1 is also able to effectively cleave non-nucleotide insertions in DNA, decanediol and diethyleneglycol moieties by the same mechanism as in the case of THF cleavage. The efficiency of Tdp1 catalyzed hydrolysis of AP-site analog correlates with the DNA helix distortion induced by the substituent. The following repair of 5′-THF and other AP-site analogs can be processed by the long-patch base excision repair pathway.     

Tyrosyl-DNA phosphodiesterase 1 initiates repair of apurinic/apyrimidinic sites

Tyrosyl-DNA phosphodiesterase 1 (Tdp1) catalyzes the hydrolysis of the phosphodiester linkage between the DNA 3' phosphate and a tyrosine residue as well as a variety of other DNA 3' damaged termini. Recently we have shown that Tdp1 can liberate the 3' DNA phosphate termini from apurinic/apyrimidinic (AP) sites. Here, we found that Tdp1 is more active in the cleavage of the AP sites inside bubble-DNA structure in comparison to ssDNA containing AP site. Furthermore, Tdp1 hydrolyzes AP sites opposite to bulky fluorescein adduct faster than AP sites located in dsDNA. Whilst the Tdp1 H493R (SCAN1) and H263A mutants retain the ability to bind an AP site-containing DNA, both mutants do not reveal endonuclease activity, further suggesting the specificity of the AP cleavage activity. We suggest that this Tdp1 activity can contribute to the repair of AP sites particularly in DNA structures containing ssDNA region or AP sites in the context of clustered DNA lesions.     

Tyrosyl-DNA phosphodiesterase 1 (TDP1) repairs DNA damage induced by topoisomerases I and II and base alkylation in vertebrate cells

Tyrosyl-DNA phosphodiesterase 1 (Tdp1) repairs topoisomerase I cleavage complexes (Top1cc) by hydrolyzing their 3'-phosphotyrosyl DNA bonds and repairs bleomycin-induced DNA damage by hydrolyzing 3'-phosphoglycolates. Yeast Tdp1 has also been implicated in the repair of topoisomerase II-DNA cleavage complexes (Top2cc). To determine whether vertebrate Tdp1 is involved in the repair of various DNA end-blocking lesions, we generated Tdp1 knock-out cells in chicken DT40 cells (Tdp1-/-) and Tdp1-complemented DT40 cells with human TDP1. We found that Tdp1-/- cells were not only hypersensitive to camptothecin and bleomycin but also to etoposide, methyl methanesulfonate (MMS), H(2)O(2), and ionizing radiation. We also show they were deficient in mitochondrial Tdp1 activity. In biochemical assays, recombinant human TDP1 was found to process 5'-phosphotyrosyl DNA ends when they mimic the 5'-overhangs of Top2cc. Tdp1 also processes 3'-deoxyribose phosphates generated from hydrolysis of abasic sites, which is consistent with the hypersensitivity of Tdp1-/- cells to MMS and H(2)O(2). Because recent studies established that CtIP together with BRCA1 also repairs topoisomerase-mediated DNA damage, we generated dual Tdp1-CtIP-deficient DT40 cells. Our results show that Tdp1 and CtIP act in parallel pathways for the repair of Top1cc and MMS-induced lesions but are epistatic for Top2cc. Together, our findings reveal a broad involvement of Tdp1 in DNA repair and clarify the role of human TDP1 in the repair of Top2-induced DNA damage.     

TDP2 promotes repair of topoisomerase I-mediated DNA damage in the absence of TDP1

The abortive activity of topoisomerases can result in clastogenic and/or lethal DNA damage in which the topoisomerase is covalently linked to the 3'- or 5'-terminus of a DNA strand break. This type of DNA damage is implicated in chromosome translocations and neurological disease and underlies the clinical efficacy of an important class of anticancer topoisomerase 'poisons'. Tyrosyl DNA phosphodiesterase-1 protects cells from abortive topoisomerase I (Top1) activity by hydrolyzing the 3'-phosphotyrosyl bond that links Top1 to a DNA strand break and is currently the only known human enzyme that displays this activity in cells. Recently, we identified a second tyrosyl DNA phosphodiesterase (TDP2; aka TTRAP/EAPII) that possesses weak 3'-tyrosyl DNA phosphodiesterase (3'-TDP) activity, in vitro. Herein, we have examined whether TDP2 contributes to the repair of Top1-mediated DNA breaks by deleting Tdp1 and Tdp2 separately and together in murine and avian cells. We show that while deletion of Tdp1 in wild-type DT40 cells and mouse embryonic fibroblasts decreases DNA strand break repair rates and cellular survival in response to Top1-induced DNA damage, deletion of Tdp2 does not. However, deletion of both Tdp1 and Tdp2 reduces rates of DNA strand break repair and cell survival below that observed in Tdp1(-)(/)(-) cells, suggesting that Tdp2 contributes to cellular 3'-TDP activity in the absence of Tdp1. Consistent with this idea, over-expression of human TDP2 in Tdp1(-)(/)(-)/Tdp2(-)(/)(-)(/)(-) DT40 cells increases DNA strand break repair rates and cell survival above that observed in Tdp1(-)(/)(-) DT40 cells, suggesting that Tdp2 over-expression can partially complement the defect imposed by loss of Tdp1. Finally, mice lacking both Tdp1 and Tdp2 exhibit greater sensitivity to Top1 poisons than do mice lacking Tdp1 alone, further suggesting that Tdp2 contributes to the repair of Top1-mediated DNA damage in the absence of Tdp1. In contrast, we failed to detect a contribution for Tdp1 to repair Top2-mediated damage. Together, our data suggest that Tdp1 and Tdp2 fulfil overlapping roles following Top1-induced DNA damage, but not following Top2-induced DNA damage, in vivo.     

Effects of DNA and protein size on substrate cleavage by human tyrosyl-DNA phosphodiesterase 1

TDP (tyrosyl-DNA phosphodiesterase) 1 catalyses the hydrolysis of phosphodiester linkages between a DNA 3' phosphate and a tyrosine residue as well as a variety of other DNA 3' substituents, and has been implicated in the repair of covalent complexes involving eukaryotic type IB topoisomerases. To better understand the substrate features that are recognized by TDP1, the size of either the DNA or protein component of the substrate was varied. Competition experiments and gel-shift analyses comparing a series of substrates with DNA lengths increasing from 6 to 28 nt indicated that, contrary to predictions based on the crystal structure of the protein, the apparent affinity for the substrate increased as the DNA length was increased over the entire range tested. It has been found previously that a substrate containing the full-length native form of human topoisomerase I protein is not cleaved by TDP1. Protein-oligonucleotide complexes containing either a 53 or 108 amino acid topoisomerase I-derived peptide were efficiently cleaved by TDP1, but similar to the full-length protein, a substrate containing a 333 amino acid topoisomerase I fragment was resistant to cleavage. Consistent with these results, evidence is presented that processing by the proteasome is required for TDP1 cleavage in vivo.     

Processing of nucleopeptides mimicking the topoisomerase I-DNA covalent complex by tyrosyl-DNA phosphodiesterase

Tyrosyl-DNA phosphodiesterase-1 (Tdp1) is the only known enzyme to remove tyrosine from complexes in which the amino acid is linked to the 3′-end of DNA fragments. Such complexes can be produced following DNA processing by topoisomerase I, and recent studies in yeast have demonstrated the importance of TDP1 for cell survival following topoisomerase I-mediated DNA damage. In the present study, we used synthetic oligodeoxynucleotide–peptide conjugates (nucleopeptides) and recombinant yeast Tdp1 to investigate the molecular determinants for Tdp1 activity. We find that Tdp1 can process nucleopeptides with up to 13 amino acid residues but is poorly active with a 70 kDa fragment of topoisomerase I covalently linked to a suicide DNA substrate. Furthermore, Tdp1 was more effective with nucleopeptides with one to four amino acids than 15 amino acids. Tdp1 was also more effective with nucleopeptides containing 15 nt than with homolog nucleopeptides containing 4 nt. These results suggest that DNA binding contributes to the activity of Tdp1 and that Tdp1 would be most effective after topoisomerase I has been proteolyzed in vivo.     

Spinocerebellar ataxia with axonal neuropathy: consequence of a Tdp1 recessive neomorphic mutation?

Tyrosyl-DNA phosphodiesterase 1 (Tdp1) cleaves the phosphodiester bond between a covalently stalled topoisomerase I (Topo I) and the 3' end of DNA. Stalling of Topo I at DNA strand breaks is induced by endogenous DNA damage and the Topo I-specific anticancer drug camptothecin (CPT). The H493R mutation of Tdp1 causes the neurodegenerative disorder spinocerebellar ataxia with axonal neuropathy (SCAN1). Contrary to the hypothesis that SCAN1 arises from catalytically inactive Tdp1, Tdp1-/- mice are indistinguishable from wild-type mice, physically, histologically, behaviorally, and electrophysiologically. However, compared to wild-type mice, Tdp1-/- mice are hypersensitive to CPT and bleomycin but not to etoposide. Consistent with earlier in vitro studies, we show that the H493R Tdp1 mutant protein retains residual activity and becomes covalently trapped on the DNA after CPT treatment of SCAN1 cells. This result provides a direct demonstration that Tdp1 repairs Topo I covalent lesions in vivo and suggests that SCAN1 arises from the recessive neomorphic mutation H493R. This is a novel mechanism for disease since neomorphic mutations are generally dominant.     

Fluorescence spectroscopy studies of vaccinia type IB DNA topoisomerase. Closing of the enzyme clamp is faster than DNA cleavage.

The prototypic type IB topoisomerase isolated from vaccinia virus cleaves the phosphodiester backbone of duplex DNA at the sequence 5'-(C/T)CCTT, forming a covalent 3'-phosphotyrosyl adduct. A precleavage conformational change in which the enzyme clamps circumferentially around the DNA has been implicated on the basis of structural and biochemical studies. However, no direct measurements to elucidate this key step have been obtained to date. To address this shortcoming we have developed two new fluorescence assays that allow detection of conformational changes in both the enzyme and substrate DNA, and allow determination of the thermodynamic and kinetic mechanism for noncovalent DNA binding and phosphodiester cleavage. The results indicate that clamp closing occurs in a rapid step (>25 s(-1)) that is at least 14-fold faster than the maximal rate of DNA cleavage. Opening of the clamp to release the noncovalently bound substrate is also 5-8-fold more rapid than DNA cleavage. We propose a model in which DNA cleavage and religation are connected through a single high energy transition state involving covalent bond breaking. Alternative models that involve a slow precleavage conformational step are not easily reconciled with the available data.     

Identification of Tyrosyl-DNA Phosphodiesterase as a Novel DNA Damage Repair Enzyme in Arabidopsis

Tyrosyl-DNA phosphodiesterase 1 (Tdp1) is a key enzyme that hydrolyzes the phosphodiester bond between tyrosine of topoisomerase and 3′-phosphate of DNA and repairs topoisomerase-mediated DNA damage during chromosome metabolism. However, functional Tdp1 has only been described in yeast and human to date. In human, mutations of the Tdp1 gene are involved in the disease spinocerebellar ataxia with axonal neuropathy. In plants, we have identified the functional nuclear protein AtTDP, homolog to human Tdp1 from Arabidopsis (Arabidopsis thaliana). The recombinant AtTDP protein certainly hydrolyzes the 3′-phosphotyrosyl DNA substrates related to repairing in vivo topoisomerase I-DNA-induced damage. The loss-of-function AtTDP mutation displays developmental defects and dwarf phenotype in Arabidopsis. This phenotype is substantially caused by decreased cell numbers without any change of individual cell sizes. The tdp plants exhibit hypersensitivities to camptothecin, a potent topoisomerase I inhibitor, and show rigorous cell death in cotyledons and rosette leaves, suggesting the failure of DNA damage repair in tdp mutants. These results indicate that AtTDP plays a clear role in the repair of topoisomerase I-DNA complexes in Arabidopsis.In all living organisms, a variety of DNA damage is constantly caused by replication errors, UV light, ionizing radiation, DNA damage agents, etc. Once DNA damage has occurred, specific DNA repair proteins, such as AP endonuclease, RAD1 (for radiation sensitive), RAD9, RAD51, XRCC2 (for x-ray repair cross-complementing), Ku80 (XRCC6), and ligase, initiate to act through the repair pathways (Wood et al., 2001). Defects in DNA damage repair have evolved into cancer or genetic diseases in mammals and affect productivity or growth in plants (Tuteja et al., 2001; Wood et al., 2001).In the repair of DNA-protein cross-links, tyrosyl-DNA phosphodiesterase 1 (Tdp1) is known as a unique protein. Tdp1 was initially reported as an active enzyme in Saccharomyces cerevisiae that specifically removes the Tyr group from the covalent intermediate between the Tyr residue and the terminal 3′- phosphate of the oligonucleotide (Yang et al., 1996). Subsequently, the yeast TDP1 gene was identified and showed highly conserved sequences with other organisms, such as Caenorhabditis elegans, Drosophila melanogaster, Mus musculus, and Homo sapiens (Pouliot et al., 1999). The Tdp1 homologs of these species are members of the phospholipase D (PLD) superfamily (Pouliot et al., 1999; Interthal et al., 2001). Yeast Tdp1 is mainly studied concerning the topoisomerase I-repair pathway using double or triple mutants. The deletion mutations of yeast Tdp1 were shown lacking in the repair of DNA damage induced by a topoisomerase inhibitor, the anticancer drug camptothecin (CPT; Pouliot et al., 2001; Liu et al., 2002; Vance and Wilson, 2002). Tdp1 has been further implicated in multiple repair pathways, including the damage repair of topoisomerase II-DNA in yeast (Nitiss et al., 2006).In multicellular eukaryotes, the defect of human Tdp1 has resulted in the neurodisorder disease spinocerebellar ataxia with axonal neuropathy (SCAN1; Takashima et al., 2002). SCAN1 is a rare autosomal recessive neurodegenerative disease, and the patients present distal muscle weakness and peripheral neuropathy (Interthal et al., 2001; Takashima et al., 2002). SCAN1 is caused by a missense mutation (His-493Arg) in the Tdp1 catalytic site. As in yeast, the human Tdp1 protein plays a role in the repair of topoisomerase I-DNA complex lesions in SCAN1 cells (El-Khamisy et al., 2005; Miao et al., 2006). SCAN1 cells are hypersensitive to CPT (Interthal et al., 2005; Miao et al., 2006) and accumulate single-strand break and double-strand break DNAs by CPT (El-Khamisy et al., 2005).At present, although the functional analysis of Tdp1 has been widely conducted in yeast and human cell lines, its role in the overall growth and development of higher plants remains unknown. Here, we investigate the function of a novel Arabidopsis (Arabidopsis thaliana) TDP, a human and yeast Tdp1 homolog. The AtTDP protein shows the DNA damage-repairing activity and substrate specificities in biochemical assay. The dwarf phenotype of the Arabidopsis tdp mutant may be due to the reduced cell number caused by the accumulation of DNA damage and progressive cell death during Arabidopsis development.     

The structure of the transition state of the heterodimeric topoisomerase I of Leishmania donovani as a vanadate complex with nicked DNA

Type IB topoisomerases are essential enzymes that are responsible for relaxing superhelical tension in DNA by forming a transient covalent nick in one strand of the DNA duplex. Topoisomerase I is a target for anti-cancer drugs such as camptothecin, and these drugs also target the topoisomerases I in pathogenic trypanosomes including Leishmania species and Trypanosoma brucei. Most eukaryotic enzymes, including human topoisomerase I, are monomeric. However, for Leishmania donovani, the DNA-binding activity and the majority of residues involved in catalysis are located in a large subunit, designated TOP1L, whereas the catalytic tyrosine residue responsible for covalent attachment to DNA is located in a smaller subunit, called TOP1S. Here, we present the 2.27A crystal structure of an active truncated L.donovani TOP1L/TOP1S heterodimer bound to nicked double-stranded DNA captured as a vanadate complex. The vanadate forms covalent linkages between the catalytic tyrosine residue of the small subunit and the nicked ends of the scissile DNA strand, mimicking the previously unseen transition state of the topoisomerase I catalytic cycle. This structure fills a critical gap in the existing ensemble of topoisomerase I structures and provides crucial insights into the catalytic mechanism.     

Design and synthesis of fluorescent substrates for human tyrosyl-DNA phosphodiesterase I

Tyrosyl-DNA phosphodiesterase 1 (Tdp1) is a DNA repair enzyme that acts upon protein–DNA covalent complexes. Tdp1 hydrolyzes 3′-phosphotyrosyl bonds to generate 3′-phosphate DNA and free tyrosine in vitro. Mutations in Tdp1 have been linked to patients with spinocerebellar ataxia, and over-expression of Tdp1 results in resistance to known anti-cancer compounds. Tdp1 has been shown to be involved in double-strand break repair in yeast, and Tdp1 has also been implicated in single-strand break repair in mammalian cells. Despite the biological importance of this enzyme and the possibility that Tdp1 may be a molecular target for new anti-cancer drugs, there are very few assays available for screening inhibitor libraries or for characterizing Tdp1 function, especially under pre-steady-state conditions. Here, we report the design and synthesis of a fluorescence-based assay using oligonucleotide and nucleotide substrates containing 3′-(4-methylumbelliferone)-phosphate. These substrates are efficiently cleaved by Tdp1, generating the fluorescent 4-methylumbelliferone reporter molecule. The kinetic characteristics determined for Tdp1 using this assay are in agreement with the previously published values, and this fluorescence-based assay is validated using the standard gel-based methods. This sensitive assay is ideal for kinetic analysis of Tdp1 function and for high-throughput screening of Tdp1 inhibitory molecules.     

Stability of the topoisomerase II closed clamp conformation may influence DNA-stimulated ATP hydrolysis

Type II DNA topoisomerases catalyze changes in DNA topology and use nucleotide binding and hydrolysis to control conformational changes required for the enzyme reaction. We examined the ATP hydrolysis activity of a bisdioxopiperazine-resistant mutant of human topoisomerase II alpha with phenylalanine substituted for tyrosine at residue 50 in the ATP hydrolysis domain of the enzyme. This substitution reduced the DNA-dependent ATP hydrolysis activity of the mutant protein without affecting the relaxation activity of the enzyme. A similar but stronger effect was seen when the homologous mutation (Tyr28 --> Phe) was introduced in yeast Top2. The ATPase activities of human TOP2alpha(Tyr50 --> Phe) and yeast Top2(Tyr28 --> Phe) were resistant to both bisdioxopiperazines and the ATPase inhibitor sodium orthovanadate. Like bisdioxopiperazines, vanadate traps the enzyme in a salt-stable closed conformation termed the closed clamp, which can be detected in the presence of circular DNA substrates. Consistent with the vanadate-resistant ATPase activity, salt-stable closed clamps were not detected in reactions containing the yeast or human mutant protein, vanadate, and ATP. Similarly, ADP trapped wild-type topoisomerase II as a closed clamp, but could not trap either the human or yeast mutant enzymes. Our results demonstrate that bisdioxopiperazine-resistant mutants exhibit a difference in the stability of the closed clamp formed by the enzyme and that this difference in stability may lead to a loss of DNA-stimulated ATPase. We suggest that the DNA-stimulated ATPase of topoisomerase II is intimately connected with steps that occur while the N-terminal domain of the enzyme is dimerized.     

ATP‐dependent DNA binding,unwinding, and resection by the Mre11/Rad50 complex

ATP‐dependent DNA end recognition and nucleolytic processing are central functions of the Mre11/Rad50 (MR) complex in DNA double‐strand break repair. However, it is still unclear how ATP binding and hydrolysis primes the MR function and regulates repair pathway choice in cells. Here, Methanococcus jannaschii MR‐ATPγS‐DNA structure reveals that the partly deformed DNA runs symmetrically across central groove between two ATPγS‐bound Rad50 nucleotide‐binding domains. Duplex DNA cannot access the Mre11 active site in the ATP‐free full‐length MR complex. ATP hydrolysis drives rotation of the nucleotide‐binding domain and induces the DNA melting so that the substrate DNA can access Mre11. Our findings suggest that the ATP hydrolysis‐driven conformational changes in both DNA and the MR complex coordinate the melting and endonuclease activity.