1
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Copp W, Karimi A, Yang T, Guarné A, Luedtke NW. Fluorescent molecular rotors detect O6-methylguanine dynamics and repair in duplex DNA. Chem Commun (Camb) 2024; 60:1156-1159. [PMID: 38190113 DOI: 10.1039/d3cc04782b] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2024]
Abstract
Alkylation at the O6 position of guanine is a common and highly mutagenic form of DNA damage. Direct repair of O6-alkylguanines by the "suicide" enzyme O6-methylguanine DNA methyltransferase (MGMT, AGT, AGAT) maintains genome stability and inhibits carcinogenesis. In this study, a fluorescent analogue of thymidine containing trans-stilbene (tsT) is quenched by O6-methylguanine residues in the opposite strand of DNA by molecular dynamics that propagate through the duplex with as much as ∼9 Å of separation. Increased fluorescence of tsT or the cytosine analogue tsC resulting from MGMT-mediated DNA repair were distinguishable from non-covalent DNA-protein binding following protease digest. To our knowledge, this is the first study utilizing molecular rotor base analogues to detect DNA damage and repair activities in duplex DNA.
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Affiliation(s)
- William Copp
- Department of Chemistry, McGill University, H3A-0B8 Montreal, Canada
- Centre de Recherche en Biologie Structural, McGill University, H3G 0B1 Montreal, Canada
| | - Ashkan Karimi
- Department of Chemistry, McGill University, H3A-0B8 Montreal, Canada
- Centre de Recherche en Biologie Structural, McGill University, H3G 0B1 Montreal, Canada
| | - Tianxiao Yang
- Centre de Recherche en Biologie Structural, McGill University, H3G 0B1 Montreal, Canada
- Department of Biochemistry, McGill University, H3G 1Y6 Montreal, Canada
| | - Alba Guarné
- Centre de Recherche en Biologie Structural, McGill University, H3G 0B1 Montreal, Canada
- Department of Biochemistry, McGill University, H3G 1Y6 Montreal, Canada
| | - Nathan W Luedtke
- Department of Chemistry, McGill University, H3A-0B8 Montreal, Canada
- Centre de Recherche en Biologie Structural, McGill University, H3G 0B1 Montreal, Canada
- Department of Pharmacology and Therapeutics, McGill University, H3A-1A3 Montreal, Canada
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2
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Tessmer I, Margison GP. The DNA Alkyltransferase Family of DNA Repair Proteins: Common Mechanisms, Diverse Functions. Int J Mol Sci 2023; 25:463. [PMID: 38203633 PMCID: PMC10779285 DOI: 10.3390/ijms25010463] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/30/2023] [Revised: 12/22/2023] [Accepted: 12/27/2023] [Indexed: 01/12/2024] Open
Abstract
DNA alkyltransferase and alkyltransferase-like family proteins are responsible for the repair of highly mutagenic and cytotoxic O6-alkylguanine and O4-alkylthymine bases in DNA. Their mechanism involves binding to the damaged DNA and flipping the base out of the DNA helix into the active site pocket in the protein. Alkyltransferases then directly and irreversibly transfer the alkyl group from the base to the active site cysteine residue. In contrast, alkyltransferase-like proteins recruit nucleotide excision repair components for O6-alkylguanine elimination. One or more of these proteins are found in all kingdoms of life, and where this has been determined, their overall DNA repair mechanism is strictly conserved between organisms. Nevertheless, between species, subtle as well as more extensive differences that affect target lesion preferences and/or introduce additional protein functions have evolved. Examining these differences and their functional consequences is intricately entwined with understanding the details of their DNA repair mechanism(s) and their biological roles. In this review, we will present and discuss various aspects of the current status of knowledge on this intriguing protein family.
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Affiliation(s)
- Ingrid Tessmer
- Rudolf Virchow Center, University of Würzburg, Josef-Schneider-Strasse 2, 97080 Würzburg, Germany
| | - Geoffrey P. Margison
- School of Health Sciences, Faculty of Biology, Medicine and Health, University of Manchester, Manchester M13 9PL, UK;
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3
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Merlo R, Mattossovich R, Genta M, Valenti A, Di Mauro G, Minassi A, Miggiano R, Perugino G. First thermostable CLIP-tag by rational design applied to an archaeal O-alkyl-guanine-DNA-alkyl-transferase. Comput Struct Biotechnol J 2022; 20:5275-5286. [PMID: 36212535 PMCID: PMC9519396 DOI: 10.1016/j.csbj.2022.09.015] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/25/2022] [Revised: 09/08/2022] [Accepted: 09/08/2022] [Indexed: 11/30/2022] Open
Abstract
Self-labelling protein tags (SLPs) are resourceful tools that revolutionized sensor imaging, having the versatile ability of being genetically fused with any protein of interest and undergoing activation with alternative probes specifically designed for each variant (namely, SNAP-tag, CLIP-tag and Halo-tag). Commercially available SLPs are highly useful in studying molecular aspects of mesophilic organisms, while they fail in characterizing model organisms that thrive in harsh conditions. By applying an integrated computational and structural approach, we designed a engineered variant of the alkylguanine-DNA-alkyl-transferase (OGT) from the hyper-thermophilic archaeon Saccharolobus solfataricus (SsOGT), with no DNA-binding activity, able to covalently react with O6-benzyl-cytosine (BC-) derivatives, obtaining the first thermostable CLIP-tag, named SsOGT-MC8. The presented construct is able to recognize and to covalently bind BC- substrates with a marked specificity, displaying a very low activity on orthogonal benzyl-guanine (BG-) substrate and showing a remarkable thermal stability that broadens the applicability of SLPs. The rational mutagenesis that, starting from SsOGT, led to the production of SsOGT-MC8 was first evaluated by structural predictions to precisely design the chimeric construct, by mutating specific residues involved in protein stability and substrate recognition. The final construct was further validated by biochemical characterization and X-ray crystallography, allowing us to present here the first structural model of a CLIP-tag establishing the molecular determinants of its activity, as well as proposing a general approach for the rational engineering of any O6-alkylguanine-DNA-alkyl-transferase turning it into a SNAP- and a CLIP-tag variant.
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4
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Farag N, Mattossovich R, Merlo R, Nierzwicki Ł, Palermo G, Porchetta A, Perugino G, Ricci F. Folding-upon-Repair DNA Nanoswitches for Monitoring the Activity of DNA Repair Enzymes. Angew Chem Int Ed Engl 2021; 60:7283-7289. [PMID: 33415794 PMCID: PMC8783695 DOI: 10.1002/anie.202016223] [Citation(s) in RCA: 23] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/07/2020] [Indexed: 09/28/2023]
Abstract
We present a new class of DNA-based nanoswitches that, upon enzymatic repair, could undergo a conformational change mechanism leading to a change in fluorescent signal. Such folding-upon-repair DNA nanoswitches are synthetic DNA sequences containing O6 -methyl-guanine (O6 -MeG) nucleobases and labelled with a fluorophore/quencher optical pair. The nanoswitches are rationally designed so that only upon enzymatic demethylation of the O6 -MeG nucleobases they can form stable intramolecular Hoogsteen interactions and fold into an optically active triplex DNA structure. We have first characterized the folding mechanism induced by the enzymatic repair activity through fluorescent experiments and Molecular Dynamics simulations. We then demonstrated that the folding-upon-repair DNA nanoswitches are suitable and specific substrates for different methyltransferase enzymes including the human homologue (hMGMT) and they allow the screening of novel potential methyltransferase inhibitors.
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Affiliation(s)
- Nada Farag
- Department of Chemistry, University of Rome Tor Vergata, Via della Ricerca Scientifica, 00133, Rome, Italy
| | - Rosanna Mattossovich
- Institute of Biosciences and BioResources, National Research Council of Italy, Via Pietro Castellino 111, 80131, Naples, Italy
| | - Rosa Merlo
- Institute of Biosciences and BioResources, National Research Council of Italy, Via Pietro Castellino 111, 80131, Naples, Italy
| | - Łukasz Nierzwicki
- Department of Bioengineering, University of California Riverside, 900 University Avenue, Riverside, CA, 52512, USA
| | - Giulia Palermo
- Department of Bioengineering, University of California Riverside, 900 University Avenue, Riverside, CA, 52512, USA
- Department of Chemistry, University of California Riverside, 900 University Avenue, Riverside, CA, 52512, USA
| | - Alessandro Porchetta
- Department of Chemistry, University of Rome Tor Vergata, Via della Ricerca Scientifica, 00133, Rome, Italy
| | - Giuseppe Perugino
- Institute of Biosciences and BioResources, National Research Council of Italy, Via Pietro Castellino 111, 80131, Naples, Italy
| | - Francesco Ricci
- Department of Chemistry, University of Rome Tor Vergata, Via della Ricerca Scientifica, 00133, Rome, Italy
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5
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Farag N, Mattossovich R, Merlo R, Nierzwicki Ł, Palermo G, Porchetta A, Perugino G, Ricci F. Folding‐upon‐Repair DNA Nanoswitches for Monitoring the Activity of DNA Repair Enzymes. Angew Chem Int Ed Engl 2021. [DOI: 10.1002/ange.202016223] [Citation(s) in RCA: 7] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Affiliation(s)
- Nada Farag
- Department of Chemistry University of Rome Tor Vergata Via della Ricerca Scientifica 00133 Rome Italy
| | - Rosanna Mattossovich
- Institute of Biosciences and BioResources National Research Council of Italy Via Pietro Castellino 111 80131 Naples Italy
| | - Rosa Merlo
- Institute of Biosciences and BioResources National Research Council of Italy Via Pietro Castellino 111 80131 Naples Italy
| | - Łukasz Nierzwicki
- Department of Bioengineering University of California Riverside 900 University Avenue Riverside CA 52512 USA
| | - Giulia Palermo
- Department of Bioengineering University of California Riverside 900 University Avenue Riverside CA 52512 USA
- Department of Chemistry University of California Riverside 900 University Avenue Riverside CA 52512 USA
| | - Alessandro Porchetta
- Department of Chemistry University of Rome Tor Vergata Via della Ricerca Scientifica 00133 Rome Italy
| | - Giuseppe Perugino
- Institute of Biosciences and BioResources National Research Council of Italy Via Pietro Castellino 111 80131 Naples Italy
| | - Francesco Ricci
- Department of Chemistry University of Rome Tor Vergata Via della Ricerca Scientifica 00133 Rome Italy
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6
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Mattossovich R, Merlo R, Miggiano R, Valenti A, Perugino G. O6-alkylguanine-DNA Alkyltransferases in Microbes Living on the Edge: From Stability to Applicability. Int J Mol Sci 2020; 21:E2878. [PMID: 32326075 PMCID: PMC7216122 DOI: 10.3390/ijms21082878] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/26/2020] [Revised: 04/09/2020] [Accepted: 04/16/2020] [Indexed: 02/06/2023] Open
Abstract
The genome of living cells is continuously exposed to endogenous and exogenous attacks, and this is particularly amplified at high temperatures. Alkylating agents cause DNA damage, leading to mutations and cell death; for this reason, they also play a central role in chemotherapy treatments. A class of enzymes known as AGTs (alkylguanine-DNA-alkyltransferases) protects the DNA from mutations caused by alkylating agents, in particular in the recognition and repair of alkylated guanines in O6-position. The peculiar irreversible self-alkylation reaction of these enzymes triggered numerous studies, especially on the human homologue, in order to identify effective inhibitors in the fight against cancer. In modern biotechnology, engineered variants of AGTs are developed to be used as protein tags for the attachment of chemical ligands. In the last decade, research on AGTs from (hyper)thermophilic sources proved useful as a model system to clarify numerous phenomena, also common for mesophilic enzymes. This review traces recent progress in this class of thermozymes, emphasizing their usefulness in basic research and their consequent advantages for in vivo and in vitro biotechnological applications.
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Affiliation(s)
- Rosanna Mattossovich
- Institute of Bioscience and BioResources, National Research Council of Italy, Via Pietro Castellino 111, 80131 Naples, Italy; (R.M.); (R.M.)
| | - Rosa Merlo
- Institute of Bioscience and BioResources, National Research Council of Italy, Via Pietro Castellino 111, 80131 Naples, Italy; (R.M.); (R.M.)
| | - Riccardo Miggiano
- Department of Pharmaceutical Sciences, University of Piemonte Orientale, Via Bovio 6, 28100 Novara, Italy;
| | - Anna Valenti
- Institute of Bioscience and BioResources, National Research Council of Italy, Via Pietro Castellino 111, 80131 Naples, Italy; (R.M.); (R.M.)
| | - Giuseppe Perugino
- Institute of Bioscience and BioResources, National Research Council of Italy, Via Pietro Castellino 111, 80131 Naples, Italy; (R.M.); (R.M.)
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7
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Copp W, O'Flaherty DK, Wilds CJ. Covalent capture of OGT's active site using engineered human-E. coli chimera and intrastrand DNA cross-links. Org Biomol Chem 2018; 16:9053-9058. [PMID: 30430154 DOI: 10.1039/c8ob02453g] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
O 6-Alkylguanine DNA alkyltransferases (AGTs) are proteins found in most organisms whose role is to remove alkylation damage from the O6- and O4-positions of 2'-deoxyguanosine (dG) and thymidine (dT), respectively. Variations in active site residues between AGTs from different organisms leads to differences in repair proficiency: The human variant (hAGT) has a proclivity for removal of alkyl groups at the O6-position of guanine and the E. coli OGT protein has activity towards the O4-position of thymine. A chimeric protein (hOGT) that our laboratory has engineered with twenty of the active site residues mutated in hAGT to those found in OGT, exhibited activity towards a broader range of substrates relative to native OGT. Among the substrates that the hOGT protein was found to act upon was interstrand cross-linked DNA connected by an alkylene linkage at the O6-position of dG to the complementary strand. In the present study the activity of hOGT towards DNA containing alkylene intrastrand cross-links (IaCL) at the O6- and O4-positions respectively of dG and dT, which lack a phosphodiester linkage between the connected residues, was evaluated. The hOGT protein exhibited proficiency at removal of an alkylene linkage at the O6-atom of dG but the O4-position of dT was refractory to protein activity. The activity of the chimeric hOGT protein towards these IaCLs to prepare well defined DNA-protein cross-linked conjugates will enable mechanistic and high resolution structural studies to address the differences observed in the repair adeptness of O4-alkylated dT by the OGT protein relative to other AGT variants.
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Affiliation(s)
- William Copp
- Department of Chemistry and Biochemistry, Concordia University, Montréal, Québec H4B1R6, Canada.
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8
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Sacre L, O'Flaherty DK, Archambault P, Copp W, Peslherbe GH, Muchall HM, Wilds CJ. O 4 -Alkylated-2-Deoxyuridine Repair by O 6 -Alkylguanine DNA Alkyltransferase is Augmented by a C5-Fluorine Modification. Chembiochem 2018; 19:575-582. [PMID: 29243336 DOI: 10.1002/cbic.201700660] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/14/2017] [Indexed: 11/10/2022]
Abstract
Oligonucleotides containing various adducts, including ethyl, benzyl, 4-hydroxybutyl and 7-hydroxyheptyl groups, at the O4 atom of 5-fluoro-O4 -alkyl-2'-deoxyuridine were prepared by solid-phase synthesis. UV thermal denaturation studies demonstrated that these modifications destabilised the duplex by approximately 10 °C, relative to the control containing 5-fluoro-2'-deoxyuridine. Circular dichroism spectroscopy revealed that these modified duplexes all adopted a B-form DNA structure. O6 -Alkylguanine DNA alkyltransferase (AGT) from humans (hAGT) was most efficient at repair of the 5-fluoro-O4 -benzyl-2'-deoxyuridine adduct, whereas the thymidine analogue was refractory to repair. The Escherichia coli AGT variant (OGT) was also efficient at removing O4 -ethyl and benzyl adducts of 5-fluoro-2-deoxyuridine. Computational assessment of N1-methyl analogues of the O4 -alkylated nucleobases revealed that the C5-fluorine modification had an influence on reducing the electron density of the O4 -Cα bond, relative to thymine (C5-methyl) and uracil (C5-hydrogen). These results reveal the positive influence of the C5-fluorine atom on the repair of larger O4 -alkyl adducts to expand knowledge of the range of substrates able to be repaired by AGT.
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Affiliation(s)
- Lauralicia Sacre
- Department of Chemistry and Biochemistry and, Centre for Research in Molecular Modeling (CERMM), Concordia University, 7141 Sherbrooke St. West, Montréal, Québec, H4B 1R6, Canada
| | - Derek K O'Flaherty
- Department of Chemistry and Biochemistry and, Centre for Research in Molecular Modeling (CERMM), Concordia University, 7141 Sherbrooke St. West, Montréal, Québec, H4B 1R6, Canada.,Present address: Howard Hughes Medical Institute, Department of Molecular Biology and, Center for Computational and Integrative Biology, Massachusetts General Hospital, 185 Cambridge Street, Boston, MA, 02114, USA
| | - Philippe Archambault
- Department of Chemistry and Biochemistry and, Centre for Research in Molecular Modeling (CERMM), Concordia University, 7141 Sherbrooke St. West, Montréal, Québec, H4B 1R6, Canada
| | - William Copp
- Department of Chemistry and Biochemistry and, Centre for Research in Molecular Modeling (CERMM), Concordia University, 7141 Sherbrooke St. West, Montréal, Québec, H4B 1R6, Canada
| | - Gilles H Peslherbe
- Department of Chemistry and Biochemistry and, Centre for Research in Molecular Modeling (CERMM), Concordia University, 7141 Sherbrooke St. West, Montréal, Québec, H4B 1R6, Canada
| | - Heidi M Muchall
- Department of Chemistry and Biochemistry and, Centre for Research in Molecular Modeling (CERMM), Concordia University, 7141 Sherbrooke St. West, Montréal, Québec, H4B 1R6, Canada
| | - Christopher J Wilds
- Department of Chemistry and Biochemistry and, Centre for Research in Molecular Modeling (CERMM), Concordia University, 7141 Sherbrooke St. West, Montréal, Québec, H4B 1R6, Canada
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9
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Miggiano R, Valenti A, Rossi F, Rizzi M, Perugino G, Ciaramella M. Every OGT Is Illuminated … by Fluorescent and Synchrotron Lights. Int J Mol Sci 2017; 18:ijms18122613. [PMID: 29206193 PMCID: PMC5751216 DOI: 10.3390/ijms18122613] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/15/2017] [Revised: 11/28/2017] [Accepted: 11/30/2017] [Indexed: 12/23/2022] Open
Abstract
O6-DNA-alkyl-guanine-DNA-alkyl-transferases (OGTs) are evolutionarily conserved, unique proteins that repair alkylation lesions in DNA in a single step reaction. Alkylating agents are environmental pollutants as well as by-products of cellular reactions, but are also very effective chemotherapeutic drugs. OGTs are major players in counteracting the effects of such agents, thus their action in turn affects genome integrity, survival of organisms under challenging conditions and response to chemotherapy. Numerous studies on OGTs from eukaryotes, bacteria and archaea have been reported, highlighting amazing features that make OGTs unique proteins in their reaction mechanism as well as post-reaction fate. This review reports recent functional and structural data on two prokaryotic OGTs, from the pathogenic bacterium Mycobacterium tuberculosis and the hyperthermophilic archaeon Sulfolobus solfataricus, respectively. These studies provided insight in the role of OGTs in the biology of these microorganisms, but also important hints useful to understand the general properties of this class of proteins.
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Affiliation(s)
- Riccardo Miggiano
- DSF-Dipartimento di Scienze del Farmaco, University of Piemonte Orientale, Via Bovio 6, 28100 Novara, Italy.
| | - Anna Valenti
- Institute of Biosciences and BioResources, National Research Council of Italy, Via Pietro Castellino 111, 80131 Naples, Italy.
| | - Franca Rossi
- DSF-Dipartimento di Scienze del Farmaco, University of Piemonte Orientale, Via Bovio 6, 28100 Novara, Italy.
| | - Menico Rizzi
- DSF-Dipartimento di Scienze del Farmaco, University of Piemonte Orientale, Via Bovio 6, 28100 Novara, Italy.
| | - Giuseppe Perugino
- Institute of Biosciences and BioResources, National Research Council of Italy, Via Pietro Castellino 111, 80131 Naples, Italy.
| | - Maria Ciaramella
- Institute of Biosciences and BioResources, National Research Council of Italy, Via Pietro Castellino 111, 80131 Naples, Italy.
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10
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Denisov AY, McManus FP, O'Flaherty DK, Noronha AM, Wilds CJ. Structural basis of interstrand cross-link repair by O6-alkylguanine DNA alkyltransferase. Org Biomol Chem 2017; 15:8361-8370. [DOI: 10.1039/c7ob02093g] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
Abstract
Conformation of the alkylene lesion may play a role in interstrand cross-link repair by O6-alkylguanine DNA alkyltransferases.
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Affiliation(s)
- Alexey Y. Denisov
- Department of Chemistry and Biochemistry
- Concordia University
- Montréal
- Canada
| | - Francis P. McManus
- Department of Chemistry and Biochemistry
- Concordia University
- Montréal
- Canada
| | | | - Anne M. Noronha
- Department of Chemistry and Biochemistry
- Concordia University
- Montréal
- Canada
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11
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McManus FP, Wilds CJ. O(6) -alkylguanine-DNA alkyltransferase-mediated repair of O(4) -alkylated 2'-deoxyuridines. Chembiochem 2014; 15:1966-77. [PMID: 25087488 DOI: 10.1002/cbic.201402169] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/16/2014] [Indexed: 11/06/2022]
Abstract
O(6) -Alkylguanine-DNA alkyltransferases (AGTs) are responsible for the removal of O(6) -alkyl 2'-deoxyguanosine (dG) and O(4) -alkyl thymidine (dT) adducts from the genome. Unlike the E. coli OGT (O(6) -alkylguanine-DNA-alkyltransferase) protein, which can repair a range of O(4) -alkyl dT lesions, human AGT (hAGT) only removes methyl groups poorly. To uncover the influence of the C5 methyl group of dT on AGT repair, oligonucleotides containing O(4) -alkyl 2'-deoxyuridines (dU) were prepared. The ability of E. coli AGTs (Ada-C and OGT), human AGT, and an OGT/hAGT chimera to remove O(4) -methyl and larger adducts (4-hydroxybutyl and 7-hydroxyheptyl) from dU were examined and compared to those relating to the corresponding dT species. The absence of the C5 methyl group resulted in an increase in repair observed for the O(4) -methyl adducts by hAGT and the chimera. The chimera was proficient at repairing larger adducts at the O(4) atom of dU. There was no observed correlation between the binding affinities of the AGT homologues to adduct-containing oligonucleotides and the amounts of repair measured.
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Affiliation(s)
- Francis P McManus
- Department of Chemistry and Biochemistry, Concordia University, Montreal, Quebec, H4B 1R6 (Canada)
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12
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Insight into the cooperative DNA binding of the O⁶-alkylguanine DNA alkyltransferase. DNA Repair (Amst) 2014; 20:14-22. [PMID: 24553127 DOI: 10.1016/j.dnarep.2014.01.006] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/01/2013] [Accepted: 01/18/2014] [Indexed: 11/22/2022]
Abstract
The O(6)-alkylguanine DNA alkyltransferase (AGT) is a highly conserved protein responsible for direct repair of alkylated guanine and to a lesser degree thymine bases. While specific DNA lesion-bound complexes in crystal structures consist of monomeric AGT, several solution studies have suggested that cooperative DNA binding plays a role in the physiological activities of AGT. Cooperative AGT-DNA complexes have been described by theoretical models, which can be tested by atomic force microscopy (AFM). Direct access to structural features of AGT-DNA complexes at the single molecule level by AFM imaging revealed non-specifically bound, cooperative complexes with limited cluster length. Implications of cooperative binding in AGT-DNA interactions are discussed.
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13
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Senthong P, Millington CL, Wilkinson OJ, Marriott AS, Watson AJ, Reamtong O, Eyers CE, Williams DM, Margison GP, Povey AC. The nitrosated bile acid DNA lesion O6-carboxymethylguanine is a substrate for the human DNA repair protein O6-methylguanine-DNA methyltransferase. Nucleic Acids Res 2013; 41:3047-55. [PMID: 23335782 PMCID: PMC3597670 DOI: 10.1093/nar/gks1476] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/01/2022] Open
Abstract
The consumption of red meat is a risk factor in human colorectal cancer (CRC). One hypothesis is that red meat facilitates the nitrosation of bile acid conjugates and amino acids, which rapidly convert to DNA-damaging carcinogens. Indeed, the toxic and mutagenic DNA adduct O6-carboxymethylguanine (O6-CMG) is frequently present in human DNA, increases in abundance in people with high levels of dietary red meat and may therefore be a causative factor in CRC. Previous reports suggested that O6-CMG is not a substrate for the human version of the DNA damage reversal protein O6-methylguanine-DNA methyltransferase (MGMT), which protects against the genotoxic effects of other O6-alkylguanine lesions by removing alkyl groups from the O6-position. We now show that synthetic oligodeoxyribonucleotides containing the known MGMT substrate O6-methylguanine (O6-MeG) or O6-CMG effectively inactivate MGMT in vitro (IC50 0.93 and 1.8 nM, respectively). Inactivation involves the removal of the O6-alkyl group and its transfer to the active-site cysteine residue of MGMT. O6-CMG is therefore an MGMT substrate, and hence MGMT is likely to be a protective factor in CRC under conditions where O6-CMG is a potential causative agent.
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Affiliation(s)
- Pattama Senthong
- Centre for Occupational and Environmental Health, Faculty of Medical & Human Sciences, University of Manchester, Manchester M13 9PL, UK
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14
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Alkyltransferase-like protein (Atl1) distinguishes alkylated guanines for DNA repair using cation-π interactions. Proc Natl Acad Sci U S A 2012; 109:18755-60. [PMID: 23112169 DOI: 10.1073/pnas.1209451109] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
Alkyltransferase-like (ATL) proteins in Schizosaccharomyces pombe (Atl1) and Thermus thermophilus (TTHA1564) protect against the adverse effects of DNA alkylation damage by flagging O(6)-alkylguanine lesions for nucleotide excision repair (NER). We show that both ATL proteins bind with high affinity to oligodeoxyribonucleotides containing O(6)-alkylguanines differing in size, polarity, and charge of the alkyl group. However, Atl1 shows a greater ability than TTHA1564 to distinguish between O(6)-alkylguanine and guanine and in an unprecedented mechanism uses Arg69 to probe the electrostatic potential surface of O(6)-alkylguanine, as determined using molecular mechanics calculations. An unexpected consequence of this feature is the recognition of 2,6-diaminopurine and 2-aminopurine, as confirmed in crystal structures of respective Atl1-DNA complexes. O(6)-Alkylguanine and guanine discrimination is diminished for Atl1 R69A and R69F mutants, and S. pombe R69A and R69F mutants are more sensitive toward alkylating agent toxicity, revealing the key role of Arg69 in identifying O(6)-alkylguanines critical for NER recognition.
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Pegg AE. Multifaceted roles of alkyltransferase and related proteins in DNA repair, DNA damage, resistance to chemotherapy, and research tools. Chem Res Toxicol 2011; 24:618-39. [PMID: 21466232 DOI: 10.1021/tx200031q] [Citation(s) in RCA: 155] [Impact Index Per Article: 11.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
Abstract
O(6)-Alkylguanine-DNA alkyltransferase (AGT) is a widely distributed, unique DNA repair protein that acts as a single agent to directly remove alkyl groups located on the O(6)-position of guanine from DNA restoring the DNA in one step. The protein acts only once, and its alkylated form is degraded rapidly. It is a major factor in counteracting the mutagenic, carcinogenic, and cytotoxic effects of agents that form such adducts including N-nitroso-compounds and a number of cancer chemotherapeutics. This review describes the structure, function, and mechanism of action of AGTs and of a family of related alkyltransferase-like proteins, which do not act alone to repair O(6)-alkylguanines in DNA but link repair to other pathways. The paradoxical ability of AGTs to stimulate the DNA-damaging ability of dihaloalkanes and other bis-electrophiles via the formation of AGT-DNA cross-links is also described. Other important properties of AGTs include the ability to provide resistance to cancer therapeutic alkylating agents, and the availability of AGT inhibitors such as O(6)-benzylguanine that might overcome this resistance is discussed. Finally, the properties of fusion proteins in which AGT sequences are linked to other proteins are outlined. Such proteins occur naturally, and synthetic variants engineered to react specifically with derivatives of O(6)-benzylguanine are the basis of a valuable research technique for tagging proteins with specific reagents.
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Affiliation(s)
- Anthony E Pegg
- Department of Cellular and Molecular Physiology, Milton S. Hershey Medical Center, Pennsylvania State University College of Medicine , Pennsylvania 17033, United States.
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16
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Alkyltransferase-like proteins: molecular switches between DNA repair pathways. Cell Mol Life Sci 2010; 67:3749-62. [PMID: 20502938 DOI: 10.1007/s00018-010-0405-8] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/03/2010] [Revised: 05/04/2010] [Accepted: 05/10/2010] [Indexed: 01/08/2023]
Abstract
Alkyltransferase-like proteins (ATLs) play a role in the protection of cells from the biological effects of DNA alkylation damage. Although ATLs share functional motifs with the DNA repair protein and cancer chemotherapy target O⁶-alkylguanine-DNA alkyltransferase, they lack the reactive cysteine residue required for alkyltransferase activity, so its mechanism for cell protection was previously unknown. Here we review recent advances in unraveling the enigmatic cellular protection provided by ATLs against the deleterious effects of DNA alkylation damage. We discuss exciting new evidence that ATLs aid in the repair of DNA O⁶-alkylguanine lesions through a novel repair cross-talk between DNA-alkylation base damage responses and the DNA nucleotide excision repair pathway.
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17
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Fang Q, Kanugula S, Tubbs JL, Tainer JA, Pegg AE. Repair of O4-alkylthymine by O6-alkylguanine-DNA alkyltransferases. J Biol Chem 2009; 285:8185-95. [PMID: 20026607 DOI: 10.1074/jbc.m109.045518] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
O(6)-Alkylguanine-DNA alkyltransferase (AGT) plays a major role in repair of the cytotoxic and mutagenic lesion O(6)-methylguanine (m(6)G) in DNA. Unlike the Escherichia coli alkyltransferase Ogt that also repairs O(4)-methylthymine (m(4)T) efficiently, the human AGT (hAGT) acts poorly on m(4)T. Here we made several hAGT mutants in which residues near the cysteine acceptor site were replaced by corresponding residues from Ogt to investigate the basis for the inefficiency of hAGT in repair of m(4)T. Construct hAGT-03 (where hAGT sequence -V(149)CSSGAVGN(157)- was replaced with the corresponding Ogt -I(143)GRNGTMTG(151)-) exhibited enhanced m(4)T repair activity in vitro compared with hAGT. Three AGT proteins (hAGT, hAGT-03, and Ogt) exhibited similar protection from killing by N-methyl-N'-nitro-N-nitrosoguanidine and caused a reduction in m(6)G-induced G:C to A:T mutations in both nucleotide excision repair (NER)-proficient and -deficient Escherichia coli strains that lack endogenous AGTs. hAGT-03 resembled Ogt in totally reducing the m(4)T-induced T:A to C:G mutations in NER-proficient and -deficient strains. Surprisingly, wild type hAGT expression caused a significant but incomplete decrease in NER-deficient strains but a slight increase in T:A to C:G mutation frequency in NER-proficient strains. The T:A to C:G mutations due to O(4)-alkylthymine formed by ethylating and propylating agents were also efficiently reduced by either hAGT-03 or Ogt, whereas hAGT had little effect irrespective of NER status. These results show that specific alterations in the hAGT active site facilitate efficient recognition and repair of O(4)-alkylthymines and reveal damage-dependent interactions of base and nucleotide excision repair.
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Affiliation(s)
- Qingming Fang
- Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033, USA.
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18
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Mutations in DNA repair genes are associated with the Haarlem lineage of Mycobacterium tuberculosis independently of their antibiotic resistance. Tuberculosis (Edinb) 2007; 87:502-8. [DOI: 10.1016/j.tube.2007.05.011] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/07/2006] [Revised: 01/09/2007] [Accepted: 05/21/2007] [Indexed: 11/18/2022]
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19
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Fang Q, Loktionova NA, Moschel RC, Javanmard S, Pauly GT, Pegg AE. Differential inactivation of polymorphic variants of human O6-alkylguanine-DNA alkyltransferase. Biochem Pharmacol 2007; 75:618-26. [PMID: 17996846 DOI: 10.1016/j.bcp.2007.09.022] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/30/2007] [Revised: 09/25/2007] [Accepted: 09/26/2007] [Indexed: 11/25/2022]
Abstract
The human DNA repair protein O(6)-alkylguanine-DNA alkyltransferase (hAGT) is an important source of resistance to some therapeutic alkylating agents and attempts to circumvent this resistance by the use of hAGT inhibitors have reached clinical trials. Several human polymorphisms in the MGMT gene that encodes hAGT have been described including L84F and the linked double alteration I143V/K178R. We have investigated the inactivation of these variants and the much rarer variant W65C by O(6)-benzylguanine, which is currently in clinical trials, and a number of other second generation hAGT inhibitors that contain folate derivatives (O(4)-benzylfolic acid, the 3' and 5' folate esters of O(6)-benzyl-2'-deoxyguanosine and the folic acid gamma ester of O(6)-(p-hydroxymethyl)benzylguanine). The I143V/K178R variant was resistant to all of these compounds. The resistance was due solely to the I143V change. These results suggest that the frequency of the I143V/K178R variant among patients in the clinical trials with hAGT inhibitors and the correlation with response should be considered.
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Affiliation(s)
- Qingming Fang
- Department of Cellular and Molecular Physiology, The Pennsylvania State University College of Medicine, Hershey, PA 17033, USA
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20
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Tubbs JL, Pegg AE, Tainer JA. DNA binding, nucleotide flipping, and the helix-turn-helix motif in base repair by O6-alkylguanine-DNA alkyltransferase and its implications for cancer chemotherapy. DNA Repair (Amst) 2007; 6:1100-15. [PMID: 17485252 PMCID: PMC1993358 DOI: 10.1016/j.dnarep.2007.03.011] [Citation(s) in RCA: 126] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/20/2023]
Abstract
O(6)-Alkylguanine-DNA alkyltransferase (AGT) is a crucial target both for the prevention of cancer and for chemotherapy, since it repairs mutagenic lesions in DNA, and it limits the effectiveness of alkylating chemotherapies. AGT catalyzes the unique, single-step, direct damage reversal repair of O(6)-alkylguanines by selectively transferring the O(6)-alkyl adduct to an internal cysteine residue. Recent crystal structures of human AGT alone and in complex with substrate DNA reveal a two-domain alpha/beta fold and a bound zinc ion. AGT uses its helix-turn-helix motif to bind substrate DNA via the minor groove. The alkylated guanine is then flipped out from the base stack into the AGT active site for repair by covalent transfer of the alkyl adduct to Cys145. An asparagine hinge (Asn137) couples the helix-turn-helix DNA binding and active site motifs. An arginine finger (Arg128) stabilizes the extrahelical DNA conformation. With this newly improved structural understanding of AGT and its interactions with biologically relevant substrates, we can now begin to unravel the role it plays in preserving genetic integrity and discover how it promotes resistance to anticancer therapies.
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Affiliation(s)
- Julie L. Tubbs
- The Scripps Research Institute, The Skaggs Institute for Chemical Biology and Department of Molecular Biology, 10550 North Torrey Pines Road, MB4, La Jolla, CA 92037
| | - Anthony E. Pegg
- Department of Cellular and Molecular Physiology, Milton S. Hershey Medical Center, Pennsylvania State University College of Medicine, Hershey, PA 17033
| | - John A. Tainer
- The Scripps Research Institute, The Skaggs Institute for Chemical Biology and Department of Molecular Biology, 10550 North Torrey Pines Road, MB4, La Jolla, CA 92037
- Life Sciences Division, Department of Molecular Biology, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
- *To whom correspondence should be addressed: Tel: +1-858-784-8119; fax: +1-858-784-2289;
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21
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Emmert B, Bünger J, Keuch K, Müller M, Emmert S, Hallier E, Westphal GA. Mutagenicity of cytochrome P450 2E1 substrates in the Ames test with the metabolic competent S. typhimurium strain YG7108pin3ERb5. Toxicology 2006; 228:66-76. [PMID: 16978761 DOI: 10.1016/j.tox.2006.08.013] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/20/2006] [Revised: 08/14/2006] [Accepted: 08/15/2006] [Indexed: 10/24/2022]
Abstract
UNLABELLED Poor metabolic competence of in vitro systems was proposed to be one of their major shortcomings accounting for false negative results in genotoxicity testing. For several "low molecular weight cancer suspects" this was specifically attributed to the lack of cytochrome P450 2E1 (CYP2E1) in conventional in vitro metabolising systems. One promising attempt to overcome this problem is the transfection of "methyltransferase-deficient"S. typhimurium strains with the plasmid pin3ERb5. This plasmid contains DNA encoding for a complete electron transport chain, comprising P450 reductase, cytochrome b5 and cytochrome P450 2E1. In order to answer the question if CYP2E1 substrates that yield negative or inconclusive results in the Ames test can be activated by metabolic competent bacterial strains, we used YG7108pin3ERb5 to investigate the following compounds: acetamide, acrylamide, acrylonitrile, allyl chloride, ethyl acrylate, ethyl carbamate, methyl-methacrylate, vinyl acetate, N-nitrosopyrrolidine, trichloroethylene and tetrachloroethylene. N-Nitrosodiethylamine served as a positive control. In addition to these known or proposed CYP2E1 substrates, we investigated the polycyclic aromatic hydrocarbon benzo[alpha]pyrene and the heterocyclic aromatic amines 2-aminofluorene and 2-aminoanthracene. RESULTS The extensive metabolic competence of the transformed strain is underlined by results showing strong mutagenicity between 10 and 500 micro g N-nitrosopyrrolidine per plate. Unexpectedly, 2-aminoanthracene was mutagenic at a concentration range between 25 and 250 micro g per plate using YG7108pin3ERb5. Moreover, we demonstrate for the first time a clear response of sufficiently characterised allyl chloride in the Ames test at a reasonably low concentration range between 300 and 1500 micro g per plate. We achieved similar results in the parent strain YG7108 with conventional metabolic activation. Without metabolic activation less pronounced mutagenicity occurred, suggesting a contribution of a direct alkylating effect. Propylene oxide is usually contained in allyl chloride as stabilizer at amounts up to 0.09%. Though YG7108 revealed to be very sensitive towards propylene oxide, allyl chloride dissolved in water was not mutagenic, showing that no water soluble compounds contribute to its mutagenicity. None of the remaining compounds showed mutagenic effects using YG7108pin3ERb5. CONCLUSION YG7108pin3ERb5 and its parent strain YG7108 are sensitive for compounds which are negative in conventional tester strains including N-nitrosodiethylamine, N-nitrosopyrrolidine, propylene oxide and allyl chloride.
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Affiliation(s)
- Birgit Emmert
- Department of Occupational and Social Health, Georg-August-University Göttingen, Waldweg 37, Göttingen, Germany.
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22
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Affiliation(s)
- Yukiko Mishina
- Department of Chemistry, The University of Chicago, 5735 South Ellis Avenue, Chicago, Illinois 60637
| | - Erica M. Duguid
- Department of Chemistry, The University of Chicago, 5735 South Ellis Avenue, Chicago, Illinois 60637
| | - Chuan He
- Department of Chemistry, The University of Chicago, 5735 South Ellis Avenue, Chicago, Illinois 60637
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23
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Duguid EM, Rice PA, He C. The structure of the human AGT protein bound to DNA and its implications for damage detection. J Mol Biol 2005; 350:657-66. [PMID: 15964013 DOI: 10.1016/j.jmb.2005.05.028] [Citation(s) in RCA: 78] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/08/2005] [Revised: 05/04/2005] [Accepted: 05/14/2005] [Indexed: 11/23/2022]
Abstract
O6-Alklyguanine-DNA alkyltransferase (AGT) is an important DNA repair protein that protects cells from mutagenesis and toxicity arising from alkylating agents. We present an X-ray crystal structure of the wild-type human protein (hAGT) bound to double-stranded DNA with a chemically modified cytosine base. The protein binds at two different sites: one at the modified base, and the other across a sticky-ended DNA junction. The protein molecule that binds the modified cytosine base flips the base and recognizes it in its active site. The one that binds ends of neighboring DNA molecules partially flips an overhanging thymine base. This base is not inserted into the active-site pocket of the protein. These two different hAGT/DNA interactions observed in the structure suggest that hAGT may not detect DNA lesions by searching for the adduct itself, but rather for weakened and/or distorted base-pairs caused by base damage in the duplex DNA. We propose that hAGT imposes a strain on the DNA duplex and searches for DNA regions where the native structure is destabilized. The structure provides implications for pyrimidine recognition, improved inhibitor design, and a possible protein/protein interaction patch on hAGT.
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Affiliation(s)
- Erica M Duguid
- Department of Chemistry, 5735 South Ellis Avenue, The University of Chicago, Chicago, IL 60637, USA
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24
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Zang H, Fang Q, Pegg AE, Guengerich FP. Kinetic analysis of steps in the repair of damaged DNA by human O6-alkylguanine-DNA alkyltransferase. J Biol Chem 2005; 280:30873-81. [PMID: 16000301 DOI: 10.1074/jbc.m505283200] [Citation(s) in RCA: 57] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Rates of individual steps in the removal of alkyl groups from O6-methyl (Me) and -benzyl (Bz) guanine in oligonucleotides by human O6-alkylguanine DNA alkyltransferase (AGT) were estimated using rapid reaction kinetic methods. The overall reaction yields hyperbolic plots of rate versus AGT concentration for O6-MeG but linear plots for the O6-BzG reaction, which is approximately 100-fold faster. The binding of AGT and DNA (double-stranded 30-mer/36-mer complex) appears to be diffusion-limited. The rate of dissociation of the complex is approximately 25-fold slower (approximately 1 s(-1)) for DNA containing O6-MeG or O6-BzG than unmodified DNA. The fluorescent dC-analog 6-methylpyrrolo[2,3-d]pyrimidine-2(3H) one deoxyribonucleoside (pyrrolo dC), which pairs with G, was positioned opposite G, O6-MeG, or O6-BzG and used as a probe of the rate of base flipping. A rapid increase of fluorescence (k approximately 200 s(-1)) was observed with O6-MeG and O6-BzG and AGT but not with a Gly mutation at Arg128, which has been implicated in base flipping with crystal structures. Only weak and slower fluorescence changes were observed with G:pyrrolo dC or T:2-aminopurine pairs. These rate estimates were used in a kinetic model in which AGT binds and scans DNA rapidly, flips O6-alkylG residues, transfers the alkyl group in a chemical step that is rate-limiting in the case of O6-MeG but not O6-BzG, and releases the dealkylated DNA. The results explain the overall patterns of rates of alkyl group removal versus AGT concentration and the effects of the mutations, as well as the greater affinity of AGT for DNA with O6-alkylG lesions.
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Affiliation(s)
- Hong Zang
- Department of Biochemistry and Center in Molecular Toxicology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-0146, USA
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25
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Duarte MP, Palma BB, Laires A, Oliveira JS, Rueff J, Kranendonk M. Escherichia coli BTC, a human cytochrome P450 competent tester strain with a high sensitivity towards alkylating agents: involvement of alkyltransferases in the repair of DNA damage induced by aromatic amines. Mutagenesis 2005; 20:199-208. [PMID: 15843388 DOI: 10.1093/mutage/gei028] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
We report here on strain BTC, a new Escherichia coli mutagenicity tester strain for the expression of human cytochrome P450 (CYP) with an enhanced sensitivity for the detection of alkylating agents. This strain was developed first through knocking out of the genes ada and ogt in our previously developed strain BMX100, resulting in PD1000. Strain PD1000 demonstrated a significantly higher detection sensitivity towards several alkylating agents such as N-nitrosodiethylamine (NNdEA), N-nitrosodi-n-propylamine (NNdPA), 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Unexpectedly, this strain also showed an enhanced sensitivity towards 2-aminoanthracene (2AA), 4-aminobiphenyl (4AbPh), 2-aminofluorene (2AF) and 2-nitroanthracene (2NA) mutagenicity. Subsequently, our previously developed bi-plasmid system for the co-expression of a specific human CYP form (CYP1A2, 2A6 or 2E1) with human NADPH-cytochrome P450 reductase (RED) was introduced in strain PD1000, resulting in strains BTC1A2, BTC2A6 and BTC2E1, respectively. The mutagenicity of NNdEA and NNK was successfully detected with strains BTC2A6 and BTC2E1 and with strains BTC1A2 and BTC2A6, respectively, in contrast to the corresponding MTC (ada+ ogt+) CYP strains. The (ada- ogt-) deficient strain BTC1A2 also showed an enhanced sensitivity towards the detection of 2AA mutagenicity, when compared with the proficient repair strain MTC1A2. This enhancement was much more pronounced with strain PD1000 using the rat liver S9 fraction than with strain BTC1A2.
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Affiliation(s)
- Maria Paula Duarte
- Department of Genetics, Faculty of Medical Sciences, Universidade Nova de Lisboa, Rua da Junqueira 96, 1349-008 Lisboa, Portugal
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26
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Meyer AS, McCain MD, Fang Q, Pegg AE, Spratt TE. O6-alkylguanine-DNA alkyltransferases repair O6-methylguanine in DNA with Michaelis-Menten-like kinetics. Chem Res Toxicol 2005; 16:1405-9. [PMID: 14615965 DOI: 10.1021/tx0341254] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
O(6)-Alkylguanine-DNA alkyltransferase (AGT) repairs O(6)-methylguanine (O(6)mG) by transferring the methyl group from the DNA to a cysteine residue on the protein. The kinetics of this reaction was examined by reacting an excess of AGT (0-300 nM) with [5'-(32)P]-labeled oligodeoxynucleotides (0.5 nM) of the sequence 5'-CGT GGC GCT YZA GGC GTG AGC-3' in which Y or Z was G or O(6)mG, annealed to its complementary strand. The reactions, conducted at 25 degrees C, were quenched by the addition of 0.1 N NaOH at various times, and the extents of reaction were monitored by ion exchange HPLC with radiochemical detection. The time courses followed first-order kinetics. The first-order rate constants were plotted against the initial concentration of AGT and fitted to the hyperbolic equation k(obs) = k(inact)[AGT](0)/(K(S) + [AGT](0)). The K(S) values for hAGT of 81-91 nM are 10-fold lower than the dissociation constants of hAGT (C145S) to unmodified and O(6)mG-containing DNA obtained by EMSA and indicate that AGT has a preference for binding to O(6)mG in DNA. The proteins reacted with DNA in which Y = O(6)mG and Z = G faster than Y = G and Z = O(6)mG due to an approximately 10-fold increase in k(inact). These results suggest that the sequence specificity in the repair of O(6)mG is manifested in the methyl transfer not in the O(6)mG recognition step.
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Affiliation(s)
- Aviva S Meyer
- Institute for Cancer Prevention, American Health Foundation Cancer Center, One Dana Road, Valhalla, New York 10595, USA
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27
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Mijal RS, Thomson NM, Fleischer NL, Pauly GT, Moschel RC, Kanugula S, Fang Q, Pegg AE, Peterson LA. The repair of the tobacco specific nitrosamine derived adduct O6-[4-Oxo-4-(3-pyridyl)butyl]guanine by O6-alkylguanine-DNA alkyltransferase variants. Chem Res Toxicol 2004; 17:424-34. [PMID: 15025514 DOI: 10.1021/tx0342417] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The tobacco specific nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a potent pulmonary carcinogen, both methylates and pyridyloxobutylates DNA. Both reaction pathways generate promutagenic O6-alkylguanine adducts. These adducts, O6-methylguanine (O6-mG) and O6-[4-oxo-4-(3-pyridyl)butyl]guanine (O6-pobG), are repaired by O6-alkylguanine-DNA alkyltransferase (AGT). In this report, we demonstrate that pyridyloxobutyl DNA adducts are repaired by AGT in a reaction that results in pyridyloxobutyl transfer to the active site cysteine. Because minor changes within the binding pocket of AGT can alter the ability of this protein to repair bulky O6-alkylguanine adducts relative to O6-mG, we explored the ability of AGTs from different species as well as several human AGT variants and mutants to discriminate between O6-mG or O6-pobG adducts. We incubated proteins with equal molar amounts of oligodeoxynucleotides containing site specifically incorporated O6-mG or O6-pobG and measured repair. Bacterial AGTs poorly repaired O6-pobG. Mouse and rat AGT repaired both adducts at comparable rates. Wild-type human AGT, variant I143V/K178R, and mutant N157H repaired O6-mG approximately twice as fast as O6-pobG. Human variant G160R and mutants P140K, Y158H, G156A, and E166G did not repair O6-pobG until all of the O6-mG was removed. To understand the role of adduct structure on relative repair rates, the competition experiments were repeated with two other bulky O6-alkylguanine adducts, O6-butylguanine (O6-buG) and O6-benzylguanine (O6-bzG). The proteins displayed similar repair preference of O6-mG relative to O6-buG as observed with O6-pobG. In contrast, all of the mammalian proteins, except the mutant P140K, preferentially repaired O6-bzG. These studies indicate that the rate of repair of O6-pobG is highly dependent on protein structure. Inefficient repair of O6-pobG by bacterial AGT explains the high mutagenic activity of this adduct in bacterial systems. In addition, differences observed in the repair of this adduct by mammalian proteins may translate into differences in sensitivity to the mutagenic and carcinogenic effects of NNK or other pyridyloxobutylating nitrosamines.
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Affiliation(s)
- Renée S Mijal
- Division of Environmental and Occupational Health and Cancer Center, University of Minnesota, Minneapolis, Minnesota 55455, USA
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28
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Kanugula S, Pegg AE. Alkylation damage repair protein O6-alkylguanine-DNA alkyltransferase from the hyperthermophiles Aquifex aeolicus and Archaeoglobus fulgidus. Biochem J 2003; 375:449-55. [PMID: 12892560 PMCID: PMC1223701 DOI: 10.1042/bj20030809] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/02/2003] [Revised: 07/24/2003] [Accepted: 08/01/2003] [Indexed: 10/27/2022]
Abstract
AGT (O6-alkylguanine DNA alkyltransferase) is an important DNA-repair protein that protects cells from killing and mutagenesis by alkylating agents. The AGT genes from two extremely thermophilic organisms, the bacterium Aquifex aeolicus and the archaeon Archaeoglobus fulgidus were PCR-derived and cloned into an expression vector. The nucleotide sequence of the Aq. aeolicus AGT encodes a 201-amino-acid protein with a molecular mass of 23000 Da and Ar. fulgidus AGT codes for a 147-amino-acid protein with a molecular mass of 16718 Da. The Aq. aeolicus and Ar. fulgidus AGTs were expressed at high levels in Escherichia coli fused to an N-terminal polyhistidine tag that allowed single-step isolation and purification by metal-affinity chromatography. Both AGTs formed inclusion bodies and were not soluble under native purification conditions. Therefore AGT isolation was performed under protein-denaturation conditions in the presence of 8.0 M urea. Soluble AGT was obtained by refolding the AGT in the presence of calf thymus DNA. Both AGTs were active in repairing O6-methylguanine and, at a lower rate, O4-methylthymine in DNA. They exhibited thermostability and optimum activity at high temperature. The thermostable AGTs, particularly that from Aq. aeolicus, were readily inactivated by the low-molecular-mass inhibitor O6-benzylguanine, which is currently in clinical trials to enhance cancer chemotherapy.
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Affiliation(s)
- Sreenivas Kanugula
- Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, The Milton S. Hershey Medical Center, P.O. Box 850, 500 University Drive, Hershey, PA 17033, USA
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29
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Guengerich FP, Fang Q, Liu L, Hachey DL, Pegg AE. O6-alkylguanine-DNA alkyltransferase: low pKa and high reactivity of cysteine 145. Biochemistry 2003; 42:10965-70. [PMID: 12974631 DOI: 10.1021/bi034937z] [Citation(s) in RCA: 51] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The active site cysteine of human O(6)-alkylguanine-DNA alkyltransferase (hAGT), Cys145, was shown to be highly reactive with model electrophiles unrelated to substrates, including 1-chloro-2,4-dinitrobenzene. The high reactivity suggested that the Cys145 thiolate anion might be stable at neutral pH. The pK(a) was estimated from plots of UV spectra (A(239)) and reactivity toward 4,4'-dithiopyridine vs pH. The estimated pK(a) for hAGT was 4-5, depending upon the method used, and near that of the extensively characterized papain Cys25. Rates of reaction with 4,4'-dithiopyridine were similar for the thiolate forms of hAGT, papain, glutathione, and the bacterial hAGT homologue Ogt (the pK(a) of the latter was 5.4). Bound Zn(2+) has previously been shown to be required for the catalytic activity of hAGT (Rasimas, J. J. et al. (2003) Biochemistry 42, 980-990). Zn(2+) was shown to be required for the low pK(a) of hAGT. The high reactivity of hAGT Cys145 is postulated to be important in normal catalytic function, in cross-linking reactions involving bis-electrophiles, and in inhibition of the DNA repair function of hAGT by electrophiles.
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Affiliation(s)
- F Peter Guengerich
- Department of Biochemistry and Center in Molecular Toxicology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232, USA.
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Rasimas JJ, Kanugula S, Dalessio PM, Ropson IJ, Fried MG, Pegg AE. Effects of zinc occupancy on human O6-alkylguanine-DNA alkyltransferase. Biochemistry 2003; 42:980-90. [PMID: 12549918 DOI: 10.1021/bi026970b] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
A recent crystallographic study of recombinant human O(6)-alkylguanine-DNA alkyltransferase (hAGT) revealed a previously unknown zinc atom [Daniels et al., (2000) EMBO J. 19, 1719-1730]. The effects of zinc on the properties of hAGT are reported here. In bacterial expression systems, recombinant hAGT was produced in increasingly larger quantities when growth media are supplemented with up to 0.1 mM ZnCl(2). Metal-enriched hAGT samples had a 5-fold increase in repair rate constant over conventionally purified protein samples and a 60-fold increase over metal-stripped hAGT. In addition, mutants of the zinc-binding residues had decreases in zinc occupancy that correlated with reductions in repair rate. Zinc modulation did not abolish the repair capacity of a fraction of the hAGT population, as evidenced by the stoichiometric reaction with an oligodeoxyribonucleotide substrate. Zinc occupancy had a similar effect on the rate of reaction with O(6)-benzylguanine, a free base substrate, as on the repair of methylated DNA. Differentially zinc-treated hAGTs showed the same affinity for binding to native DNA and substrate oligodeoxyribonucleotides. Metal content manipulations had little effect upon the CD spectrum of hAGT, but fluorescence studies revealed a small conformational change based upon metal binding, and zinc occupancy correlated with enhanced hAGT stability as evidenced by resistance to the denaturing effects of urea. These results indicate that the presence of zinc confers a mechanistic enhancement to repair activity that does not result from an increase in substrate binding affinity. Zinc also provides conformational stability to hAGT that may influence its regulation.
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Affiliation(s)
- Joseph J Rasimas
- Department of Biochemistry and Molecular Biology, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033, USA
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31
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Liu L, Pegg AE, Williams KM, Guengerich FP. Paradoxical enhancement of the toxicity of 1,2-dibromoethane by O6-alkylguanine-DNA alkyltransferase. J Biol Chem 2002; 277:37920-8. [PMID: 12151404 DOI: 10.1074/jbc.m205548200] [Citation(s) in RCA: 42] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The presence of the DNA repair protein O(6)-alkylguanine-DNA alkyltransferase (AGT) paradoxically increases the mutagenicity and cytotoxicity of 1,2-dibromoethane (DBE) in Escherichia coli. This enhancement of genotoxicity did not occur when the inactive C145A mutant of human AGT (hAGT) was used. Also, hAGT did not enhance the genotoxicity of S-(2-haloethyl)glutathiones that mimic the reactive product of the reaction of DBE with glutathione, which is catalyzed by glutathione S-transferase. These experiments support a mechanism by which hAGT activates DBE. Studies in vitro showed a direct reaction between purified recombinant hAGT and DBE resulting in a loss of AGT repair activity and a formation of an hAGT-DBE conjugate at Cys(145). A 2-hydroxyethyl adduct was found by mass spectrometry to be present in the Gly(136)-Arg(147) peptide from tryptic digests of AGT reacted with DBE. Incubation of AGT with DBE and oligodeoxyribonucleotides led to the formation of covalent AGT-oligonucleotide complexes. These results indicate that DBE reacts at the active site of AGT to generate an S-(2-bromoethyl) intermediate, which forms a highly reactive half-mustard at Cys(145). In the presence of DNA, the DNA-binding function of AGT facilitates formation of DNA adducts. In the absence of DNA, the intermediate undergoes hydrolytic decomposition to form AGT-Cys(145)-SCH(2)CH(2)OH.
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Affiliation(s)
- Liping Liu
- Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033, USA
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32
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Luu KX, Kanugula S, Pegg AE, Pauly GT, Moschel RC. Repair of oligodeoxyribonucleotides by O(6)-alkylguanine-DNA alkyltransferase. Biochemistry 2002; 41:8689-97. [PMID: 12093287 DOI: 10.1021/bi025857i] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Activity of the DNA repair protein O(6)-alkylguanine-DNA alkyltransferase (AGT) is an important source of tumor cell resistance to alkylating agents. AGT inhibitors may prove useful in enhancing chemotherapy. AGT is inactivated by reacting stoichiometrically with O(6)-benzylguanine (b(6)G), which is currently in clinical trials for this purpose. Short oligodeoxyribonucleotides containing a central b(6)G are more potent inactivators of AGT than b(6)G. We examined whether human AGT could react with oligodeoxyribonucleotides containing multiple b(6)G residues. The single-stranded 7-mer 5'-d[T(b(6)G)(5)G]-3' was an excellent AGT substrate with all five b(6)G adducts repaired although one adduct was repaired much more slowly. The highly b(6)G-resistant Y158H and P140K AGT mutants were also inactivated by 5'-d[T(b(6)G)(5)G]-3'. Studies with 7-mers containing a single b(6)G adduct showed that 5'-d[TGGGG(b(6)G)G]-3' was more poorly repaired by wild-type AGT than 5'-d[T(b(6)G)GGGGG]-3' and 5'-d[TGG(b(6)G)GGG]-3' and was even less repairable by mutants Y158H and P140K. This positional effect was unaffected by interchanging the terminal 5'- or 3'-nucleotides and was also observed with single-stranded 16-mer oligodeoxyribonucleotides containing O(6)-methylguanine, where a minimum of four nucleotides 3' to the lesion was required for the most efficient repair. Annealing with the reverse complementary strands to produce double-stranded substrates increased the ability of AGT to repair adducts at all positions except at positions 2 and 15. Our results suggest that AGT recognizes the polarity of single-stranded DNA, with the best substrates having an adduct adjacent to the 5'-terminal residue. These findings will aid in designing novel AGT inhibitors that incorporate O(6)-alkylguanine adducts in oligodeoxyribonucleotide contexts.
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Affiliation(s)
- Kieu X Luu
- Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, The Milton S. Hershey Medical Center, P.O. Box 850, 500 University Drive, Hershey, Pennsylvania 17033-0850, USA
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33
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Seitz EM, Haseltine CA, Kowalczykowski SC. DNA recombination and repair in the archaea. ADVANCES IN APPLIED MICROBIOLOGY 2002; 50:101-69. [PMID: 11677683 DOI: 10.1016/s0065-2164(01)50005-2] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Affiliation(s)
- E M Seitz
- Sections of Microbiology and of Molecular and Cellular Biology, Center for Genetics and Development, University of California, Davis, Davis, California 95616-8665, USA
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34
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Woodside AM, Guengerich FP. Effect of the O6 substituent on misincorporation kinetics catalyzed by DNA polymerases at O(6)-methylguanine and O(6)-benzylguanine. Biochemistry 2002; 41:1027-38. [PMID: 11790127 DOI: 10.1021/bi011495n] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Misincorporation at a DNA-carcinogen adduct may contribute to formation of mutations if a polymerase proceeds past the lesion, compromising fidelity, as in the G:C to A:T mutations caused by O(6)-alkylguanine. Replication of primer/templates containing guanine (G), O(6)-methylguanine (O(6)-MeG), or O(6)-benzylguanine (O(6)-BzG) was assessed using T7 DNA polymerase exo(-) (T7(-)) and HIV-1 reverse transcriptase (RT). The steady-state parameters indicated that T7(-) and RT preferentially incorporated dTTP opposite O(6)-MeG and O(6)-BzG. The incorporation efficiencies (k(cat)/K(m)) were less for O(6)-BzG than O(6)-MeG for both dCTP and dTTP insertion. Pre-steady-state analysis indicated that the product formed during the burst phase, i.e., the burst amplitude, differed significantly between the unmodified 24-mer/36-G-mer and the O(6)-alkylG-containing substrates. Extension of the O(6)-BzG-containing duplexes was much more difficult for both polymerases as compared to O(6)-MeG, except when RT easily extended the O(6)-BzG:T base pair. The for binding of dCTP or dTTP to a RT*DNA complex containing O(6)-MeG was 8-fold greater than for dNTP binding to a complex containing unmodified DNA. The for a RT*DNA complex containing O(6)-BzG was 50-fold greater. In conclusion, the bulkier O(6)-BzG is a greater block to polymerization by T7(-) and RT than is O(6)-MeG, but some polymerization does occur with an O(6)-BzG substrate. Pre-steady-state analysis indicates that neither dCTP nor dTTP insertion is strongly preferred during polymerization of O(6)-BzG-containing DNA, unlike the case of O(6)-MeG. These results and others regarding polymerase stalling opposite O(6)-MeG and O(6)-BzG are discussed in the following paper in this issue [Woodside, A. M., and Guengerich, F. P. (2002) Biochemistry 41, 1039-1050].
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Affiliation(s)
- Adrienne M Woodside
- Department of Biochemistry and Center in Molecular Toxicology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-0146, USA
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35
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Damoiseaux R, Keppler A, Johnsson K. Synthesis and applications of chemical probes for human O6-alkylguanine-DNA alkyltransferase. Chembiochem 2001; 2:285-7. [PMID: 11828457 DOI: 10.1002/1439-7633(20010401)2:4<285::aid-cbic285>3.0.co;2-n] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
Affiliation(s)
- R Damoiseaux
- Institut de Chimie Organique, BCH Université de Lausanne, 1015 Lausanne, Switzerland
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36
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Liu H, Xu-Welliver M, Pegg AE. The role of human O(6)-alkylguanine-DNA alkyltransferase in promoting 1,2-dibromoethane-induced genotoxicity in Escherichia coli. Mutat Res 2000; 452:1-10. [PMID: 10894884 DOI: 10.1016/s0027-5107(00)00062-2] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The expression of the DNA repair protein human O(6)-alkylguanine-DNA alkyltransferase (AGT) in Escherichia coli strains GWR109 or TRG8 that lack endogenous AGT greatly increased the toxicity and mutagenicity of 1,2-dibromoethane (DBE). Pretreatment of strain TRG8 expressing human AGT, which is permeable to exogenous drugs, with the AGT inhibitor O(6)-benzylguanine (BG) abolished the lethal and mutagenic effects of DBE, indicating that an active AGT is required for promoting DBE genotoxicity. This was confirmed by the observation that E. coli expressing either the C145A AGT mutant, which is inactive due to loss of the alkyl acceptor site, or mutants Y114E and R128A, which are inactive due to alteration of the DNA binding domain, did not enhance the action of DBE. However, the AGT mutant protein P138M/V139L/P140K, which is active in repairing methylated DNA but is totally resistant to inactivation by BG due to alterations in the active site pocket, was unable to enhance the genotoxicity of DBE. Similarly, other mutants, G156P, Y158H and K165R that are strongly resistant to BG, were much less effective than wild type AGT in mediating the genotoxicity of DBE. Mutant P140A, which is moderately resistant to BG, did increase mutations in response to DBE but was less active than wild type. These results suggest that human AGT is able to interact with a DNA lesion produced by DBE but, instead of repairing it, converts it to a more genotoxic adduct. This interaction is prevented by mutations that modify the active site of AGT to exclude BG.
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Affiliation(s)
- H Liu
- Department of Cellular and Molecular Physiology, Milton S. Hershey Medical Center, Pennsylvania State University College of Medicine, 17033-0850, USA
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37
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Abstract
This article describes the five main classes of DNA repair processes that occur in humans with respect to their mechanism of action, major substrates, and role in protection against endogenous and environmental DNA damaging agents. The importance of all of these processes in protection from the initiation of neoplastic growth has been established either in studies of inheritable diseases affecting DNA repair or experiments with transgenic animals or both. The capacity of DNA repair pathways to deal with DNA damage is therefore a critical factor in the cellular response to environmental, and dietary carcinogens. DNA repair activity and factors affecting this activity either directly or indirectly must be taken into account in risk assessment.
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Affiliation(s)
- A E Pegg
- Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Milton S. Hershey Medical Center, Hershey 17033, USA
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38
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Xu-Welliver M, Pegg AE. Point mutations at multiple sites including highly conserved amino acids maintain activity, but render O6-alkylguanine-DNA alkyltransferase insensitive to O6-benzylguanine. Biochem J 2000; 347:519-26. [PMID: 10749682 PMCID: PMC1220985 DOI: 10.1042/0264-6021:3470519] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
The DNA repair protein, O(6)-alkylguanine-DNA alkyltransferase (AGT), is inactivated by reaction with the pseudosubstrate, O(6)-benzylguanine (BG). This inactivation sensitizes tumour cells to chemotherapeutic alkylating agents, and BG is aimed at enhancing cancer treatment in clinical trials. Point mutations in a 24 amino acid sequence likely to form the BG-binding pocket were identified using a screening method designed to identify BG-resistant mutants. It was found that alterations in 21 of these residues were able to render AGT resistant to BG. These included mutations at the highly conserved residues Lys(165), Leu(168) and Leu(169). The two positions at which changes led to the largest increase in resistance to BG were Gly(156) and Lys(165). Eleven mutants at Gly(156) were identified, with increases in resistance ranging from 190-fold (G156V) to 4400-fold (G156P). Two mutants at Lys(165) found in the screen (K165S and K165A) showed 620-fold and 100-fold increases in resistance to BG. Two mutants at the Ser(159) position (S159I and S159V) were >80-fold more resistant than wild-type AGT. Eleven active mutants at Leu(169) were also resistant to BG, but with lower increases (5-86-fold). Fourteen BG-resistant mutants were found for position Cys(150), with 3-26-fold increases in the amount of inhibitor needed to produce a 50% loss of activity in a 30 min incubation. Six BG-resistant mutants at Asn(157) were found with increases of 4-13-fold. These results show that many changes can render human AGT resistant to BG without preventing the ability to protect tumour cells from therapeutic alkylating agents.
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Affiliation(s)
- M Xu-Welliver
- Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, The Milton S. Hershey Medical Center, P.O. Box 850, 500 University Drive, Hershey, PA 17033-0850, U.S.A
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39
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Xu-Welliver M, Kanugula S, Loktionova NA, Crone TM, Pegg AE. Conserved residue lysine165 is essential for the ability of O6-alkylguanine-DNA alkyltransferase to react with O6-benzylguanine. Biochem J 2000; 347:527-34. [PMID: 10749683 PMCID: PMC1220986 DOI: 10.1042/0264-6021:3470527] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022]
Abstract
The role of lysine(165) in the activity of the DNA repair protein, O(6)-alkylguanine-DNA alkyltransferase (AGT), and the ability of AGT to react with the pseudosubstrate inhibitor, O(6)-benzylguanine (BG), was investigated by changing this lysine to all other 19 possibilities. All of these mutants (except for K165T, which could not be tested as it was too poorly active for assay in crude cell extracts) gave BG-resistant AGTs with increases in the amount of inhibitor needed to produce a 50% loss of activity in a 30 min incubation (ED(50)) from 100-fold (K165A) to 2400-fold (K165F). Lys(165) is a completely conserved residue in AGTs from many species, and all of the mutations at this site also reduced the ability to repair methylated DNA. The least deleterious change was that to arginine, which reduced the rate constant for DNA repair by approx. 2.5-fold. Mutant K165R resembled all of the other mutants in being highly resistant to BG, with an ED(50) value for inactivation by BG>200-fold greater than wild-type. Detailed studies of purified K165A AGT showed that the rate constant for repair and the binding to methylated DNA substrates were reduced by 10-20-fold. Despite this, the K165A mutant AGT was able to protect cells from alkylating agents and this protection was not abolished by BG. These results show that, firstly, lysine at position 165 is needed for optimal activity of AGT towards methylated DNA substrates and is essential for efficient reaction with BG; and second, even if the AGT activity towards methylated DNA substrates is impaired by mutations at codon 165, such mutants can protect tumour cells from therapeutic alkylating agents. These results raise the possibility that the conservation of Lys(165) is due to the need for AGT activity towards substrates containing more bulky adducts than O(6)-methylguanine. They also suggest that alterations at Lys(165) may occur during chemotherapy with BG and alkylating agents and could limit the effectiveness of this therapy.
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Affiliation(s)
- M Xu-Welliver
- Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, The Milton S. Hershey Medical Center, P.O. Box 850, 500 University Drive, Hershey, PA 17033-0850, USA
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40
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Abstract
The predominant pathway for the repair of O(6)-methylguanine in DNA is via the activity of an alkyltransferase protein that transfers the methyl group to a cysteine acceptor site on the protein itself. This review article describes recent studies on this alkyltransferase. The protein repairs not only methyl groups but also 2-chloroethyl-, benzyl- and pyridyloxobutyl-adducts. It acts on double-stranded DNA by flipping the O(6)-guanine adduct out of the DNA helix and into a binding pocket. The free base, O(6)-benzylguanine, is able to bind in this pocket and react with the cysteine, rendering it an effective inactivator of mammalian alkyltransferases. The alkylated form of the protein is rapidly degraded by the ubiquitin/proteasomal system. Some tumor cells do not express alkyltransferase despite having an intact gene. Methylation of key sites in CpG-rich islands in the promoter region are involved in this silencing and a change in the nuclear localization of an enhancer binding protein may also contribute. The alkyltransferase promoter contains Sp1, GRE and AP-1 sites and is slightly inducible by glucocorticoids and protein kinase C activators. There is a complex relationship between p53 and alkyltransferase expression with p53 mediating a rise in alkyltransferase in response to ionizing radiation but having no clear effect on basal levels. DNA adducts at the O(6)-position of guanine are a major factor in the carcinogenic, mutagenic, apoptopic and clastogenic actions of methylating agents and chloroethylating agents. Studies with transgenic mice in which alkyltransferase levels are increased or decreased confirm the importance of this repair pathway in protecting against carcinogenesis. Alkyltransferase activity in tumors protects them from therapeutic agents such as temozolomide and BCNU. This resistance is abolished by O(6)-benzylguanine and this drug is currently in clinical trials to enhance cancer chemotherapy by these agents. Studies are in progress to reduce the toxicity of such therapy towards the bone marrow by gene therapy to express alkyltransferases with mutations imparting resistance to O(6)-benzylguanine at high levels in marrow stem cells. Several polymorphisms in the human alkyltransferase gene have been identified but the significance of these in terms of alkyltransferase action is currently unknown.
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Affiliation(s)
- A E Pegg
- Departments of Cellular and Molecular Physiology and Pharmacology, Milton S. Hershey Medical Center, Pennsylvania State University College of Medicine, P.O. Box 850, 500 University Drive, Hershey, PA, USA.
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41
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Wibley JE, Pegg AE, Moody PC. Crystal structure of the human O(6)-alkylguanine-DNA alkyltransferase. Nucleic Acids Res 2000; 28:393-401. [PMID: 10606635 PMCID: PMC102527 DOI: 10.1093/nar/28.2.393] [Citation(s) in RCA: 94] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/08/1999] [Revised: 11/24/1999] [Accepted: 11/24/1999] [Indexed: 11/14/2022] Open
Abstract
The mutagenic and carcinogenic effects of simple alkylating agents are mainly due to O(6)-alkylation of guanine in DNA. This lesion results in transition mutations. In both prokaryotic and eukaryotic cells, repair is effected by direct reversal of the damage by a suicide protein, O(6)-alkylguanine-DNA alkyltransferase. The alkyltransferase removes the alkyl group to one of its own cysteine residues. However, this mechanism for preserving genomic integrity limits the effectiveness of certain alkylating anticancer agents. A high level of the alkyltransferase in many tumour cells renders them resistant to such drugs. Here we report the X-ray structure of the human alkyltransferase solved using the technique of multiple wavelength anomalous dispersion. This structure explains the markedly different specificities towards various O(6)-alkyl lesions and inhibitors when compared with the Escherichia coli protein (for which the structure has already been determined). It is also used to interpret the behaviour of certain mutant alkyltransferases to enhance biochemical understanding of the protein. Further examination of the various models proposed for DNA binding is also permitted. This structure may be useful for the design and refinement of drugs as chemoenhancers of alkylating agent chemotherapy.
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Affiliation(s)
- J E Wibley
- Department of Biochemistry, University of Leicester, University Road, Leicester LE1 7RH, UK
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42
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Xu-Welliver M, Leitão J, Kanugula S, Meehan WJ, Pegg AE. Role of codon 160 in the sensitivity of human O6-alkylguanine-DNA alkyltransferase to O6-benzylguanine. Biochem Pharmacol 1999; 58:1279-85. [PMID: 10487529 DOI: 10.1016/s0006-2952(99)00216-6] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
O6-Alkylguanine-DNA alkyltransferase (AGT) is a DNA repair protein that provides protection from alkylating agents such as dacarbazine, temozolomide, and 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU), which are used for cancer chemotherapy. O6-Benzylguanine (BG) is an inhibitor of AGT that sensitizes tumors to these agents. BG is currently in clinical trials. It is possible that the presence of resistant forms of AGT may limit the effectiveness of this strategy. Previous studies have shown that the AGT mutant G160R, which may occur naturally as a result of a polymorphism in the AGT gene, is resistant to BG, whereas the mutants G160W and G160A are actually more sensitive to the inhibitor. To examine other mutations at this site, a random sequence was placed at codon 160 in the AGT cDNA, and a plasmid library was constructed to express these sequences in Escherichia coli. After selection with BG and N-methyl-N'-nitro-N-nitrosoguanidine, BG-resistant mutants were obtained and analyzed. Eleven different amino acid substitutions were found to impart BG resistance by this assay. The most resistant mutants contained histidine or arginine, which had EC50 values of 12 and 4.7 microM, respectively, compared with the wild-type EC50 of 0.08 microM, but nine other alterations led to at least a 10-fold rise in the EC50 value. Three additional mutations at codon 160 were constructed by site-directed mutagenesis, and these led to 6- to 11-fold increases in resistance to BG. Comparisons of the properties of mutants G160R and G160E showed that the presence of DNA enhanced the reaction with BG much more strongly when an acidic residue was present at this position. This may account for the lack of selection of the G160E mutation even though it did impart resistance to BG. These results indicate that many alterations of AGT at position 160 can lead to significant resistance to BG.
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Affiliation(s)
- M Xu-Welliver
- Department of Cellular and Molecular Physiology, Milton S. Hershey Medical Center, Pennsylvania State University College of Medicine, Hershey 17033, USA
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43
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Loktionova NA, Xu-Welliver M, Crone TM, Kanugula S, Pegg AE. Protection of CHO cells by mutant forms of O6-alkylguanine-DNA alkyltransferase from killing by 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU) plus O6-benzylguanine or O6-benzyl-8-oxoguanine. Biochem Pharmacol 1999; 58:237-44. [PMID: 10423163 DOI: 10.1016/s0006-2952(99)00095-7] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/16/2022]
Abstract
O6-Benzylguanine (BG) is an inactivator of human O6-alkylguanine-DNA alkyltransferase (AGT) currently undergoing clinical trials to enhance cancer chemotherapy by alkylating agents. Mutant forms of AGT resistant to BG in vitro were expressed in CHO cells to determine if they could impart resistance to killing by the combination of BG and 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU). All the BG-resistant mutant proteins tested (P140A, P140K, P138M/V139L/P140K, G156A, P140A/G160R, and G160R) showed a reduced rate of reaction with methylated DNA substrates in vitro. However, when expressed in equal amounts in CHO cells, mutants P140A, P140K, P138M/V139L/P140K, and G160R gave levels of protection from the chloroethylating agent BCNU equivalent to that of wild-type AGT. This indicates that a 10-fold reduction in rate constant did not prevent their ability to repair chloroethylated DNA in the cell. AGT activity was readily lost when CHO cells expressing wild-type AGT were exposed to BG or its 8-oxo metabolite (O6-benzyl-8-oxoguanine), but cells expressing mutants P140A or G160R required 30-fold higher concentrations and cells expressing mutants P140K or P138M/V139L/P140K were totally resistant. When cells were treated with 80 microM BCNU plus BG or 8-oxo-BG, those expressing wild-type AGT were killed when inhibitor concentrations of up to 500 microM were used, whereas cells expressing P140K or P138M/V139L/P140K showed no effect, and cells expressing P140A or G160R showed an intermediate resistance. These results suggest that: (i) appearance of BG-resistant mutant AGTs may be a problem during therapy, and (ii) the P140K mutant AGT is an excellent candidate for gene therapy approaches where expression of a BG-resistant AGT in hematopoietic cells is used to reduce toxicity.
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Affiliation(s)
- N A Loktionova
- Department of Cellular and Molecular Physiology, Milton S. Hershey Medical Center, Pennsylvania State University College of Medicine, Hershey, 17033, USA
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44
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45
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Pegg AE, Kanugula S, Edara S, Pauly GT, Moschel RC, Goodtzova K. Reaction of O6-benzylguanine-resistant mutants of human O6-alkylguanine-DNA alkyltransferase with O6-benzylguanine in oligodeoxyribonucleotides. J Biol Chem 1998; 273:10863-7. [PMID: 9556560 DOI: 10.1074/jbc.273.18.10863] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Inactivation of the human DNA repair protein, O6-alkylguanine-DNA alkyltransferase (AGT), by O6-benzylguanine renders tumor cells susceptible to killing by alkylating agents. AGT mutants resistant to O6-benzylguanine can be made by converting Pro140 to an alanine (P140A) or Gly156 to an alanine (G156A). These mutations had a much smaller effect on the reaction with O6-benzylguanine when it was incorporated into a short single-stranded oligodeoxyribonucleotide. Such oligodeoxyribonucleotides could form the basis for the design of improved AGT inhibitors. AGT and mutants P140A and G156A preferentially reacted with O6-benzylguanine when incubated with a mixture of two 16-mer oligodeoxyribonucleotides, one containing O6-benzylguanine and the other, O6-methylguanine. When the 6 amino acids located in positions 159-164 in AGT were replaced by the equivalent sequence from the Escherichia coli Ada-C protein (mutant AGT/6ada) the preference for benzyl repair was eliminated. Further mutation incorporating the P140A change into AGT/6ada giving mutant P140A/6ada led to a protein that resembled Ada-C in preference for the repair of methyl groups, but P140A/6ada did not differ from P140A in reaction with the free base O6-benzylguanine. Changes in the AGT active site pocket can therefore affect the preference for repair of O6-benzyl or -methyl groups when present in an oligodeoxyribonucleotide without altering the reaction with free O6-benzylguanine.
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Affiliation(s)
- A E Pegg
- Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, The Milton S. Hershey Medical Center, Hershey, Pennsylvania 17033-0850, USA.
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Kanugula S, Goodtzova K, Pegg AE. Probing of conformational changes in human O6-alkylguanine-DNA alkyl transferase protein in its alkylated and DNA-bound states by limited proteolysis. Biochem J 1998; 329 ( Pt 3):545-50. [PMID: 9445381 PMCID: PMC1219075 DOI: 10.1042/bj3290545] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
Human O6-alkylguanine-DNA alkyl transferase (hAGT) is a DNA repair protein that protects cells from alkylation damage by transferring an alkyl group from the O6-position of guanine to a cysteine residue in the active site (-PCHR-) of the protein. The structure of the hAGT protein (23 kDa) has been probed by limited proteolysis with trypsin and Glu-C endoproteases and analysis of the polypeptide fragments by SDS/PAGE. The native hAGT protein had limited accessibility to digestion with trypsin and Glu-C in spite of a number of potential cleavage sites. Initial cleavage by trypsin occurred at residue Lys-193 to give a 21 kDa polypeptide fragment, and this polypeptide underwent further cleavage at residues Arg-128 and Lys-165. These trypsin-cleavage sites became more accessible to digestion in the presence of double-stranded DNA (dsDNA), indicating that hAGT undergoes a change in its conformation on binding to DNA. However, the trypsin cutting site at the Arg-128 position was less available for digestion in the presence of single-stranded DNA (ssDNA), suggesting that the hAGT protein has a different conformation when bound to ssDNA compared with dsDNA. When protease digestion was carried out on wild-type protein, preincubated with the low-molecular-mass pseudosubstrate O6-benzylguanine, increased susceptibility to proteases was observed. A mutant C145A hAGT protein, which cannot repair O6-alkylguanine because the Cys-145 acceptor site in the active site of the protein is changed to Ala, showed identical trypsin cleavage to the wild type, but its digestion was not affected by O6-benzylguanine. These results suggest that alkylation of hAGT leads to an altered conformation. The acquisition of increased susceptibility to proteases upon DNA binding and alkylation demonstrates that hAGT undergoes considerable conformational changes in its structure upon binding to DNA and after repair of alkylation damage.
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Affiliation(s)
- S Kanugula
- Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Hershey 17033-0850, USA
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Verdemato PE, Moody PCE. Repair of Alkylated DNA by the E. coli Ada Protein. DNA Repair (Amst) 1998. [DOI: 10.1007/978-3-642-48770-5_1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
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