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The Werner Syndrome Helicase/Exonuclease Processes Mobile D-Loops through Branch Migration and Degradation

By Patricia L. Opresko, Gregory Sowd and Hong Wang


RecQ DNA helicases are critical for preserving genome integrity. Of the five RecQ family members identified in humans, only the Werner syndrome protein (WRN) possesses exonuclease activity. Loss of WRN causes the progeroid disorder Werner syndrome which is marked by cancer predisposition. Cellular evidence indicates that WRN disrupts potentially deleterious intermediates in homologous recombination (HR) that arise in genomic and telomeric regions during DNA replication and repair. Precisely how the WRN biochemical activities process these structures is unknown, especially since the DNA unwinding activity is poorly processive. We generated biologically relevant mobile D-loops which mimic the initial DNA strand invasion step in HR to investigate whether WRN biochemical activities can disrupt this joint molecule. We show that WRN helicase alone can promote branch migration through an 84 base pair duplex region to completely displace the invading strand from the D-loop. However, substrate processing is altered in the presence of the WRN exonuclease activity which degrades the invading strand both prior to and after release from the D-loop. Furthermore, telomeric D-loops are more refractory to disruption by WRN, which has implications for tighter regulation of D-loop processing at telomeres. Finally, we show that WRN can recognize and initiate branch migration from both the 5′ and 3′ ends of the invading strand in the D-loops. These findings led us to propose a novel model for WRN D-loop disruption. Our biochemical results offer an explanation for the cellular studies that indicate both WRN activities function in processing HR intermediates

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Publisher: Public Library of Science
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    1. (1998). A unified view of polymer, dumbbell, and oligonucleotide DNA nearest-neighbor thermodynamics.
    2. (2003). Analysis of helicase activity and substrate specificity of Drosophila RECQ5.
    3. (2006). Analysis of the DNA unwinding activity of RecQ family helicases.
    4. (2004). Biochemical and kinetic characterization of the DNA helicase and exonuclease activities of werner syndrome protein.
    5. (2002). Biochemical characterization of the DNA substrate specificity of werner syndrome helicase.
    6. (1994). Catalysis of ATP-dependent homologous DNA pairing and strand exchange by yeast RAD51 protein.
    7. (2001). Coordinate Action of the Helicase and 39 to 59 Exonuclease of Werner Syndrome Protein.
    8. (2004). Defective telomere lagging strand synthesis in cells lacking WRN helicase activity.
    9. (2006). DNA helicases required for homologous recombination and repair of damaged replication forks.
    10. (2002). DNase I footprinting and enhanced exonuclease function of the bipartite Werner syndrome protein (WRN) bound to partially melted duplex DNA.
    11. (2003). Drosophila BLM in double-strand break repair by synthesis-dependent strand annealing.
    12. (2005). Elevated telomeretelomere recombination in WRN-deficient, telomere dysfunctional cells promotes escape from senescence and engagement of the ALT pathway.
    13. (1999). Functional and physical interaction between WRN helicase and human replication protein A.
    14. (2005). Functional human telomeres are recognized as DNA damage in g2 of the cell cycle.
    15. (2007). Functional role of the Werner syndrome RecQ helicase in human fibroblasts.
    16. (2004). Homologous recombination generates T-loop-sized deletions at human telomeres.
    17. (2002). Homologous recombination resolution defect in werner syndrome.
    18. (2005). Human DNA polymerase eta promotes DNA synthesis from strand invasion intermediates of homologous recombination.
    19. (2007). Human premature aging, DNA repair and RecQ helicases.
    20. (1996). Human Rad51 protein promotes ATPdependent homologous pairing and strand transfer reactions in vitro.
    21. (2006). Human telomeres have different overhang sizes at leading versus lagging strands.
    22. (1995). Identification of double Holliday junctions as intermediates in meiotic recombination.
    23. (2003). Inhibition of Werner syndrome helicase activity by benzo[c]phenanthrene diol epoxide dA adducts in DNA is both strand-and stereoisomer-dependent.
    24. (2004). Junction of RecQ helicase biochemistry and human disease.
    25. (2001). Loss of Werner syndrome protein function promotes aberrant mitotic recombination.
    26. (2002). Machwe A
    27. (1999). Mammalian telomeres end in a large duplex loop.
    28. (2008). Mechanism and substrate specificity of telomeric protein POT1 stimulation of the Werner syndrome helicase.
    29. (2006). Mechanism of homologous recombination: mediators and helicases take on regulatory functions.
    30. (2006). Mobile D-loops are a preferred substrate for the Bloom’s syndrome helicase.
    31. (2007). Novel pro- and antirecombination activities of the Bloom’s syndrome helicase.
    32. (2007). Nucleic acid unwinding by hepatitis C virus and bacteriophage t7 helicases is sensitive to base pair stability.
    33. (1996). Positional cloning of the Werner’s syndrome gene.
    34. (2005). POT1 stimulates RecQ helicases WRN and BLM to unwind telomeric DNA substrates.
    35. (2007). Probing the roles of active site residues in the 39-59 exonuclease of the Werner syndrome protein.
    36. (2005). Recombination at mammalian telomeres: an alternative mechanism for telomere protection and elongation.
    37. (2001). Recombination-mediated lengthening of terminal telomeric repeats requires the
    38. (2000). Reconstitution of the strand invasion step of double-strand break repair using human Rad51 Rad52 and RPA proteins.
    39. (2003). RecQ helicases: caretakers of the genome.
    40. (2003). RecQ helicases: suppressors of tumorigenesis and premature aging.
    41. (2007). RECQL5/Recql5 helicase regulates homologous recombination and suppresses tumor formation via disruption of Rad51 presynaptic filaments.
    42. (2006). Replisome assembly and the direct restart of stalled replication forks.
    43. (2005). Shelterin: the protein complex that shapes and safeguards human telomeres.
    44. (2000). Somatic mutation rates and specificities at TC/AG and GT/CA microsatellite sequences in nontumorigenic human lymphoblastoid cells.
    45. (2003). Telomere instability in a human tumor cell line expressing a dominant-negative WRN protein.
    46. (2007). Telomere ResQue and preservation-Roles for the Werner syndrome protein and other RecQ helicases.
    47. (2001). The Bloom’s and Werner’s syndrome proteins are DNA structure-specific helicases.
    48. (2003). The Bloom’s syndrome helicase suppresses crossing over during homologous recombination.
    49. (2008). The Human RecQ helicases, BLM and RECQ1, display distinct DNA substrate specificities.
    50. (1998). The premature ageing syndrome protein, WRN, is a 39–.59 exonuclease.
    51. (2001). The Saccharomyces cerevisiae WRN homolog Sgs1p participates in telomere maintenance in cells lacking telomerase.
    52. (2004). The Werner Syndrome Helicase and Exonuclease Cooperate to Resolve Telomeric D Loops in a Manner Regulated by TRF1 and TRF2.
    53. (2008). The Werner syndrome protein binds replication fork and holliday junction DNAs as an oligomer.
    54. (2004). The Werner syndrome protein has separable recombination and survival functions.
    55. (2002). Werner and Bloom Syndrome Helicases.
    56. (2007). Werner and HutchinsonGilford progeria syndromes: mechanistic basis of human progeroid diseases.
    57. (1998). Werner Syndrome Protein. Ii. characterization of the integral 39–.59 dna exonuclease.
    58. (2000). Werner’s syndrome protein (WRN) migrates Holliday junctions and co- localizes with RPA upon replication arrest.
    59. (2008). WRN controls formation of extrachromosomal telomeric circles and is required for TRF2DeltaB-mediated telomere shortening.
    60. (2006). WRN exonuclease structure and molecular mechanism imply an editing role in DNA end processing.

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