51
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Xie P. Dynamics of monomeric and hexameric helicases. Biophys Chem 2016; 211:49-58. [DOI: 10.1016/j.bpc.2016.02.003] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/24/2015] [Revised: 02/11/2016] [Accepted: 02/12/2016] [Indexed: 01/19/2023]
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52
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Sun B, Wang MD. Single-molecule perspectives on helicase mechanisms and functions. Crit Rev Biochem Mol Biol 2015; 51:15-25. [DOI: 10.3109/10409238.2015.1102195] [Citation(s) in RCA: 31] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022]
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53
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Sicard F, Destainville N, Manghi M. DNA denaturation bubbles: free-energy landscape and nucleation/closure rates. J Chem Phys 2015; 142:034903. [PMID: 25612729 DOI: 10.1063/1.4905668] [Citation(s) in RCA: 33] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/04/2023] Open
Abstract
The issue of the nucleation and slow closure mechanisms of non-superhelical stress-induced denaturation bubbles in DNA is tackled using coarse-grained MetaDynamics and Brownian simulations. A minimal mesoscopic model is used where the double helix is made of two interacting bead-spring rotating strands with a prescribed torsional modulus in the duplex state. We demonstrate that timescales for the nucleation (respectively, closure) of an approximately 10 base-pair bubble, in agreement with experiments, are associated with the crossing of a free-energy barrier of 22 kBT (respectively, 13 kBT) at room temperature T. MetaDynamics allows us to reconstruct accurately the free-energy landscape, to show that the free-energy barriers come from the difference in torsional energy between the bubble and duplex states, and thus to highlight the limiting step, a collective twisting, that controls the nucleation/closure mechanism, and to access opening time scales on the millisecond range. Contrary to small breathing bubbles, those more than 4 base-pair bubbles are of biological relevance, for example, when a pre-existing state of denaturation is required by specific DNA-binding proteins.
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Affiliation(s)
- François Sicard
- Université de Toulouse, UPS, Laboratoire de Physique Théorique (IRSAMC), F-31062 Toulouse, France
| | - Nicolas Destainville
- Université de Toulouse, UPS, Laboratoire de Physique Théorique (IRSAMC), F-31062 Toulouse, France
| | - Manoel Manghi
- Université de Toulouse, UPS, Laboratoire de Physique Théorique (IRSAMC), F-31062 Toulouse, France
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54
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Xie P. A unified model of nucleic acid unwinding by the ribosome and the hexameric and monomeric DNA helicases. J Theor Biol 2015; 380:359-66. [PMID: 26092375 DOI: 10.1016/j.jtbi.2015.06.011] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/03/2015] [Accepted: 06/05/2015] [Indexed: 11/18/2022]
Abstract
DNA helicases are enzymes that use the chemical energy to separate DNA duplex into their single-stranded forms. The ribosome, which catalyzes the translation of messenger RNAs (mRNAs) into proteins, can also unwind mRNA duplex. According to their structures, the DNA helicases can fall broadly into hexameric and monomeric forms. A puzzling issue for the monomeric helicases is that although they have similar structures, in vitro biochemical data showed convincingly that in the monomeric forms some have very weak DNA unwinding activities, some have relatively high unwinding activities while others have high unwinding activities. However, in the dimeric or oligomeric forms all of them have high unwinding activities. In addition, in the monomeric forms all of them can translocate efficiently along the single-stranded DNA (ssDNA). Here, we propose a model of the translocation along the ssDNA and DNA unwinding by the monomeric helicases, providing a consistent explanation of these in vitro experimental data. Moreover, by comparing the present model for the monomeric helicases with the model for the hexameric helicases and that for the ribosome which were proposed before, a unified model of nucleic acid unwinding by the three enzymes is proposed.
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Affiliation(s)
- Ping Xie
- Key Laboratory of Soft Matter Physics and Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.
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55
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Qiu Y, Niu H, Vukovic L, Sung P, Myong S. Molecular mechanism of resolving trinucleotide repeat hairpin by helicases. Structure 2015; 23:1018-27. [PMID: 26004439 DOI: 10.1016/j.str.2015.04.006] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/20/2014] [Revised: 03/19/2015] [Accepted: 04/01/2015] [Indexed: 10/23/2022]
Abstract
Trinucleotide repeat (TNR) expansion is the root cause for many known congenital neurological and muscular disorders in human including Huntington's disease, fragile X syndrome, and Friedreich's ataxia. The stable secondary hairpin structures formed by TNR may trigger fork stalling during replication, causing DNA polymerase slippage and TNR expansion. Srs2 and Sgs1 are two helicases in yeast that resolve TNR hairpins during DNA replication and prevent genome expansion. Using single-molecule fluorescence, we investigated the unwinding mechanism by which Srs2 and Sgs1 resolves TNR hairpin and compared it with unwinding of duplex DNA. While Sgs1 unwinds both structures indiscriminately, Srs2 displays repetitive unfolding of TNR hairpin without fully unwinding it. Such activity of Srs2 shows dependence on the folding strength and the total length of TNR hairpin. Our results reveal a disparate molecular mechanism of Srs2 and Sgs1 that may contribute differently to efficient resolving of the TNR hairpin.
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Affiliation(s)
- Yupeng Qiu
- Bioengineering Department, University of Illinois, 1304 West Springfield Avenue, Urbana, IL 61801, USA
| | - Hengyao Niu
- Department of Molecular Biophysics and Biochemistry, Yale University, 333 Cedar Street, PO Box 208024, New Haven, CT 06520, USA
| | - Lela Vukovic
- Department of Physics, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA; Physics Frontier Center (Center of Physics for Living Cells), University of Illinois, 1110 West Green Street, Urbana, IL 61801, USA
| | - Patrick Sung
- Department of Molecular Biophysics and Biochemistry, Yale University, 333 Cedar Street, PO Box 208024, New Haven, CT 06520, USA
| | - Sua Myong
- Bioengineering Department, University of Illinois, 1304 West Springfield Avenue, Urbana, IL 61801, USA; Institute for Genomic Biology, University of Illinois, 1206 West Gregory Street, Urbana, IL 61801, USA; Physics Frontier Center (Center of Physics for Living Cells), University of Illinois, 1110 West Green Street, Urbana, IL 61801, USA; Biophysics and Computational Biology, University of Illinois, 1110 West Green Street, Urbana, IL 61801, USA.
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56
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Arslan S, Khafizov R, Thomas CD, Chemla YR, Ha T. Protein structure. Engineering of a superhelicase through conformational control. Science 2015; 348:344-7. [PMID: 25883358 DOI: 10.1126/science.aaa0445] [Citation(s) in RCA: 74] [Impact Index Per Article: 8.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
Abstract
Conformational control of biomolecular activities can reveal functional insights and enable the engineering of novel activities. Here we show that conformational control through intramolecular cross-linking of a helicase monomer with undetectable unwinding activity converts it into a superhelicase that can unwind thousands of base pairs processively, even against a large opposing force. A natural partner that enhances the helicase activity is shown to achieve its stimulating role also by selectively stabilizing the active conformation. Our work provides insight into the regulation of nucleic acid unwinding activity and introduces a monomeric superhelicase without nuclease activities, which may be useful for biotechnological applications.
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Affiliation(s)
- Sinan Arslan
- Physics Department and Center for the Physics of Living Cells, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
| | - Rustem Khafizov
- Physics Department and Center for the Physics of Living Cells, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
| | - Christopher D Thomas
- Astbury Centre for Structural Molecular Biology, University of Leeds, Leeds LS2 9JT, UK
| | - Yann R Chemla
- Physics Department and Center for the Physics of Living Cells, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
| | - Taekjip Ha
- Physics Department and Center for the Physics of Living Cells, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA. Institute for Genomic Biology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA. Howard Hughes Medical Institute, University of Illinois, Urbana, IL 61801, USA.
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57
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Comstock MJ, Whitley KD, Jia H, Sokoloski J, Lohman TM, Ha T, Chemla YR. Protein structure. Direct observation of structure-function relationship in a nucleic acid-processing enzyme. Science 2015; 348:352-4. [PMID: 25883359 DOI: 10.1126/science.aaa0130] [Citation(s) in RCA: 133] [Impact Index Per Article: 14.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/08/2014] [Accepted: 02/24/2015] [Indexed: 12/24/2022]
Abstract
The relationship between protein three-dimensional structure and function is essential for mechanism determination. Unfortunately, most techniques do not provide a direct measurement of this relationship. Structural data are typically limited to static pictures, and function must be inferred. Conversely, functional assays usually provide little information on structural conformation. We developed a single-molecule technique combining optical tweezers and fluorescence microscopy that allows for both measurements simultaneously. Here we present measurements of UvrD, a DNA repair helicase, that directly and unambiguously reveal the connection between its structure and function. Our data reveal that UvrD exhibits two distinct types of unwinding activity regulated by its stoichiometry. Furthermore, two UvrD conformational states, termed "closed" and "open," correlate with movement toward or away from the DNA fork.
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Affiliation(s)
- Matthew J Comstock
- Department of Physics, Center for the Physics of Living Cells, and Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
| | - Kevin D Whitley
- Department of Physics, Center for the Physics of Living Cells, and Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
| | - Haifeng Jia
- Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, MO 63110, USA
| | - Joshua Sokoloski
- Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, MO 63110, USA
| | - Timothy M Lohman
- Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, MO 63110, USA
| | - Taekjip Ha
- Department of Physics, Center for the Physics of Living Cells, and Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA. Howard Hughes Medical Institute, Urbana, IL 61801, USA. Institute for Genomic Biology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
| | - Yann R Chemla
- Department of Physics, Center for the Physics of Living Cells, and Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
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58
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Camera-based three-dimensional real-time particle tracking at kHz rates and Ångström accuracy. Nat Commun 2015; 6:5885. [PMID: 25565216 PMCID: PMC4338538 DOI: 10.1038/ncomms6885] [Citation(s) in RCA: 83] [Impact Index Per Article: 9.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/18/2014] [Accepted: 11/18/2014] [Indexed: 01/30/2023] Open
Abstract
Optical and magnetic tweezers are widely employed to probe the mechanics and activity of individual biomolecular complexes. They rely on micrometer-sized particles to detect molecular conformational changes from the particle position. Real-time particle tracking with Ångström accuracy has so far been only achieved using laser detection through photodiodes. Here we demonstrate that camera-based imaging can provide a similar performance for all three dimensions. Particle imaging at kHz rates is combined with real-time data processing being accelerated by a graphics processing unit. For particles that are fixed in the sample cell we can detect 3 Å sized steps that are introduced by cell translations at rates of 10 Hz, while for DNA-tethered particles 5 Å steps at 1 Hz can be resolved. Moreover, 20 particles can be tracked in parallel with comparable accuracy. Our approach provides a simple and robust way for high-resolution tweezers experiments using multiple particles at a time.
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59
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Ghoneim M, Spies M. Direct correlation of DNA binding and single protein domain motion via dual illumination fluorescence microscopy. NANO LETTERS 2014; 14:5920-31. [PMID: 25204359 PMCID: PMC4189620 DOI: 10.1021/nl502890g] [Citation(s) in RCA: 32] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/06/2023]
Abstract
We report a dual illumination, single-molecule imaging strategy to dissect directly and in real-time the correlation between nanometer-scale domain motion of a DNA repair protein and its interaction with individual DNA substrates. The strategy was applied to XPD, an FeS cluster-containing DNA repair helicase. Conformational dynamics was assessed via FeS-mediated quenching of a fluorophore site-specifically incorporated into XPD. Simultaneously, binding of DNA molecules labeled with a spectrally distinct fluorophore was detected by colocalization of the DNA- and protein-derived signals. We show that XPD undergoes thermally driven conformational transitions that manifest in spatial separation of its two auxiliary domains. DNA binding does not strictly enforce a specific conformation. Interaction with a cognate DNA damage, however, stabilizes the compact conformation of XPD by increasing the weighted average lifetime of this state by 140% relative to an undamaged DNA. Our imaging strategy will be a valuable tool to study other FeS-containing nucleic acid processing enzymes.
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Affiliation(s)
- Mohamed Ghoneim
- Center
for Biophysics and Computational Biology, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States
- Department
of Biochemistry, University of Iowa, Iowa City, Iowa 52242, United States
| | - Maria Spies
- Department
of Biochemistry, University of Iowa, Iowa City, Iowa 52242, United States
- E-mail: . Phone +1-319-335-3221
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60
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Bharti SK, Khan I, Banerjee T, Sommers JA, Wu Y, Brosh RM. Molecular functions and cellular roles of the ChlR1 (DDX11) helicase defective in the rare cohesinopathy Warsaw breakage syndrome. Cell Mol Life Sci 2014; 71:2625-39. [PMID: 24487782 PMCID: PMC4537069 DOI: 10.1007/s00018-014-1569-4] [Citation(s) in RCA: 41] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/26/2013] [Revised: 01/07/2014] [Accepted: 01/16/2014] [Indexed: 02/08/2023]
Abstract
In 2010, a new recessive cohesinopathy disorder, designated Warsaw breakage syndrome (WABS), was described. The individual with WABS displayed microcephaly, pre- and postnatal growth retardation, and abnormal skin pigmentation. Cytogenetic analysis revealed mitomycin C (MMC)-induced chromosomal breakage; however, an additional sister chromatid cohesion defect was also observed. WABS is genetically linked to bi-allelic mutations in the ChlR1/DDX11 gene which encodes a protein of the conserved family of Iron-Sulfur (Fe-S) cluster DNA helicases. Mutations in the budding yeast ortholog of ChlR1, known as Chl1, were known to cause sister chromatid cohesion defects, indicating a conserved function of the gene. In 2012, three affected siblings were identified with similar symptoms to the original WABS case, and found to have a homozygous mutation in the conserved Fe-S domain of ChlR1, confirming the genetic linkage. Significantly, the clinically relevant mutations perturbed ChlR1 DNA unwinding activity. In addition to its genetic importance in human disease, ChlR1 is implicated in papillomavirus genome maintenance and cancer. Although its precise functions in genome homeostasis are still not well understood, ongoing molecular studies of ChlR1 suggest the helicase plays a critically important role in cellular replication and/or DNA repair.
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Affiliation(s)
- Sanjay Kumar Bharti
- Laboratory of Molecular Gerontology, National Institute on Aging, National Institutes of Health, NIH Biomedical Research Center, Baltimore, MD 21224 USA
| | - Irfan Khan
- Laboratory of Molecular Gerontology, National Institute on Aging, National Institutes of Health, NIH Biomedical Research Center, Baltimore, MD 21224 USA
| | - Taraswi Banerjee
- Laboratory of Molecular Gerontology, National Institute on Aging, National Institutes of Health, NIH Biomedical Research Center, Baltimore, MD 21224 USA
| | - Joshua A. Sommers
- Laboratory of Molecular Gerontology, National Institute on Aging, National Institutes of Health, NIH Biomedical Research Center, Baltimore, MD 21224 USA
| | - Yuliang Wu
- Department of Biochemistry, University of Saskatchewan, Saskatoon, SK S7N 5E5 Canada
| | - Robert M. Brosh
- Laboratory of Molecular Gerontology, National Institute on Aging, National Institutes of Health, NIH Biomedical Research Center, Baltimore, MD 21224 USA
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61
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Koh HR, Xing L, Kleiman L, Myong S. Repetitive RNA unwinding by RNA helicase A facilitates RNA annealing. Nucleic Acids Res 2014; 42:8556-64. [PMID: 24914047 PMCID: PMC4117756 DOI: 10.1093/nar/gku523] [Citation(s) in RCA: 32] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/04/2023] Open
Abstract
Helicases contribute to diverse biological processes including replication, transcription and translation. Recent reports suggest that unwinding of some helicases display repetitive activity, yet the functional role of the repetitiveness requires further investigation. Using single-molecule fluorescence assays, we elucidated a unique unwinding mechanism of RNA helicase A (RHA) that entails discrete substeps consisting of binding, activation, unwinding, stalling and reactivation stages. This multi-step process is repeated many times by a single RHA molecule without dissociation, resulting in repetitive unwinding/rewinding cycles. Our kinetic and mutational analysis indicates that the two double stand RNA binding domains at the N-terminus of RHA are responsible for such repetitive unwinding behavior in addition to providing an increased binding affinity to RNA. Further, the repetitive unwinding induces an efficient annealing of a complementary RNA by making the unwound strand more accessible. The complex and unusual mechanism displayed by RHA may help in explaining how the repetitive unwinding of helicases contributes to their biological functions.
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Affiliation(s)
- Hye Ran Koh
- Department of Physics, University of Illinois, Urbana, IL 61801, USA Institute for Genomic Biology, University of Illinois, Urbana, IL 61801, USA
| | - Li Xing
- Lady Davis Institute for Medical Research and McGill AIDS Centre, Jewish General Hospital, Montreal, Quebec, H3T 1E2, Canada Department of Medicine, McGill University, Montreal, Quebec, H3G 1Y6, Canada
| | - Lawrence Kleiman
- Lady Davis Institute for Medical Research and McGill AIDS Centre, Jewish General Hospital, Montreal, Quebec, H3T 1E2, Canada Department of Medicine, McGill University, Montreal, Quebec, H3G 1Y6, Canada
| | - Sua Myong
- Institute for Genomic Biology, University of Illinois, Urbana, IL 61801, USA Department of Bioengineering, University of Illinois, Urbana, IL 61801, USA Physics Frontier Center (Center of Physics for Living Cells), University of Illinois, Urbana, IL 61801, USA Biophysics and Computational Biology, 1110 W. Green St., Urbana, IL 61801, USA
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62
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Hughes CD, Simons M, Mackenzie CE, Van Houten B, Kad NM. Single molecule techniques in DNA repair: a primer. DNA Repair (Amst) 2014; 20:2-13. [PMID: 24819596 DOI: 10.1016/j.dnarep.2014.02.003] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/31/2014] [Accepted: 02/03/2014] [Indexed: 10/25/2022]
Abstract
A powerful new approach has become much more widespread and offers insights into aspects of DNA repair unattainable with billions of molecules. Single molecule techniques can be used to image, manipulate or characterize the action of a single repair protein on a single strand of DNA. This allows search mechanisms to be probed, and the effects of force to be understood. These physical aspects can dominate a biochemical reaction, where at the ensemble level their nuances are obscured. In this paper we discuss some of the many technical advances that permit study at the single molecule level. We focus on DNA repair to which these techniques are actively being applied. DNA repair is also a process that encompasses so much of what single molecule studies benefit--searching for targets, complex formation, sequential biochemical reactions and substrate hand-off to name just a few. We discuss how single molecule biophysics is poised to transform our understanding of biological systems, in particular DNA repair.
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Affiliation(s)
- Craig D Hughes
- School of Biological Sciences, University of Essex, Wivenhoe Park, Colchester CO4 3SQ, UK
| | - Michelle Simons
- School of Biological Sciences, University of Essex, Wivenhoe Park, Colchester CO4 3SQ, UK
| | - Cassidy E Mackenzie
- School of Biological Sciences, University of Essex, Wivenhoe Park, Colchester CO4 3SQ, UK
| | - Bennett Van Houten
- Department of Pharmacology and Chemical Biology, University of Pittsburgh Cancer Institute, University of Pittsburgh, Pittsburgh, PA, USA
| | - Neil M Kad
- School of Biological Sciences, University of Essex, Wivenhoe Park, Colchester CO4 3SQ, UK.
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63
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Single-molecule fluorescence reveals the unwinding stepping mechanism of replicative helicase. Cell Rep 2014; 6:1037-1045. [PMID: 24630993 PMCID: PMC3988844 DOI: 10.1016/j.celrep.2014.02.022] [Citation(s) in RCA: 45] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/26/2013] [Revised: 01/10/2014] [Accepted: 02/16/2014] [Indexed: 11/21/2022] Open
Abstract
Bacteriophage T7 gp4 serves as a model protein for replicative helicases that couples deoxythymidine triphosphate (dTTP) hydrolysis to directional movement and DNA strand separation. We employed single-molecule fluorescence resonance energy transfer methods to resolve steps during DNA unwinding by T7 helicase. We confirm that the unwinding rate of T7 helicase decreases with increasing base pair stability. For duplexes containing >35% guanine-cytosine (GC) base pairs, we observed stochastic pauses every 2–3 bp during unwinding. The dwells on each pause were distributed nonexponentially, consistent with two or three rounds of dTTP hydrolysis before each unwinding step. Moreover, we observed backward movements of the enzyme on GC-rich DNAs at low dTTP concentrations. Our data suggest a coupling ratio of 1:1 between base pairs unwound and dTTP hydrolysis, and they further support the concept that nucleic acid motors can have a hierarchy of different-sized steps or can accumulate elastic energy before transitioning to a subsequent phase. Single DNA unwinding assay recapitulates sequence-dependent unwinding High-resolution data reveal an unwinding step size of 2–3 bp Two or three hidden steps precede the unwinding step, suggesting 1:1 chemical coupling 1:1 coupling is maintained at low dNTP, but helicase often slips backward
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64
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Spies M. Two steps forward, one step back: determining XPD helicase mechanism by single-molecule fluorescence and high-resolution optical tweezers. DNA Repair (Amst) 2014; 20:58-70. [PMID: 24560558 DOI: 10.1016/j.dnarep.2014.01.013] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/13/2013] [Revised: 12/31/2013] [Accepted: 01/13/2014] [Indexed: 11/17/2022]
Abstract
XPD-like helicases constitute a prominent DNA helicase family critical for many aspects of genome maintenance. These enzymes share a unique structural feature, an auxiliary domain stabilized by an iron-sulphur (FeS) cluster, and a 5'-3' polarity of DNA translocation and duplex unwinding. Biochemical analyses alongside two single-molecule approaches, total internal reflection fluorescence microscopy and high-resolution optical tweezers, have shown how the unique structural features of XPD helicase and its specific patterns of substrate interactions tune the helicase for its specific cellular function and shape its molecular mechanism. The FeS domain forms a duplex separation wedge and contributes to an extended DNA binding site. Interactions within this site position the helicase in an orientation to unwind the duplex, control the helicase rate, and verify the integrity of the translocating strand. Consistent with its cellular role, processivity of XPD is limited and is defined by an idiosyncratic stepping kinetics. DNA duplex separation occurs in single base pair steps punctuated by frequent backward steps and conformational rearrangements of the protein-DNA complex. As such, the helicase in isolation mainly stabilizes spontaneous base pair opening and exhibits a limited ability to unwind stable DNA duplexes. The presence of a cognate ssDNA binding protein converts XPD into a vigorous helicase by destabilizing the upstream dsDNA as well as by trapping the unwound strands. Remarkably, the two proteins can co-exist on the same DNA strand without competing for binding. The current model of the XPD unwinding mechanism will be discussed along with possible modifications to this mechanism by the helicase interacting partners and unique features of such bio-medically important XPD-like helicases as FANCJ (BACH1), RTEL1 and CHLR1 (DDX11).
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Affiliation(s)
- Maria Spies
- Department of Biochemistry, University of Iowa Carver College of Medicine, IA 52242, United States.
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65
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Abstract
A prerequisite for DNA replication is the unwinding of duplex DNA catalyzed by a replicative hexameric helicase. Despite a growing body of research, key elements of helicase mechanism remain under substantial debate. In particular, the number of DNA strands encircled by the helicase ring during unwinding and the ring orientation at the replication fork completely contrast in contemporary mechanistic models. Here we use single-molecule and ensemble assays to address these questions for the papillomavirus E1 helicase. We find that E1 unwinds DNA with a strand-exclusion mechanism, with the N-terminal side of the helicase ring facing the replication fork. We show that E1 generates strikingly heterogeneous unwinding patterns stemming from varying degrees of repetitive movements, which is modulated by the DNA-binding domain. Together, our studies reveal previously unrecognized dynamic facets of replicative helicase unwinding mechanisms.
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66
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Jackson RN, Lavin M, Carter J, Wiedenheft B. Fitting CRISPR-associated Cas3 into the helicase family tree. Curr Opin Struct Biol 2014; 24:106-14. [PMID: 24480304 DOI: 10.1016/j.sbi.2014.01.001] [Citation(s) in RCA: 53] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/06/2013] [Revised: 12/30/2013] [Accepted: 01/04/2014] [Indexed: 01/03/2023]
Abstract
Helicases utilize NTPs to modulate their binding to nucleic acids and many of these enzymes also unwind DNA or RNA duplexes in an NTP-dependent fashion. These proteins are phylogenetically related but functionally diverse, with essential roles in virtually all aspects of nucleic acid metabolism. A new class of helicases associated with RNA-guided adaptive immune systems in bacteria and archaea has recently been identified. Prokaryotes acquire resistance to invading genetic parasites by integrating short fragments of foreign nucleic acids into repetitive loci in the host chromosome known as CRISPRs (Clustered Regularly Interspaced Short Palindromic Repeats). CRISPR-associated gene 3 (cas3) encodes a conserved helicase protein that is essential for phage defense. Here we review recent advances in Cas3 biology, and provide a new phylogenetic framework that positions Cas3 in the helicase family tree. We anticipate that this Cas3 phylogeny will guide future biochemical and structural studies.
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Affiliation(s)
- Ryan N Jackson
- Department of Microbiology and Immunology, Montana State University, Bozeman, MT 59718, United States
| | - Matthew Lavin
- Department of Plant Sciences and Plant Pathology, Montana State University, Bozeman, MT 59718, United States
| | - Joshua Carter
- Department of Microbiology and Immunology, Montana State University, Bozeman, MT 59718, United States
| | - Blake Wiedenheft
- Department of Microbiology and Immunology, Montana State University, Bozeman, MT 59718, United States.
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