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      Crystal structure of a c-kit promoter quadruplex reveals the structural role of metal ions and water molecules in maintaining loop conformation

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          Abstract

          We report here the 1.62 Å crystal structure of an intramolecular quadruplex DNA formed from a sequence in the promoter region of the c-kit gene. This is the first reported crystal structure of a promoter quadruplex and the first observation of localized magnesium ions in a quadruplex structure. The structure reveals that potassium and magnesium ions have an unexpected yet significant structural role in stabilizing particular quadruplex loops and grooves that is distinct from but in addition to the role of potassium ions in the ion channel at the centre of all quadruplex structures. The analysis also shows how ions cluster together with structured water molecules to stabilize the quadruplex arrangement. This particular quadruplex has been previously studied by NMR methods, and the present X-ray structure is in accord with the earlier topology assignment. However, as well as the observations of potassium and magnesium ions, the crystal structure has revealed a highly significant difference in the dimensions of the large cleft in the structure, which is a plausible target for small molecules. This difference can be understood by the stabilizing role of structured water networks.

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          Human telomeric sequence forms a hybrid-type intramolecular G-quadruplex structure with mixed parallel/antiparallel strands in potassium solution

          Attila Ambrus, Ding Chen, Jixun Dai … (2006)
          Human telomeric DNA consists of tandem repeats of the sequence d(TTAGGG). The formation and stabilization of DNA G-quadruplexes in the human telomeric sequence have been shown to inhibit the activity of telomerase, thus the telomeric DNA G-quadruplex has been considered as an attractive target for cancer therapeutic intervention. However, knowledge of the intact human telomeric G-quadruplex structure(s) formed under physiological conditions is a prerequisite for structure-based rational drug design. Here we report the folding structure of the human telomeric sequence in K+ solution determined by NMR. Our results demonstrate a novel, unprecedented intramolecular G-quadruplex folding topology with hybrid-type mixed parallel/antiparallel G-strands. This telomeric G-quadruplex structure contains three G-tetrads with mixed G-arrangements, which are connected consecutively with a double-chain-reversal side loop and two lateral loops, each consisting of three nucleotides TTA. This intramolecular hybrid-type telomeric G-quadruplex structure formed in K+ solution is distinct from those reported on the 22 nt Tel22 in Na+ solution and in crystalline state in the presence of K+, and appears to be the predominant conformation for the extended 26 nt telomeric sequence Tel26 in the presence of K+, regardless of the presence or absence of Na+. Furthermore, the addition of K+ readily converts the Na+-form conformation to the K+-form hybrid-type G-quadruplex. Our results explain all the reported experimental data on the human telomeric G-quadruplexes formed in the presence of K+, and provide important insights for understanding the polymorphism and interconversion of various G-quadruplex structures formed within the human telomeric sequence, as well as the effects of sequence and cations. This hybrid-type G-quadruplex topology suggests a straightforward pathway for the secondary structure formation with effective packing within the extended human telomeric DNA. The hybrid-type telomeric G-quadruplex is most likely to be of pharmacological relevance, and the distinct folding topology of this G-quadruplex suggests that it can be specifically targeted by G-quadruplex interactive small molecule drugs.
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            Solution structure of the human telomeric repeat d[AG3(T2AG3)3] G-tetraplex.

            Jigna Patel, Jian Wang (1993)
            Repeats of Gn sequences are detected as single strand overhangs at the ends of eukaryotic chromosomes together with associated binding proteins. Such telomere sequences have been implicated in the replication and maintenance of chromosomal termini. They may also mediate chromosomal organization and association during meiosis and mitosis. We have determined the three-dimensional solution structure of the human telomere sequence, d[AG3(T2AG3)3] in Na(+)-containing solution using a combined NMR, distance geometry and molecular dynamics approach (including relaxation matrix refinement). The sequence, which contains four AG3 repeats, folds intramolecularly into a G-tetraplex stabilized by three stacked G-tetrads which are connected by two lateral loops and a central diagonal loop. Of the four grooves that are formed, one is wide, two are of medium width and one is narrow. The alignment of adjacent G-G-G segments in parallel generates the two grooves of medium width whilst the antiparallel arrangement results in one wide and one narrow groove. Three of the four adenines stack on top of adjacent G-tetrads while the majority of the thymines sample multiple conformations. The availability of the d[AG3(T2AG3)3] solution structure containing four AG3 human telomeric repeats should permit the rational design of ligands that recognize and bind with specificity and affinity to the individual grooves of the G-tetraplex, as well as to either end containing the diagonal and lateral loops. Such ligands could modulate the equilibrium between folded G-tetraplex structures and their unfolded extended counterparts.
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              The PyMOL Molecular Graphics System

              W. L. Delano, W.L. DeLano, WL DeLano … (2002)
              Article 0 comments Cited 197 times – based on 0 reviews     Review now
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                Author and article information

                Journal
                Journal ID (nlm-ta): Nucleic Acids Res
                Journal ID (iso-abbrev): Nucleic Acids Res
                Journal ID (publisher-id): nar
                Journal ID (hwp): nar
                Title: Nucleic Acids Research
                Publisher: Oxford University Press
                ISSN (Print): 0305-1048
                ISSN (Electronic): 1362-4962
                Publication date Collection: May 2012
                Publication date (Print): May 2012
                Publication date (Electronic): 28 January 2012
                Publication date PMC-release: 28 January 2012
                Volume: 40
                Issue: 10
                Pages: 4691-4700
                Affiliations
                CRUK Biomolecular Structure Group, UCL School of Pharmacy, University College London, 29-39 Brunswick Square, WC1N 1AX, London, UK
                Author notes
                *To whom correspondence should be addressed. Tel: +44 207 753 5969; Fax: +44 207 753 5970; Email: s.neidle@ 123456ucl.ac.uk
                Article
                Publisher ID: gks023
                DOI: 10.1093/nar/gks023
                PMC ID: 3378867
                PubMed ID: 22287624
                SO-VID: 3e8bb805-fc8e-4d11-8838-3c278df19105
                Copyright © © The Author(s) 2012. Published by Oxford University Press.
                License:

                This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License ( http://creativecommons.org/licenses/by-nc/3.0), which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

                History
                Date received : 22 November 2011
                Date revision received : 27 December 2011
                Date accepted : 4 January 2012
                Page count
                Pages: 10
                Categories
                Subject: Structural Biology

                ScienceOpen disciplines: Genetics
                Data availability:
                ScienceOpen disciplines: Genetics

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