Human telomeric DNA consists of tandem repeats of the sequence 5-TTAGGG-3, including a 3 terminal single-stranded overhang of 100C200 nucleotides that can fold into quadruplex structures in the presence of suitable metal ions. lay the foundation for utilizing the -HL ion channel as a simple tool for monitoring how small molecules and physical context shift the equilibrium between the many G-quadruplex folds of the human telomere sequence. Introduction The human telomere is composed of a telomere-specific protein complex and long stretches of a repetitive sequence, 5-TTAGGG-3, that is terminated with a 100C200 nucleotide single-stranded overhang.1C3 This complex protects the ends of linear chromosomes from deterioration via the DNA damage response pathway.4 Telomere length regulation is complicated by the end-replication problem that results from incomplete telomerase activity and homologous recombination.5,6 Despite the cells incessant efforts to maintain the telomere structure, they shorten with age as a result of chronic stress, oxidation and inflammation.7,8 Consequently, telomere length serves as a molecular clock for cellular lifespan, and severely truncated ends lead to cell senescence and death.9,10 Furthermore, ~85% of human cancer cells maintain excellent telomeres by means of upregulating telomerase providing a state of immortality.11,12 This discovery highlights telomere maintenance in cancer cells as a ubiquitous druggable target with high potential for controlling this disease.13,14 Essential to telomere function, the 3 terminal single-stranded overhang can be hidden by forming higher order secondary structures Boceprevir that interact with various proteins to retain genomic stability.15,16 One of the proposed secondary Boceprevir structures is folding of the overhang to a G-quadruplex, which features Hoogsteen hydrogen-bonded assemblies of four guanine bases (G-tetrad) with adjacent G-tetrads assembled together by – stacking interactions and coordination of appropriate cations within the quadruplex channel.17 Recent studies have detected G-quadruplexes in mammalian cells and have confirmed that small molecules do bind these structures in vivo with high affinity.18 The G-quadruplexes that fold from the human telomere sequence Boceprevir can adopt myriad structures that are dependent on the cation, medium, and the presence or absence of overhanging ends.19 Cells maintain K+ cations in the highest concentration (140 mM);20 therefore, G-quadruplex folds observed in the presence of K+ are thought to dominate in vivo. The truncated human telomere sequence 5-TAGGG(TTAGGG)3-3 folds unimolecularly in an antiparallel fashion to yield a three-tetrad G-quadruplex with two K+ ions coordinated within the interior channel. NMR analysis has characterized the dominant fold to have a lateral loop closest to the 5-end, followed by two edgewise loops; this topology has been termed the hybrid-1 fold (Figure 1a). However, in the natural human telomere sequence 5-TAGGG(TTAGGG)3TT-3 that includes a 3 TT overhang, the hybrid-2 conformation is thought to dominate according to NMR studies.21C24 The key difference for this fold lies in the loop orientation, in which the lateral loop is closest to the 3-end (Figure 1a). Biophysical studies typically avoid using telomere sequences that contain a 3-TT tail to simplify the model system, because this tail causes increased structural diversity.22 Boceprevir Furthermore, these hybrid structures are polymorphic and interconvertible in solution due to the small energetic differences between them.25 Moreover, a significant amount of a triplex-folded intermediate has recently been observed,25,26 which has compounded the challenge in studying the human telomere sequence. In a last set of studies concerning the structure of the human telomere sequence by X-ray crystallography and by NMR in solutions with dehydrating agents the propeller fold was observed.17,27 This fold is unimolecular with parallel strands and all G nucleotides adopting the conformation that forces all three loops to be oriented laterally. Figure 1 Structures of the G-quadruplexes formed by the human telomere sequence The -hemolysin (-HL) ion channel-forming protein has advanced as a potential next-generation DNA sequencing platform because of its ability to thread single-stranded, but not double-stranded, DNA through its narrow -barrel.28C33 Additionally, it has been used as a tool to provide a wealth of biophysical data concerning DNA and RNA.34C39 By monitoring the duration and current signatures while electrically drawing the DNA molecules through the nanopore ion channel embedded in a lipid bilayer, the kinetics of the duplex unzipping processes and DNA-protein interactions can be evaluated. Recently, Shim, et al. reported studies of the thrombin binding aptamer, which is a simple G-quadruplex comprising one cation Rabbit Polyclonal to Shc (phospho-Tyr349). and two tetrads, by encapsulating it in the -HL nanocavity.40,41 Furthermore, Rotem, et al. utilized Boceprevir an -HL equipped with the TBA to interrogate the interactions between thrombin and its aptamer as a protein detection platform that also provided thermodynamic and kinetic properties.
Human telomeric DNA consists of tandem repeats of the sequence 5-TTAGGG-3,