title: Human telomeric sequence forms a hybrid-type intramolecular G-quadruplex structure with mixed parallel/antiparallel strands in potassium solution.
abstract: 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.
authors.affiliation: College of Pharmacy, The University of Arizona, 1703 E. Mabel St, Tucson, AZ 85721, USA.
authors.forename: Attila
authors.initials: A
authors.lastname: Ambrus
authors.forename: Ding
authors.initials: D
authors.lastname: Chen
authors.forename: Jixun
authors.initials: J
authors.lastname: Dai
authors.forename: Tiffanie
authors.initials: T
authors.lastname: Bialis
authors.forename: Roger A
authors.initials: RA
authors.lastname: Jones
authors.forename: Danzhou
authors.initials: D
authors.lastname: Yang
mesh_terms: D002942:Circular Dichroism; D004247:DNA / Q000737:chemistry*; D054856:G-Quadruplexes; D006147:Guanine / Q000737:chemistry*; D006801:Humans; D019906:Nuclear Magnetic Resonance, Biomolecular; D009690:Nucleic Acid Conformation; D011188:Potassium / Q000737:chemistry*; D011522:Protons; D012091:Repetitive Sequences, Nucleic Acid; D012964:Sodium / Q000737:chemistry; D012996:Solutions; D016615:Telomere / Q000737:chemistry*
chemical_list: D011522:Protons; D012996:Solutions; D006147:Guanine; D004247:DNA; D012964:Sodium; D011188:Potassium