title: Potassium-induced loop conformational transition of a potent anti-HIV oligonucleotide.
abstract: Spectroscopic, thermal denaturation and kinetic studies have revealed that DNA oligonucleotides 5'-d(GGGTGGGTGGGTGGGT) (T30695) and 5'-d(GTGGTGGGTGGGTGGGT) (T30177) from extremely stable intramolecular G-tetrads via a two-step process that involves the binding of one K+ ion to a central pair of G-quartets and two additional K+ ions, presumably, to the loops (Jing et al., (1997) Biochemistry in press). In that these oligonucleotides are potent HIV-1 inhibitors and among the most active HIV-1 integrase inhibitors yet identified, we have sought to further characterize the K(+)-induced folding process for the purpose of rational chemical modification of these anti-HIV agents. Our NMR investigation demonstrates that in the presence of Li+ ions, T30695 forms an unimolecular tetrad fold, stabilized by a pair of syn-anti-syn-anti G-quartets comprising a central core. The NMR spectrum of T30695 as a function of K+ titration reveals a well-defined transition that saturates upon addition of three K+ ions per oligomer. During this process, the initial Li(+)-dependent G-quartet structure converts into a highly symmetrical, stable form (the NMR detected melting transition temperature is increased by approximately 20 degrees C). The conformation of the G-quartet core remains unchanged, while the loosely structured loop residues become organized in a fashion which is stabilized by K+ ion binding and by interactions with the core. To explain these data, we propose a model wherein K+ binding to the loops induces structural rearrangement, to yield a planar array of loop bases in proximity to the underlying G-quartets. By reference to closely related homologues, which lack activity as an HIV-1 or integrase inhibitor, the possibility is discussed that this ion-coordinated loop structure is crucial to the biological activity of T30695.
authors.affiliation: Department of Molecular Physiology and Biophysics, Baylor College of Medicine, Houston, TX 77030, USA. njing@bcm.tmc.edu
authors.forename: N
authors.initials: N
authors.lastname: Jing
authors.forename: X
authors.initials: X
authors.lastname: Gao
authors.forename: R F
authors.initials: RF
authors.lastname: Rando
authors.forename: M E
authors.initials: ME
authors.lastname: Hogan
mesh_terms: D019380:Anti-HIV Agents / Q000737:chemistry*; D019428:HIV Integrase Inhibitors / Q000737:chemistry*; D006801:Humans; D007477:Ions; D008094:Lithium; D019906:Nuclear Magnetic Resonance, Biomolecular; D009690:Nucleic Acid Conformation; D009838:Oligodeoxyribonucleotides / Q000737:chemistry*; D011188:Potassium
chemical_list: D019380:Anti-HIV Agents; D019428:HIV Integrase Inhibitors; D007477:Ions; D009838:Oligodeoxyribonucleotides; D008094:Lithium; D011188:Potassium