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G-quadruplexes (G4) are non-canonical secondary structures consisting in stacked tetrads of hydrogen-bonded guanines bases. An essential feature of G4 is their intrinsic polymorphic nature, which is characterized by the equilibrium between several conformations (also called topologies) and the presence of different types of loops with variable lengths. In cells, G4 functions rely on protein or enzymatic factors that recognize and promote or resolve these structures. In order to characterize new G4-dependent mechanisms, extensive researches aimed at identifying new G4 binding proteins. Using G-rich single-stranded oligonucleotides that adopt non-controlled G4 conformations, a large number of G4-binding proteins have been identified in vitro, but their specificity towards G4 topology remained unknown. Constrained G4 structures are biomolecular objects based on the use of a rigid cyclic peptide scaffold as a template for directing the intramolecular assembly of the anchored oligonucleotides into a single and stabilized G4 topology. Here, using various constrained RNA or DNA G4 as baits in human cell extracts, we establish the topology preference of several well-known G4-interacting factors. Moreover, we identify new G4-interacting proteins such as the NELF complex involved in the RNA-Pol II pausing mechanism, and we show that it impacts the clastogenic effect of the G4-ligand pyridostatin. [ABSTRACT FROM AUTHOR]

Low-dose hyper-radiosensitivity (HRS) (below 0.5 Gy) has been extensively documented in the past few years. The molecular basis of this phenomenon remains largely unknown and the purpose of this study was to investigate the possible implication of the DNA repair DNA-PK complex. The activity of the DNA-PK complex, i.e. Ku DNA-end binding activity and kinase activity of the whole complex, was studied in 10 human cancer cell lines, 2 h after 0.2, 0.5 and 1 Gy irradiation. After low-dose irradiation (0.2 Gy), a marked decrease in DNA-PK activity was found in all six cell lines exhibiting HRS, whereas the DNA-PK activity was increased in the four cell lines which did not exhibit HRS. This modulation of DNA-PK activity was a rapid phenomenon occurring within the 2 h following low-dose radiation exposure. These data strongly suggest the implication of the DNA-PK repair complex in the HRS phenomenon. [ABSTRACT FROM AUTHOR]