The solution structures of higher-order human telomere G-quadruplex multimers

Human telomeres contain the repeat DNA sequence 5'-d(TTAGGG), with duplex regions that are several kilobases long terminating in a 3' single-stranded overhang. The structure of the single-stranded overhang is not known with certainty, with disparate models proposed in the literature. We report here the results of an integrated structural biology approach that combines small-angle X-ray scattering, circular dichroism (CD), analytical ultracentrifugation, sizeexclusion column chromatography and molecular dynamics simulations that provide the most detailed characterization to date of the structure of the telomeric overhang. We find that the single-stranded sequences 5'-d(TTAGGG)(n), with n = 8, 12 and 16, fold into multimeric structures containing the maximal number (2, 3 and 4, respectively) of contiguous G4 units with no long gaps between units. The G4 units are a mixture of hybrid-1 and hybrid-2 conformers. In the multimeric structures, G4 units interact, at least transiently, at the interfaces between units to produce distinctive CD signatures. Global fitting of our hydrodynamic and scattering data to a worm-like chain (WLC) model indicates that these multimeric G4 structures are semi-flexible, with a persistence length of similar to 34 angstrom. Investigations of its flexibility using MD simulations reveal stacking, unstacking, and coiling movements, which yield unique sites for drug targeting.

Automated summary

The study used an integrated structural biology approach to characterize the structure of telomeric overhang, revealing that single-stranded sequences fold into multimeric structures with a maximum number of G4 units. The flexibility of these structures was investigated through molecular dynamics simulations, identifying unique sites for drug targeting.