A Dual-App Nucleoside Probe Provides Structural Insights into the Human Telomeric Overhang in Live Cells.

Understanding the topology adopted by individual G-quadruplex (GQ)-forming sequences in vivo and targeting a specific GQ motif among others in the genome will have a profound impact on GQ-directed therapeutic strategies. However, this remains a major challenge as most of the tools poorly distinguish different GQ conformations and are not suitable for both cell-free and in-cell analysis. Here, we describe an innovative probe design to investigate GQ conformations and recognition in both cell-free and native cellular environments by using a conformation-sensitive dual-app nucleoside analogue probe. The nucleoside probe, derived by conjugating fluorobenzofuran at the 5-position of 2'-deoxyuridine, is composed of a microenvironment-sensitive fluorophore and an in-cell NMR compatible 19 F label. This noninvasive nucleoside, incorporated into the human telomeric DNA oligonucleotide repeat, serves as a common probe to distinguish different GQ topologies and quantify topology-specific binding of ligands by fluorescence and NMR techniques. Importantly, unique signatures displayed by the 19 F-labeled nucleoside for different GQs enabled a systematic study in Xenopus laevis oocytes to provide new structural insights into the GQ topologies adopted by human telomeric overhang in cells, which so far has remained unclear. Studies using synthetic cell models, immunostaining on fixed cells, and crystallization conditions suggest that parallel GQ is the preferred conformation of telomeric DNA repeat. However, our findings using the dual-app probe clearly indicate that multiple structures including hybrid-type parallel-antiparallel and parallel GQs are formed in the cellular environment. Taken together, our findings open new experimental strategies to investigate topology, recognition, and therapeutic potential of individual GQ-forming motifs in a biologically relevant context.