Recognition and Binding of Human Telomeric G‑Quadruplex DNA by Unfolding Protein 1

• Published in: Biochemistry (Easton) Vol. 53; no. 20; pp. 3347 - 3356
• Main Authors: Hudson, Jason S, Ding, Lei, Le, Vu, Lewis, Edwin, Graves, David
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 27.05.2014
• Subjects: G-Quadruplexes; Heterogeneous Nuclear Ribonucleoprotein A1; Heterogeneous-Nuclear Ribonucleoprotein Group A-B - chemistry; Heterogeneous-Nuclear Ribonucleoprotein Group A-B - genetics; Heterogeneous-Nuclear Ribonucleoprotein Group A-B - metabolism; Humans; Potassium - chemistry; Protein Binding - physiology; Protein Unfolding; Sodium - chemistry; Telomere - chemistry; Telomere - metabolism
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi500351u

The specific recognition by proteins of G-quadruplex structures provides evidence of a functional role for in vivo G-quadruplex structures. As previously reported, the ribonucleoprotein, hnRNP Al, and it is proteolytic derivative, unwinding protein 1 (UP1), bind to and destabilize G-quadruplex structures formed by the human telomeric repeat d­(TTAGGG) n . UP1 has been proposed to be involved in the recruitment of telomerase to telomeres for chain extension. In this study, a detailed thermodynamic characterization of the binding of UP1 to a human telomeric repeat sequence, the d­[AG­G­G­(TT­A­G­GG)3] G-quadruplex, is presented and reveals key insights into the UP1-induced unfolding of the G-quadruplex structure. The UP1–G-quadruplex interactions are shown to be enthalpically driven, exhibiting large negative enthalpy changes for the formation of both the Na+ and K+ G-quadruplex–UP1 complexes (ΔH values of −43 and −19 kcal/mol, respectively). These data reveal three distinct enthalpic contributions from the interactions of UP1 with the Na+ form of G-quadruplex DNA. The initial interaction is characterized by a binding affinity of 8.5 × 108 M–1 (strand), 200 times stronger than the binding of UP1 to a single-stranded DNA with a comparable but non-quadruplex-forming sequence [4.1 × 106 M–1 (strand)]. Circular dichroism spectroscopy reveals the Na+ form of the G-quadruplex to be completely unfolded by UP1 at a binding ratio of 2:1 (UP1:G-quadruplex DNA). The data presented here demonstrate that the favorable energetics of the initial binding event are closely coupled with and drive the unfolding of the G-quadruplex structure.