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Human POT1 unfolds G-quadruplexes by conformational selection.

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The shelterin protein POT1 (Protection of Telomeres 1) unfolds telomeric G-quadruplex structures via a conformational selection mechanism, coupled with obligatory unfolding, as revealed by kinetic and thermodynamic studies.

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Area of Science:

  • Biochemistry
  • Molecular Biology
  • Structural Biology

Background:

  • Telomeres protect chromosome ends, and their G-rich sequences can form G-quadruplex structures.
  • The shelterin protein POT1 (Protection of Telomeres 1) plays a crucial role in telomere maintenance.
  • The precise mechanism by which POT1 interacts with and unfolds G-quadruplex structures remains unclear.

Purpose of the Study:

  • To elucidate the reaction mechanism of POT1 unfolding human telomeric G-quadruplexes.
  • To characterize the kinetics and thermodynamics of POT1-G-quadruplex interactions.
  • To determine the structural basis of POT1's interaction with telomeric DNA.

Main Methods:

  • Stopped-flow kinetics and spectroscopic titrations.
  • Isothermal titration calorimetry (ITC).
  • Fluorescence, circular dichroism (CD), and analytical ultracentrifugation (AUC).
  • Molecular dynamics (MD) simulations.

Main Results:

  • POT1 binding to single-stranded DNA is fast and strong, while binding to pre-folded G-quadruplexes is significantly slower.
  • POT1 binding is coupled to G-quadruplex unfolding, forming a 2:1 POT1:DNA complex.
  • A conformational selection model accurately describes the experimental binding data.
  • POT1 specifically unfolds and binds various telomeric G-quadruplex conformations but not duplex DNA.

Conclusions:

  • The most plausible mechanism for POT1 unfolding telomeric G-quadruplexes is conformational selection coupled to unfolding.
  • POT1 exhibits high specificity for telomeric G-quadruplex structures over other DNA forms.
  • MD simulations provide a structural model consistent with experimental findings, detailing the 2:1 POT1:DNA complex.