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Potassium channels maintain selectivity through a constrained filter conformation stabilized by potassium ions. Ion concentration and mutations dynamically alter this structure, impacting ion permeability and selectivity.

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Area of Science:

  • Biophysics
  • Structural Biology
  • Ion Channel Function

Background:

  • Potassium channels are crucial for cellular function, exhibiting high selectivity for K+ over Na+.
  • Archetypal selectivity filter (SF) structures show perfect coordination of dehydrated K+ ions.
  • Understanding the dynamic conformational changes in SFs is key to explaining ion selectivity.

Purpose of the Study:

  • To investigate the conformational dynamics of the selectivity filter loop (SF-loop) in KirBac1.1.
  • To determine the role of ion concentration and specific ions (K+, Na+) in stabilizing SF conformations.
  • To elucidate the structural basis for K+ selectivity and the anomalous mole fraction effect.

Main Methods:

  • Single-molecule Förster Resonance Energy Transfer (smFRET) to monitor SF-loop dynamics.
  • Ion flux assays using radioactive isotopes (86Rb+ and 22Na+) to measure ion permeability.
  • Site-directed mutagenesis to alter channel selectivity.

Main Results:

  • KirBac1.1 SF-loop transitions between constrained and dilated conformations based on ion concentration.
  • K+ ions stabilize the constrained conformation essential for selectivity; Na+ does not.
  • Mutations causing non-selectivity lead to unstable, dilated conformations.
  • Differential ion fluxes (86Rb+ vs. 22Na+) observed under K+ and Na+ gradients, respectively.

Conclusions:

  • The dynamic conformational states of the SF-loop are critical for potassium channel selectivity.
  • Ion-dependent stabilization of the constrained SF conformation underlies K+ selectivity.
  • Observed differential ion selectivity in distinct conformational states provides a structural basis for the anomalous mole fraction effect.