Molecular Events behind the Selectivity and Inactivation Properties of Model NaK-Derived Ion Channels

Area of Science:

• Biophysics
• Ion channel function
• Molecular dynamics

Background:

• Non-selective NaK and partly K+-selective NaK2K channels play crucial roles in cellular ion transport.
• Understanding the conformational dynamics of these channels is key to elucidating their ion selectivity mechanisms.

Purpose of the Study:

• To investigate the pore conformational flexibility of NaK and NaK2K channels in response to Na+ and K+ binding.
• To elucidate the molecular basis for the poor ion selectivity observed in these channels.

Main Methods:

• Utilized Y55W mutants of NaK and NaK2K channels.
• Employed Homo-Förster Resonance Energy Transfer (FRET) measurements to assess intersubunit distances.
• Conducted ion binding experiments.

Main Results:

• Demonstrated remarkable pore conformational flexibility in NaK and NaK2K channels.
• Observed distinct selectivity filter (SF) conformations: wide-open in Na+ and tight induced-fit in K+.
• Identified altered interactions between the SF and protein scaffold contributing to flexibility.
• Binding experiments revealed low K+ affinity and lack of Na+-induced collapse, explaining poor selectivity.

Conclusions:

• NaK and NaK2K channels exhibit significant conformational flexibility, allowing them to accommodate different cations.
• This flexibility underlies their inability to effectively select K+ over Na+.
• The channels lack C-type inactivation due to sustained K+ binding, leading to permanently open states.