Sodium Ions Do Not Stabilize the Selectivity Filter of a Potassium Channel

•The non-selective ion channel NaK has multiple stable selectivity filter conformations.•The selectivity filter of NaK adapts to the K+ or Na+ ions present.•K+-selective mutants of NaK lose selectivity filter stability under Na+ conditions.•Entire pore region of K+-selective mutants shows ion dependent structural changes.•K+-selective mutants exhibit stronger link between selectivity filter and pore helix.