Dynamic Change of Electrostatic Field in TMEM16F Permeation Pathway Shifts Its Ion Selectivity

TMEM16F is activated by elevated intracellular Ca²⁺, and functions both as a small-conductance ion channel permeable to Ca²⁺ and as a phospholipid scramblase. It is important to hold this positive feedback in check to prevent prolonged Ca²⁺-overloading in cells. We hypothesize that TMEM16F shifts its ion selectivity so that it is more permeable to Cl⁻ than cations at high intracellular Ca²⁺ concentration. We tested this hypothesis with the Q559K mutant that shows no current rundown in excised patch with prolonged Ca²⁺ elevation. Recorded in NaCl⁻based solution, the channel shifted its ion selectivity from Na⁺-selective to Cl⁻-selective when intracellular Ca²⁺ was increased. The ion selectivity switch did not correlate with changes of channel open state. Rather, it was indicative of an alteration of electrostatic field in the permeation pathway. Shifting ion-selectivity synergistically by intracellular divalent ions and membrane potential could work as a built-in mechanism that allows TMEM16F to brake the positive feedback.