TMEM16F is a component of a Ca²⁺-activated Cl^- channel but not a volume-sensitive outwardly rectifying Cl^- channel

TMEM16 (transmembrane protein 16) proteins, which possess eight putative transmembrane domains with intracellular NH2- and COOH-terminal tails, are thought to comprise a Cl- channel family. The function of TMEM16F, a member of the TMEM16 family, has been greatly controversial. In the present study, we performed whole cell patch-clamp recordings to investigate the function of human TMEM16F. In TMEMl6F-transfected HEK293T cells but not TMEM16K- and mock-transfected cells, activation of membrane currents with strong outward rectification was found to be induced by application of a Ca²⁺ ionophore, ionomycin, or by an increase in the intracellular free Ca²⁺ concentration. The free Ca²⁺ concentration for half-maximal activation of TMEM16F currents was 9.6 μM, which is distinctly higher than that for TMEM16A/B currents. The outwardly rectifying current-voltage relationship for TMEM16F currents was not changed by an increase in the intracellular Ca²⁺ level, in contrast to TMEM16A/B currents. The Ca²⁺-activated TMEM16F currents were anion selective, because replacing Cl- with aspartate- in the bathing solution without changing cation concentrations caused a positive shift of the reversal potential. The anion selectivity sequence of the TMEM16F channel was I > Br- > Cl- > F > aspartate-. Niflumic acid, a Ca²⁺-activated Cl- channel blocker, inhibited the TMEMl6F-dependent Cl- currents. Neither overexpression nor knockdown of TMEM16F affected volume-sensitive outwardly rectifying Cl- channel (VSOR) currents activated by osmotic swelling or apoptotic stimulation. These results demonstrate that human TMEM16F is an essential component of a Ca²⁺-activated Cl- channel with a Ca²⁺ sensitivity that is distinct from that of TMEM16A/B and that it is not related to VSOR activity.