Ion Channel Function of Human TMEM16F Is Associated with Phospholipid Transport through Its Subunit Cavity

Transmembrane protein 16F (TMEM16F), identified as the causative gene for Scott syndrome, which causes blood coagulation disorders, is known to function as not only a scramblase that bi-directionally transports phospholipids in the lipid bilayer but also a Ca²⁺-activated ion channel with low intracellular Ca²⁺ sensitivity. However, how the dual functions of TMEM16F are controlled remains poorly understood. In this study, we investigated the properties of amino acid residues in human TMEM16F involved in the linkage between phospholipid and ion transports and the regulation of their transports using flow cytometry and whole-cell patch-clamp recordings. We demonstrated that ion and phospholipid transports induced by elevation of intracellular Ca²⁺ concentration were tightly coupled in human embryonic kidney HEK293T cells overexpressing wild-type TMEM16F or its mutants. Mutations of amino acid residues in the hydrophilic subunit cavity of TMEM16F indicated that both substrates were transported through its subunit cavity. Importantly, the tail current analysis suggests that conformational changes of TMEM16F by the channel gating are required for its phospholipid transport. These results suggest that ion channel activities of human TMEM16F modulate its scramblase activities.