Massive surface membrane expansion without involvement of classical exocytic mechanisms.

Activation of TMEM16F, a Ca2+ -dependent ion channel and lipid scramblase, causes massive surface membrane expansion in multiple cell types by unresolved mechanisms. We describe here that membrane expansion reflects opening of deeply invaginating surface membrane compartments when anionic phospholipids are lost from the cytoplasmic membrane leaflet. Compartments that open contain vesicle-associated membrane proteins (VAMPs) and can open with as little as one micromolar free Cai2+. Cationic peptides that sequester anionic phospholipids open the compartments from the cytoplasmic side without Ca2+. Monovalent cations facilitate membrane expansion via coupled permeation with anionic phospholipids through TMEM16F. When monovalent cation concentrations are reduced, membrane expansion can be reversed by changing ion gradients and membrane voltage. Depolarization closes the compartments by generating inward cation gradients through TMEM16F that promote influx of anionic phospholipids. In summary, TMEM16F-mediated membrane expansion likely does not reflect exocytosis but rather the relaxation of constrictions that close surface membrane invaginations.\n\nSummaryThe surface membrane of diverse cell types can be remodeled by opening and closing surface invaginations that are held shut by proteins that bind negatively charged lipids and constrict the orifices of these compartments.