Phospholipid flipping facilitates annexin translocation across membranes

Abstract

Annexins are phospholipid binding proteins that somehow translocate from the inner leaflet of the plasma membrane to the outer leaflet. For example, Annexin A2 is known to localise to the outer leaflet of the plasma membrane (cell surface) where it is involved in plasminogen activation leading to fibrinolysis and cell migration, among other functions. Despite having well described extracellular functions, the mechanism of annexin transport from the cytoplasmic inner leaflet to the extracellular outer leaflet of the plasma membrane remains unclear. Here, we show that phospholipid flipping activity is crucial for the transport of annexins A2 and A5 across membranes in cells and in liposomes. We identified TMEM16F (anoctamin-6) as a lipid scramblase required for transport of these annexins to the outer leaflet of the plasma membrane. This work reveals a mechanism for annexin translocation across membranes which depends on plasma membrane phospholipid flipping.