ATP11C and CDC50A flippase activity prevents phosphatidylserine externalization and phagocytosis.
Major finding: ATP11C and CDC50A flippase activity prevents phosphatidylserine externalization and phagocytosis.
Mechanism: Apoptotic cell engulfment requires flippase inactivation via caspase-driven ATP11C cleavage.
Impact: Phosphatidylserine exposure is sufficient to induce macrophage engulfment of viable cells.
Exposure of phosphatidylserine (PtdSer) on the surface of apoptotic cells is an important signal triggering phagocytosis and clearance of dying cells by macrophages. In healthy cells, PtdSer primarily resides in the inner leaflet of the plasma membrane and its asymmetrical distribution is maintained by flippase activity, which mediates the translocation of PtdSer from the outer to the inner leaflet of the plasma membrane. However, the identity of this flippase and its regulation remain unclear. Using a genetic screen in human cells, Segawa and colleagues identified ATPase type 11C (ATP11C), a member of the type 4 P-type ATPase subfamily, and its β subunit cell division cycle protein 50A (CDC50A, also known as TMEM30A) as PtdSer flippases required for PtdSer internalization at the cell surface. Induction of apoptosis by Fas ligand resulted in cleavage of ATP11C by caspases; mutation of caspase recognition sites in ATP11C prevented its cleavage but did not induce PtdSer exposure or phagocytosis by macrophages, suggesting that inactivation of flippase activity via caspase-mediated cleavage of ATP11C is necessary for PtdSer externalization and engulfment of apoptotic cells. CDC50A functioned as a chaperone for ATP11C, as mutation of CDC50A impaired localization of ATP11C at the plasma membrane, augmented cell surface exposure of PtdSer and other phospholipids, and triggered phagocytosis of both live and dead cells by macrophages. In addition, expression of mutant CDC50A suppressed the induction of tumor formation in nude mice, suggesting that CDC50A disruption enhances the clearing of viable cells by macrophages in vivo. These findings define the mechanism by which ATP11C and CDC50A regulate PtdSer localization and indicate that PtdSer exposure on the cell surface is a sufficient signal for cell engulfment by macrophages.