ABSTRACT
Phosphatidylserine (PS) is a phospholipid that is abundant in eukaryotic plasma membranes,has crucial biological functions.Under cell apoptosis, cells can not generate enough ATP for energy and the concentration of cytoplasmic Ca 2 +increases, resulting in PS eversion.Apoptosis and the clearance of apoptotic cells are essential processes in animal development and homeostasis.For apoptotic cells to be cleared, they must display aneat me signal, most likely PS exposure, which prompts phagocytes to engulf the cells.PS is exposed by the action of scramblase on the cell's surface in biological processes such as apoptosis and platelet activation.Once exposed to the cell surface, PS acts as an eat me signal on dead cells, and creates a scaffold for blood-clotting factors on activated platelets.The molecular identities of the flippase and scramblase that work at plasma membranes have long eluded researchers.Indeed, their identity as well as the mechanism of the PS exposure to the cell surface has only recently been revealed.We describe how PS is exposed in activated platelets and in apoptotic cells, and discuss the clearance of apoptotic cells.