The loss of plasma asymmetry is a key feature of apoptosis (reviewed in 1). In normal cells, membrane phospholipids are distributed asymmetrically between the inner and outer leaflets of the plasma membrane. For example, phosphatidylserine (PS), an aminophospholipid, is normally present in the inner leaflet of the plasma membrane. Early in apoptosis, before the loss of membrane integrity, PS translocates from the inner to the outer leaflet of the plasma membrane, exposing it to the external cellular environment at the surface of the plasma membrane. Annexin V is a 35-36 kDa, calcium-dependent, phospholipid-binding protein with a high affinity for PS and is used to indirectly monitor PS translocation. The Annexin V assay is perhaps the most widely used assay today, facilitated by the observation that PS translocation appears to be a universal apoptotic phenomenon. It has been detected in mammalian, insect, and plant cells under the action of most, if not all, triggers of apoptosis.
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