The human vascular anticoagulant, annexin V is a 35-36KDa Ca2+ dependent phospholipids binding protein that has a high affinity for PS. In healthy cells, PS is located on the cytoplasmic surface of the plasma membrane.
However, during the earlies apoptosis, phosphatidylserine (PS), normally located on the cytoplasmic surface of the cell membrane, becomes exposed to the extracellular environment, causing changes in the membrane asymmetry and permeability.