Early in apoptosis, before the loss of membrane integrity,phosphatidylserine (PS) translocates from the inner to outer leaflet of plasma membrane, exposing it to external cellular environment at surface of plasma membrane
Annexin V is 35-36kDa, calcium-dependent, phospholipid-binding protein with a high affinity for PS and is used to indirectly monitor PS translocation
Annexin V assay is facilitated by the observation that PS translocation appears to be a universal apoptotic phenomenon
Binds to negatively charged phospholipid surfaces Defined calcium and salt concentrations are required Used in Fluorescence microscopy