Apoptosin

Apoptosin is a term used to describe a protein that plays a role in programmed cell death, also known as apoptosis. While not a single, universally defined protein, it generally refers to molecules that are either induced or activated during the apoptotic process and can be detected on the surface of dying cells. One of the most well-known examples of a protein considered an apoptosin is the annexin A5 (or AnxA5).

Annexin A5 is a calcium-dependent phospholipid-binding protein. Under normal physiological conditions, annexin A5 is located in the cytoplasm. However, during apoptosis, a critical event occurs: the translocation of phosphatidylserine (PS) from the inner leaflet to the outer leaflet of the plasma membrane. PS is a negatively charged phospholipid that is typically found on the inner membrane surface. Its exposure on the outer surface serves as an "eat me" signal for phagocytic cells, marking the cell for clearance.

Annexin A5 has a high affinity for phosphatidylserine. Consequently, when PS is exposed on the outer membrane during apoptosis, annexin A5 binds to it. This binding of annexin A5 to the surface of apoptotic cells is a hallmark of cellular demise and is exploited in various diagnostic and research applications. For instance, labeled annexin A5 is commonly used in flow cytometry and imaging techniques to identify and quantify cells undergoing apoptosis. The presence of annexin A5 binding indicates that a cell is in the early to middle stages of apoptosis, before the complete disintegration of the cell membrane. Therefore, "apoptosin" broadly refers to such surface-exposed apoptotic markers, with annexin A5 being the most prominent example.