Annexin A5

The annexins are a large family of related proteins which share the property of binding to phophospholipid containing membranes in a Calcium dependent manner. Different members of the family were discovered by different laboratories and as a result the various members have many alternate names, such as lipocortin, calpactin, calelectrin and very many others. The widely used current nomenclature is now based on a letter to indicate membership in a particular one of several annexin sub-families and a number for individual gene products, hence the name annexin A5. The annexin family is defined by a compact disc structure formed from 16 closely packed α-helices which co-ordinate multiple calcium ions with phospholipid containing membranes. This domain is defined by 4 imperfect repeats of a ~77 amino acid sequence, each repeat forming 4 α-helices. Annexin A5 is expressed widely in tissues and has been used as a marker of apoptosis, as apoptotic cells may express binding sites for this protein on their cell surface. The protein binds to phosphatidylserine, a membrane lipid normally not found on the external surface of cells which becomes expressed on the cell surface during apoptosis. As a result fluorescent annexin A5 or annexin A5 antibody can be use to isolate apoptotic cells by fluorescence activated cell sorting.