|
Annexin is a common name for a group of cellular proteins. The annexins are a family of proteins, first described in the 1980's, that bind calcium dependently to phospholipid membranes. They are found in all kingdoms (animal, plant and fungi) with the exception of the bacteria. In humans the annexins are found inside the Cell. However some annexins ( annexin A1, Annexin A2 and Annexin A5) have also been found outside the cellular environment for example in blood. How the annexins are transported out of the cell into the blood is a mystery because they lack a signal peptide which is necessary for proteins to be transported out of the cell.
Types Annexin A1: Function: Annexin A1 plays a role in inflammation processes. Annexin A1 as a diagnostic marker of disease. The gene for annexin A1 (ANXA1) is upregulated in hairy cell leukemia. ANXA1 protein expression is specific to hairy cell leukemia. Detection of ANXA1 (by immunocytochemical means) reportedly provides a simple, highly sensitive and specific assay for the diagnosis of hairy cell leukemia. Annexin A2: Function: Annexin A2 is a pleiotropic protein meaning that its function is dependent on place and time in the body. Annexin A2 has been proposed to function inside the cell in sorting of endosomes and outside the cell in anti coagulant reactions. Annexin A5: Function: The function of annexin A5 is unknown, however annexin A5 has been proposed to play a role in the inhibition of blood coagulation by competing for phosphatidylserine binding sites with prothrombin and also to inhibit the activity of phospholipase A1 also by competing for phosphatidylserine binding sites. These properties have been found by in vitro experiments (in a test tube) and they have not been confirmed by experiments with laboratory annimals Annexin A5 (or Annexin V) in pathology: Antibodies directed against annexin A5 are the cause of a syndrome called the Antiphospholipid syndrome. Annexin A5 forms a shield around negatively charged phospholipid molecules. The formation of an annexin A5 shield blocks the entry of phospholipids into coagulation (clotting) reactions. In the antiphospholipid antibody syndrome, the formation of the shield is disrupted by antibodies. Without the shield, there is an increased quantity of phospholipid molecules on cell membranes, speeding up coagulation reactions and causing the blood clotting characteristic of the antiphospholipid antibody syndrome. Laboratory use of annexin A5: Annexin A5 is used as a probe in the annexin A5 affinity assay to detect cells which have expressed phosphatidylserine on the cell surface, a feature found in apoptosis as well as other forms of cell death. | ||||||||
|
| |||||||||
 |
|
| |