Annexin

Annexin is a common name for a group of cellular proteins. They are mostly found in eukaryotic organisms (animal, plant and fungi). In humans, the annexins are found inside the cell. However some annexins (Annexin A1, Annexin A2, and Annexin A5) can be secreted from the cytoplasm to outside cellular environments, such as blood. Annexin is also known as lipocortin. Lipocortins suppress phospholipase A2. Increased expression of the gene coding for annexin-1 is one of the mechanisms by which glucocorticoids (such as cortisol) inhibit inflammation. The protein family of annexins has continued to grow since their association with intracellular membranes was first reported in 1977. The recognition that these proteins were members of a broad family first came from protein sequence comparisons and their cross-reactivity with antibodies. One of these workers (Geisow) coined the name Annexin shortly after. As of 2002 160 annexin proteins have been identified in 65 different species. The criteria that a protein has to meet to be classified as an annexin are: it has to be capable of binding negatively charged phospholipids in a calcium dependent manner and must contain a 70 amino acid repeat sequence called an annexin repeat. Several proteins consist of annexin with other domains like gelsolin. The basic structure of an annexin is composed of two major domains. The first is located at the COOH terminal and is called the “core” region. The second is located at the NH2 terminal and is called the “head” region. The core region consists of an alpha helical disk. The convex side of this disk has type 2 calcium-binding sites. They are important for allowing interaction with the phospholipids at the plasma membrane. The N terminal region is located on the concave side of the core region and is important for providing a binding site for cytoplasmic proteins. In some annexins it can become phosphorylated and can cause affinity changes for calcium in the core region or alter cytoplasmic protein interaction.