| Type: | Family | Name: | Annexin, type VI |
| Description: | The annexins (or lipocortins) are a family of proteins that bind to phospholipids in a calcium-dependent manner []. They are distributed ubiquitously in different tissues and cell types of higher and lower eukaryotes, including mammals, fish, birds, Drosophila melanogaster(Fruit fly), Xenopus laevis(African clawed frog), Caenorhabditis elegans, Dictyostelium discoideum(Slime mold) and Neurospora crassa[, ]. Annexins are absent from yeasts and prokaryotes []. The plant annexins are somewhat distinct from those found in other taxa [].Most eukaryotic species have 1-20 annexin (ANX) genes. All annexins share a core domain made up of four similar repeats, each approximately 70 amino acids long []. Each individual annexin repeat (sometimes referred to as endonexin folds) is folded into five alpha-helices, and in turn are wound into a right-handed super-helix; they usually contain a characteristic 'type 2' motif for binding calcium ions with the sequence 'GxGT-[38residues]-D/E'. Animal and fungal annexins also have variable amino-terminal domains. The core domains of most vertebrate annexins have been analysed by X-ray crystallography, revealing conservation of their secondary and tertiary structures despite only 45-55% amino-acid identity among individual members. The four repeats pack into a structure that resembles a flattened disc, with a slightly convex surface on which the Ca 2+ -binding loops are located and a concave surface at which the amino and carboxyl termini come into close apposition.Annexins are traditionally thought of as calcium-dependent phospholipid-binding proteins, but recent work suggests a more complex set of functions. The famiy has been linked with inhibition of phospholipase activity, exocytosis and endoctyosis, signal transduction, organisation of the extracellular matrix, resistance to reactive oxygen species and DNA replication [].This entry represents Type VI annexins that are found in various secretory cells, e.g. B- and T-cells (where it is found in greater concentrations in mature cells), and the lactation ducts of non-lactating human breasts. The observation that the protein is absent in lactating breasts suggests that it inhibits secretion. The type VI class may also play a part in the regulation of some calcium channels, and its presence may cause arrest of cell growth, before the DNA-replication stage, in cells growing at low serum concentrations. This annexin class is unusual in containing eight repeats of the conserved domain rather than the usual four. It is thus believed that the protein has arisen from a gene duplication event. | Short Name: | AnnexinVI |
