Protein Domain : Saposin B type domain IPR008139

Type	Domain
Description	The saposin B-type domain is a ~80 amino acid domain present in saposins and related proteins that interact with lipids. The domain is named after thesmall lysosomal proteins, saposins, which serve as sphingolipid hydrolase activator proteins in vertebrates. The mammalian saposins are synthesized as asingle precursor molecule (prosaposin) which contains two saposin A-type domains in the extremities that are removed in the activation reaction, and four saposin B-type domains yielding the active saposins A,B, C and D after proteolytic cleavage. Saposin-like proteins (SAPLIPs) can have different functions, such as enzymatic activities, as cofactors ofenzymes involved in lipid metabolism, as components of lung surfactant reducing the surface tension, as part of a complex involved in stageregulation of Dictyostelium, as antimicrobial effector molecules, or as a stimulator of dendritic outgrowth [, , , ].The 3D structures of different SAPLIPs have been resolved, and show that the saposin B-type domain is formed by a four/five helical bundle. The saposin B-type domain is characterised by six conservedcysteine residues involved in three disulfide bridges: one between helices 2 and 3, one between the first and the last helix and one from the N-terminalpart of the first helix to the C terminus. In plant aspartic proteinases the two subdomains that are connected by the disulfide bridges occur in inversedorder, these are called "swaposin"domains [ , , ]. In these phytepsin proteinsthe two half saposin B-type domains occur in combination with the aspartyl protease signature [, ].
Short Name	SaposinB_dom