Domain

Saposin B type domain (IPR008139)

Short name: SaposinB_dom

Domain relationships

None.

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 the small lysosomal proteins, saposins, which serve as sphingolipid hydrolase activator proteins in vertebrates. The mammalian saposins are synthesized as a single 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 of enzymes involved in lipid metabolism, as components of lung surfactant reducing the surface tension, as part of a complex involved in stage regulation of Dictyostelium, as antimicrobial effector molecules, or as a stimulator of dendritic outgrowth [PMID: 8003971, PMID: 7595087, PMID: 11513801, PMID: 12826659].

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 conserved cysteine 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-terminal part of the first helix to the C terminus. In plant aspartic proteinases the two subdomains that are connected by the disulfide bridges occur in inversed order, these are called "swaposin" domains [PMID: 7610480, PMID: 8868085, PMID: 10406799]. In these phytepsin proteins the two half saposin B-type domains occur in combination with the aspartyl protease signature [PMID: 11513801, PMID: 12518053].

Contributing signatures

Signatures from InterPro member databases are used to construct an entry.
SMART
PROSITE profiles