InterPro domain: IPR008139

The saposin B-type domain is a ~80 amino acid domain present in saposins andrelated proteins that interact with lipids. The domain is named after thesmall lysosomal proteins, saposins, which serve as sphingolipid hydrolaseactivator proteins in vertebrates. The mammalian saposins are synthesized as asingle precursor molecule (prosaposin) which contains two saposin A-typedomains 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) canhave different functions, such as enzymatic activities, as cofactors ofenzymes involved in lipid metabolism, as components of lung surfactantreducing the surface tension, as part of a complex involved in stageregulation of Dictyostelium, as antimicrobial effector molecules, or as astimulator of dendritic outgrowth [ 1 , 2 , 3 , 4 ].

The 3D structures of different SAPLIPs have been resolved, and show that thesaposin 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 2and 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 thetwo subdomains that are connected by the disulfide bridges occur in inversedorder, these are called "swaposin" domains [ 5 , 6 , 7 ]. In these phytepsin proteinsthe two half saposin B-type domains occur in combination with the aspartylprotease signature [ 8 , 8 ].

1. Acid sphingomyelinase possesses a domain homologous to its activator proteins: saposins B and D. Protein Sci. 3, 359-61
2. Saposin-like proteins (SAPLIP) carry out diverse functions on a common backbone structure. J. Lipid Res. 36, 1653-63
3. Novel putative saposin-like proteins of Entamoeba histolytica different from amoebapores. Biochim. Biophys. Acta 1514, 14-20
4. MSAP is a novel MIR-interacting protein that enhances neurite outgrowth and increases myosin regulatory light chain. J. Biol. Chem. 278, 35412-20
5. Swaposins: circular permutations within genes encoding saposin homologues. Trends Biochem. Sci. 20, 179-80
6. Cytotoxic T cells: more weapons for new targets? Trends Microbiol. 4, 91-4
7. Crystal structure of plant aspartic proteinase prophytepsin: inactivation and vacuolar targeting. EMBO J. 18, 3947-55
8. Crystal structure of saposin B reveals a dimeric shell for lipid binding. Proc. Natl. Acad. Sci. U.S.A. 100, 38-43