Saposin B is a small non-enzymatic glycoprotein required for the breakdown of cerebroside sulphates (sulphatides) in lysosomes. Saposin B contains three intramolecular disulphide bridges, exists as a dimer and is remarkably heat, protease, and pH stable. The crystal structure of human saposin B reveals an unusual shell-like dimer consisting of a monolayer of alpha-helices enclosing a large hydrophobic cavity [1,2]. It is one of the most studied members of the saposin protein family and it is involved in the hydrolysis of glycolipids and glycerolipids. SapB is unique in the saposin family in that it facilitates degradation by interacting with the substrate, not the enzymes [3].

1: Swaposins: circular permutations within genes encoding saposin homologues. Ponting CP, Russell RB; Trends Biochem Sci 1995;20:179-180. PMID:7610480

2: Crystal structure of saposin B reveals a dimeric shell for lipid binding. Ahn VE, Faull KF, Whitelegge JP, Fluharty AL, Prive GG; Proc Natl Acad Sci U S A. 2003;100:38-43. PMID:12518053

3: The Lysosomal Protein Saposin B Binds Chloroquine. Huta BP, Mehlenbacher MR, Nie Y, Lai X, Zubieta C, Bou-Abdallah F, Doyle RP; ChemMedChem. 2016;11:277-282. PMID:26616259