Saposin-A and saposin-C stimulate the hydrolysis of glucosylceramide by beta-glucosylceramidase (EC 3.2.1.45) and galactosylceramide by beta-galactosylceramidase (EC 3.2.1.46). Saposin-C apparently acts by combining with the enzyme and acidic lipid to form an activated complex, rather than by solubilizing the substrate. Saposin-B stimulates the hydrolysis of galacto-cerebroside sulfate by arylsulfatase A (EC 3.1.6.8), GM1 gangliosides by beta-galactosidase (EC 3.2.1.23) and globotriaosylceramide by alpha-galactosidase A (EC 3.2.1.22). Saposin-B forms a solubilizing complex with the substrates of the sphingolipid hydrolases. Saposin-D is a specific sphingomyelin phosphodiesterase activator (EC 3.1.4.12). Behaves as a myelinotrophic and neurotrophic factor, these effects are mediated by its G-protein-coupled receptors, GPR37 and GPR37L1, undergoing ligand-mediated internalization followed by ERK phosphorylation signaling. Saposins are specific low-molecular mass non-enzymic proteins, they participate in the lysosomal degradation of sphingolipids, which takes place by the sequential action of specific hydrolases. Saposin-B is a homodimer. Prosaposin exists as a roughly half-half mixture of monomers and disulfide-linked dimers (PubMed:10406958, PubMed:12510003, PubMed:7730378, PubMed:21835174). Monomeric prosaposin interacts (via C-terminus) with sortilin/SORT1, the interaction is required for targeting to lysosomes (PubMed:14657016, PubMed:22431521). Interacts with GRN; facilitates lysosomal delivery of progranulin from the extracellular space and the biosynthetic pathway (PubMed:26370502). Secreted as a fully glycosylated 70 kDa protein composed of complex glycans. Additional isoforms seem to exist. The lysosomal precursor is proteolytically processed to 4 small peptides, which are similar to each other and are sphingolipid hydrolase activator proteins. N-linked glycans show a high degree of microheterogeneity. The one residue extended Saposin-B-Val is only found in 5% of the chains. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. Defects in PSAP saposin-D region are found in a variant of Tay-Sachs disease (GM2-gangliosidosis). Disease susceptibility is associated with variants affecting the gene represented in this entry. Saposin-B co-purifies with 1 molecule of phosphatidylethanolamine.