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. 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). Saposins are specific low-molecular mass non-enzymatic 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. Monomeric prosaposin interacts (via C-terminus) with sortilin/SORT1, the interaction is required for targeting to lysosomes. Interacts with GRN; facilitates lysosomal delivery of progranulin from the extracellular space and the biosynthetic pathway (By similarity). Secreted as a fully glycosylated 70 kDa protein composed of complex glycans.