The function of Ensembl gene identifier ENSG00000197746 (PSAP, prosaposin) is as follows. Saposins are specific low-molecular mass non-enzymatic glycoproteins that act as activator proteins for lysosomal sphingolipid-degrading enzymes, facilitating the hydrolysis of sphingolipids by extracting lipid substrates from membranes and presenting them to their respective enzymes. They are derived from a common precursor protein, prosaposin, which is proteolytically cleaved to yield four homologous proteins (saposin A, B, C, and D), each with distinct but partially overlapping lipid and enzyme specificities. . [Prosaposin]: 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. {ECO:0000250|UniProtKB:Q61207, ECO:0000269|PubMed:10383054}. [Saposin-A]: acts as an essential lysosomal cofactor that activates galactocerebrosidase (GALC, EC 3.2.1.46) by facilitating enzyme access to its membrane-embedded galactosylceramide substrate, the principal lipid component of myelin. . [Saposin-B]: Stimulates the hydrolysis of galacto-cerebroside sulfate by arylsulfatase A ARSA (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). Forms a solubilizing complex with the substrates of the sphingolipid hydrolases. . [Saposin-C]: Acts as an essential lysosomal cofactor that activates acid beta-glucosylceramidases (EC 3.2.1.45) by altering lipid bilayer properties to facilitate enzyme access to its membrane-embedded glucosylceramide substrate, while also protecting glucocerebrosidase from intralysosomal proteolytic degradation. . [Saposin-D]: Acts as a lysosomal activator protein that stimulates acid ceramidase (EC 3.5.1.23) by solubilizing and presenting ceramide substrates at the lipid-water interface, facilitating their hydrolysis into sphingosine and fatty acids. It also activates acid sphingomyelinase (EC 3.1.4.12), though ceramide catabolism represents its primary physiological role. {ECO:0000269|PubMed:2845979, ECO:0000269|PubMed:8185598, ECO:0000269|PubMed:8203897}.