The lysosomal degradation of sphingolipids takes place by the sequential action of specific hydrolases. Some of these enzymes require specific low-molecular mass, non-enzymic proteins: the sphingolipids activator proteins (coproteins). Defects in PSAP are the cause of combined saposin deficiency (CSAPD); also known as prosaposin deficiency. CSAPD is due to absence of all saposins, leading to a fatal storage disorder with hepatosplenomegaly and severe neurological involvement. Defects in PSAP saposin-B region are the cause of leukodystrophy metachromatic due to saposin-B deficiency (MLD- SAPB). MLD-SAPB is an atypical form of metachromatic leukodystrophy. It is characterized by tissue accumulation of cerebroside-3-sulfate, demyelination, periventricular white matter abnormalities, peripheral neuropathy. Additional neurological features include dysarthria, ataxic gait, psychomotr regression, seizures, cognitive decline and spastic quadriparesis. Defects in PSAP saposin-C region are the cause of atypical Gaucher disease (AGD). Affected individuals have marked glucosylceramide accumulation in the spleen without having a deficiency of glucosylceramide-beta glucosidase characteristic of classic Gaucher disease, a lysosomal storage disorder. Defects in PSAP saposin-A region are the cause of atypical Krabbe disease (AKRD). AKRD is a disorder of galactosylceramide metabolism. AKRD features include progressive encephalopathy and abnormal myelination in the cerebral white matter resembling Krabbe disease. Defects in PSAP saposin-D region are found in a variant of Tay-Sachs disease (GM2-gangliosidosis). 3 isoforms of the human protein are produced by alternative splicing. Note: This description may include information from UniProtKB.
Chromosomal Location of Human Ortholog: 10q21-q22
Cellular Component: lysosomal lumen; extracellular space; mitochondrion; lysosomal membrane; extracellular region; integral to membrane
Molecular Function: protein binding; enzyme activator activity; lipid binding
Biological Process: positive regulation of catalytic activity; platelet activation; regulation of lipid metabolic process; platelet degranulation; sphingolipid metabolic process; regulation of MAPKKK cascade; glycosphingolipid metabolic process; blood coagulation; lipid transport
SS: The number of records in which this modification site was determined using site-specific methods. SS methods include amino acid sequencing, site-directed mutagenesis, modification site-specific antibodies, specific MS strategies, etc.