Definition
Saposins A, B, C, and D are a group of structurally similar glycoproteins derived by proteolytic processing from a single precursor protein, prosaposin. All saposins contain about 80 amino acids and have 6 identically placed cysteine residues 1.
Related peptides
Processing of prosaposin (70 kDa) generates saposin proteins, A, B, C, and D, from its first, second, third, and fourth domains 2. It has been noted that other proteins have the same polypeptide motif characterized by the similar location of six cysteines. These saposin-like (SAPLIP) proteins are surfactant protein B (SP-B), Entamoeba histolytica pore forming peptide, NK-lysin, acid sphingomyelinase and acyloxyacyl hydrolase. The structural homology and the conserved disulfide bridges suggest for all SAPLIPs a common fold, called saposin fold 3.
Discovery
The first saposin (now called saposin B) was described by Jatzkewitz and his colleagues in 1964 as a required heat-stable factor for hydrolysis of sulfatides by arylsulfatase A. The second saposin (saposin C) was discovered in 1971 by Ho and OBrien and was found to stimulate the hydrolysis of glucosylceramidase and to accumulate in tissues of Gaucher disease patients. The discovery of the two remaining saposins, A and D, resulted from cloning and sequencing of the prosaposin cDNA. The discovery that all four saposins are part of a single precursor protein, prosaposin, began with the cloning of a cDNA encoding saposin B and the discovery that the human cDNA sequence was homologous with a protein in rat testis, SGP-1 4.
Structural Characteristics
All four saposins are structurally similar to one another, including placement of six cysteines, a glycosylation site, and conserved proline residues in identical positions 1. A three-dimensional model comparing saposins A and C reveals significant sequence homology between them, especially preservation of conserved acidic and basic residues in their middle regions. Each appears to possess a conformationally rigid hydrophobic pocket stabilized by three internal disulfide bridges, with amphipathic helical regions interrupted by helix breakers 2. The different activities of the saposins must reside within the module of the a-helices and/or in additional specific regions of the molecule 3. The trophic activity of prosaposin on human neuroblastoma cells resides in the aminoterminal hydrophilic sequence (LIDNNRTEEILY) of human saposin C. Nanomolar concentrations of a 22-mer peptide encompassing this region stimulate neurite outgrowth and choline acetyltransferase activity, and prevents cell death in neuroblastoma cells. In primary cerebellar granule cells, the 22-mer also stimulates neurite outgrowth. Peptides encompassing the neurotrophic sequence in prosaposin have been termed prosaptides 5.
Mode of Action
Saposins, through their interaction with glycosphingolipid hydrolases and their substrates, increase lysosomal hydrolytic activities. Saposins and prosaposin are expressed by various cell types and as a secretory protein in body fluids including blood, seminal plasma, seminiferous tubular fluid, and prostatic secretions. Prosaposin and its active domain, saposin C, are known for their potent neurotrophic activities and are involved in neuro-embryological development. The neurotrophic activity of prosaposin has been attributed to the NH2-terminal portion of the saposin C domain of the molecule which is the source for a number of biologically active synthetic peptides such as prosaptides TX14A. Prosaptides (i.e., TX14A), saposin C, and prosaposin exert their biological effects by binding to a partially characterized single high-affinity G-protein coupled receptor (GPCR) 6.
Functions
The apparent function of the saposins is that of promoting the enzymatic degradation of sphingolipids in lysosomes. It has been reported that saposin A (Sap A) and saposin C (Sap C) stimulate the hydrolysis of glucosylceramide and galactosylceramide, saposin B (Sap B) that of several sphingolipids including sulfatide and G-ganglioside, and saposin D (Sap D) that of sphingomyelin. The physiological significance of Sap B and C has been unequivocally assessed by the discovery that a defect of Sap B results in a variant form of metachromatic leukodistrophy, and a defect of Sap C causes a variant form of Gaucher disease. The tissue accumulation of sulfatides in the first case and of glucosylceramides in the second indicates that in vivo Sap B has a fundamental role in the regulation of arylsulfatase A activity and Sap C in that of glucosylceramidase. The functional properties of Sap B and C have been extensively investigated in vitro. The mechanism by which Sap B promotes sphingolipid degradation has been well characterized by showing that it forms water-soluble complexes with several sphingolipids (sulfatide, globotriaosylceramide, ganglioside G, etc.) thus making them accessible to the respective sphingolipid hydrolases 1.
References
1. Vaccaro AM, Salvioli R, Barca A, Tatti M, Ciaffoni F, Maras B, Siciliano R, Zappacosta F, Amoresano A, Pucci P (1995). Structural Analysis of Saposin C and B. Complete Localization Of Disulfide Bridges. J. Biol. Chem., 270(17):9953-9960.
2. Morimoto S, Martin BM, Yamamoto Y, Kretz KA, O'Brien JS, Kishimoto Y (1989). Saposin A: second cerebrosidase activator protein. PNAS., 86(9):3389-3393
3. Vaccaro AM, Salvioli R, Tatti M, Ciaffoni F (1999). Saposins and Their Interaction with Lipids. Neurochemical Research, 24(2):307-314.
4. Kishimoto Y, Hiraiwa M, O'Brien' JS. 1992. Saposins: structure, function, distribution, and molecular genetics. Journal of Lipid Research, 33:1255-1267
5. O'Brien JS, Carson GS, Seo HC, Hiraiwa M, Weiler S, Tomich JM, Barranger JA, Kahn M, Azuma N, Kishimoto Y (1995). Identification of the neurotrophic factor sequence of prosaposin. Faseb., 9(8):681-685.
6. Lee TJ, Sartor O, Luftig RB, Koochekpour S (2004). Saposin C promotes survival and prevents apoptosis via PI3K/Akt-dependent pathway in prostate cancer cells. Molecular Cancer, 3:31