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Prosaposin, also known as PSAP, is a protein which in humans is encoded by the ''PSAP'' gene.〔(【引用サイトリンク】 url = http://www.ncbi.nlm.nih.gov/sites/entrez?Db=gene&Cmd=ShowDetailView&TermToSearch=5660 )〕 This highly conserved glycoprotein is a precursor for 4 cleavage products: saposins A, B, C, and D. Saposin is an acronym for Sphingolipid Activator PrO()teINs. Each domain of the precursor protein is approximately 80 amino acid residues long with nearly identical placement of cysteine residues and glycosylation sites. Saposins A-D localize primarily to the lysosomal compartment where they facilitate the catabolism of glycosphingolipids with short oligosaccharide groups. The precursor protein exists both as a secretory protein and as an integral membrane protein and has neurotrophic activities.〔 Saposins A-D are required for the hydrolysis of certain sphingolipids by specific lysosomal hydrolases. ==Family members== * Saposin A was identified as an N-terminal domain in the prosaposin cDNA prior to its isolation. It is known to stimulate the enzymatic hydrolysis of 4-methlyumbelliferyl-β-glucoside, glucocerebroside, and galactocerebroside. * Saposin B was the first to be discovered and was found to be required as a heat-stable factor for hydrolysis of sulfatides by arylsulfatase A. It is known by many different names, such as, sphingolipid activator protein-1 (SAP-1), sulfatide activator protein, GM1 ganglioside activator, dispersin, and nonspecific. It has been observed that this particular saposin activates many enzymes through interaction with the substrates not the enzymes themselves. * Saposin C was the second saposin to be discovered and stimulates the hydrolysis of glycocerebroside by glycosylceramidase and galactocerebroside by galactoslyceramidase. * Saposin D is not well known to due lack of investigation at this point in time. It was predicted from the cDNA sequence of prosaposin, like saposin A. Ezymatic stimulation is very specific for this particular glycoprotein and it not understood completely.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Prosaposin」の詳細全文を読む スポンサード リンク
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