Leonova T, Qi X, Bencosme A, Ponce E, Sun Y, Grabowski G A
Division of Human Genetics, Children's Hospital Research Foundation at Children's Hospital Medical Center, Cincinnati, Ohio 45229-3039, USA.
J Biol Chem. 1996 Jul 19;271(29):17312-20. doi: 10.1074/jbc.271.29.17312.
Prosaposin is a multifunctional protein encoded at a single locus in humans and mice. The precursor contains, in tandem, four glycoprotein activators or saposins, termed A, B, C, and D, that are essential for specific glycosphingolipid hydrolase activities. Prosaposin appears to be a potent neurotrophic factor. To explore the proteolytic processing from prosaposin to mature activator proteins, metabolic labeling was done with human prosaposin expressed in insect cells, human fibroblasts, neuronal stem cells (NT2) and retinoic acid-differentiated NT2 neurons. In all cell types, the major processing pathway was through a tetrasaposin, A-B-C-D, from which saposin A was then removed. In mammalian cells monosaposins were derived from the trisaposin B-C-D by cleavage to the disaposins, B-C and C-D, that were processed to monosaposins. In insect cells the major end products were the disaposins, with A-B and C-D derived from the tetrasaposin, A-B-C-D, or with B-C and C-D derived from the trisaposin, B-C-D. In insect and mammalian cells, the nonsignal NH2-terminal peptide preceding saposin A (termed Nter) was usually removed prior to saposin A cleavage. In NT2-derived differentiated neurons, precursor tetrasaposins containing A-B-C-D were secreted with and without Nter. Immunofluorescence studies using prosaposin-specific antisera showed large steady state amounts of uncleaved prosaposin in Purkinje cells, cortical neurons, and other specific cell types in adult mice. These studies indicate that prosaposin processing is highly regulated at a proteolytic level to produce prosaposin, tetrasaposins, or mature monosaposins in specific mammalian cells.
Prosaposin是一种多功能蛋白质，在人类和小鼠中由单个基因座编码。前体包含串联的四种糖蛋白激活剂或鞘脂激活蛋白，称为A、B、C和D，它们对于特定的糖鞘脂水解酶活性至关重要。Prosaposin似乎是一种有效的神经营养因子。为了探索从Prosaposin到成熟激活蛋白的蛋白水解加工过程，对在昆虫细胞、人成纤维细胞、神经干细胞（NT2）和视黄酸分化的NT2神经元中表达的人Prosaposin进行了代谢标记。在所有细胞类型中，主要的加工途径是通过四聚体鞘脂激活蛋白A-B-C-D，然后从中去除鞘脂激活蛋白A。在哺乳动物细胞中，单体鞘脂激活蛋白是由三聚体鞘脂激活蛋白B-C-D切割成二聚体鞘脂激活蛋白B-C和C-D后加工而成的。在昆虫细胞中，主要的终产物是二聚体鞘脂激活蛋白，A-B和C-D来自四聚体鞘脂激活蛋白A-B-C-D，或者B-C和C-D来自三聚体鞘脂激活蛋白B-C-D。在昆虫和哺乳动物细胞中，鞘脂激活蛋白A之前的非信号NH2末端肽（称为Nter）通常在鞘脂激活蛋白A切割之前被去除。在NT2衍生的分化神经元中，含有A-B-C-D的前体四聚体鞘脂激活蛋白在有和没有Nter的情况下都会分泌。使用Prosaposin特异性抗血清的免疫荧光研究表明，成年小鼠的浦肯野细胞、皮质神经元和其他特定细胞类型中存在大量稳定状态的未切割Prosaposin。这些研究表明，Prosaposin的加工在蛋白水解水平上受到高度调节，以在特定的哺乳动物细胞中产生Prosaposin、四聚体鞘脂激活蛋白或成熟的单体鞘脂激活蛋白。
