Trafficking of sulfated glycoprotein-1 (prosaposin) to lysosomes or to the extracellular space in rat Sertoli cells.
Sulfated glycoprotein-1 (prosaposin) exists in 2 forms: a 65kDa form targeted to lysosomes and a 70kDa form secreted extracellularly. In order to understand the sorting and targeting mechanisms of the two forms of SGP-1, we have compared their maturation, processing, and secretion in rat Sertoli cells in vivo. Metabolic labeling experiments in vivo demonstrated that the 65kDa form is synthesized first, then post-translationally modified to the 70kDa form of SGP-1. Subcellular fractionation of testicular homogenate was used to obtain Golgi fractions containing up to 50-fold enrichment in galactosyltransferase. Permeabilization of enriched Golgi fractions with saponin released the 70kDa form, but did not affect the 65kDa protein. While excess free mannose 6-phosphate did not release lysosomal SGP-1, it released the 35kDa cathepsin L from Golgi membranes. Using quantitative electron-microscopic immunocytochemistry, the lysosomal contents of SGP-1 were shown to increase significantly after the administration of tunicamycin in vivo. Therefore, the trafficking of the 65kDa form of SGP-1 to the lysosomes appears to be independent of the M6P-receptor pathway. The 70kDa form of SGP-1 was found to aggregate within perforated Golgi fractions in a process which depends on low pH and calcium ions. We conclude that the targeting of the 65kDa form of SGP-1 to the lysosomes involves an early association with Golgi membrane that is independent of mannose 6-phosphate receptors.
Igdoura
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Department of Anatomy and Cell Biology, McGill University, 3640 University Street, Montreal, Canada H3A 2B2.
Rasky
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Morales
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D000135:Acid Phosphatase / Q000378:metabolism; D000818:Animals; D001692:Biological Transport; D004797:Enzyme-Linked Immunosorbent Assay; D005110:Extracellular Space / Q000378:metabolism*; D005952:Glucose-6-Phosphatase / Q000378:metabolism; D006023:Glycoproteins / Q000493:pharmacokinetics*; D006056:Golgi Apparatus / Q000201:enzymology / Q000648:ultrastructure; D006863:Hydrogen-Ion Concentration; D008247:Lysosomes / Q000378:metabolism*; D008297:Male; D008360:Mannosephosphates / Q000494:pharmacology; D008861:Microsomes / Q000201:enzymology; D008928:Mitochondria / Q000201:enzymology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012503:Saponins / Q000494:pharmacology; D049231:Saposins; D012708:Sertoli Cells / Q000378:metabolism* / Q000648:ultrastructure*; D013462:Sulfur Radioisotopes; D014415:Tunicamycin / Q000494:pharmacology
D006023:Glycoproteins; D008360:Mannosephosphates; C483347:Psap protein, rat; D012503:Saponins; D049231:Saposins; D013462:Sulfur Radioisotopes; D014415:Tunicamycin; C027693:mannose-6-phosphate; D000135:Acid Phosphatase; D005952:Glucose-6-Phosphatase