Lysosomal enzyme trafficking in mannose 6-phosphate receptor-positive mouse L-cells: demonstration of a steady state accumulation of phosphorylated acid hydrolases.
During their transport from the endoplasmic reticulum to lysosomes, newly synthesized acid hydrolases are phosphorylated in the Golgi apparatus to generate a common recognition marker, mannose 6-phosphate (Man 6-P). The phosphorylated acid hydrolases then bind to the Man 6-P receptor and are transported by an unknown route to lysosomes. To learn more about the delivery pathway, we examined the fate of the phosphorylated oligosaccharides synthesized by a Man 6-P receptor-positive line of mouse L-cells. In contrast to the rapid degradation of the recognition marker previously observed in mouse lymphoma cells (Gabel, C. A., D. E. Goldberg, and S. Kornfield. 1982. J. Cell Biol., 95:536-542), the number of high mannose oligosaccharides phosphorylated by the L-cells after a 30-min pulse labeling with [2-3H]mannose increased continuously during a subsequent 4-h chase period to a maximum of 9.3% of the total cell-associated structures. After 19 h of chase the absolute number of phosphorylated oligosaccharides declined, but the loss was accompanied by a general loss of cellular oligosaccharides such that 7.4% of the cell-associated high mannose oligosaccharides remained phosphorylated. The longevity of the Man 6-P recognition marker in the L-cells was verified by analyzing the ability of an individual acid hydrolase, beta-glucuronidase, to serve as a ligand for the Man 6-P receptor. At least 60% of the steady state beta-glucuronidase molecules isolated from the L-cells could undergo receptor-mediated endocytosis into enzyme-deficient human fibroblasts. Dense lysosomal granules isolated by metrizamide gradient centrifugation from [3H]mannose-labeled L-cells were found to be highly enriched in their content of phosphomonoester-containing oligosaccharides. The data indicate that acid hydrolases may retain their Man 6-P recognition markers within lysosomes, and suggest the possibility that dephosphorylation occurs at a nonlysosomal location through which the newly synthesized enzymes pass en route to lysosomes.
D000818:Animals; D001692:Biological Transport; D002352:Carrier Proteins / Q000378:metabolism*; D002499:Centrifugation, Density Gradient; D004705:Endocytosis; D005347:Fibroblasts / Q000378:metabolism; D005966:Glucuronidase / Q000378:metabolism; D006801:Humans; D006867:Hydrolases / Q000378:metabolism*; D007739:L Cells / Q000378:metabolism; D008247:Lysosomes / Q000201:enzymology*; D051379:Mice; D009844:Oligosaccharides / Q000032:analysis; D010766:Phosphorylation; D010770:Phosphotransferases / Q000378:metabolism; D011499:Protein Processing, Post-Translational; D017527:Receptor, IGF Type 2; D017855:Transferases (Other Substituted Phosphate Groups)
D002352:Carrier Proteins; D009844:Oligosaccharides; D017527:Receptor, IGF Type 2; D010770:Phosphotransferases; D017855:Transferases (Other Substituted Phosphate Groups); C030012:UDP-N-acetylglucosamine-lysosomal-enzyme-N-acetylglucosaminephosphotransferase; D006867:Hydrolases; D005966:Glucuronidase