Postendocytic maturation of acid hydrolases: evidence of prelysosomal processing

The mannose 6-phosphate (Man 6-P) receptor operates to transport both endogenous newly synthesized acid hydrolases and extracellular enzymes to the lysosomal compartment. In a previous study (Gabel, C. A., and S. A. Foster, 1986, J. Cell Biol., 103:1817-1827), we noted that beta-glucuronidase molecules internalized by mouse L-cells via the Man 6-P receptor undergo a proteolytic cleavage and a limited dephosphorylation. In this report, we present evidence that indicates that the postendocytic alterations of the acid hydrolase molecules occur at a site through which the enzymes pass en route to the lysosomal compartment. Mouse L-cells incubated at 20 degrees C with beta-glucuronidase (isolated from mouse macrophage secretions) internalize the enzyme in a process that is inhibited by Man 6-P but unaffected by cycloheximide. As such, the linear accumulation of the ligand observed at 20 degrees C appears to occur through the continued recycling of the cell surface Man 6-P receptor. The subcellular distribution of the internalized ligands was assessed after homogenization of the cells and fractionation of the extracts by density gradient centrifugation. In contrast to the accumulation of the ligand within lysosomes at 37 degrees C, the beta-glucuronidase molecules internalized by the L cells at 20 degrees C accumulate within a population of vesicles that sediment at the same density as endocytic vesicles. Biochemical analysis of the internalized ligands indicates that: (a) the subunit molecular mass of both beta-glucuronidase and beta-galactosidase decrease upon cell association relative to the input form of the enzymes, and (b) the beta-glucuronidase molecules experience a limited dephosphorylation such that high-mannose-type oligosaccharides containing two phosphomonoesters are converted to single phosphomonoester forms. The same two post-endocytic alterations occur after the internalization of beta-glucuronidase by human I-cell disease fibroblasts, despite the low acid hydrolase content of these cells. The results indicate, therefore, that acid hydrolases internalized via the Man 6-P receptor are processed within the endocytic compartment. In that endogenous newly synthesized acid hydrolases display similar alterations during their maturation, the results further suggest that the endosomal compartment is involved in the sorting of ligands transported via both the cell surface and intracellular Man 6-P receptor.