Glycosylation and sorting pathways of lysosomal enzymes in mussel digestive cells

Our aim was to contribute to the understanding of the synthesis, maturation and activation of lysosomal enzymes in an invertebrate cellular model: the endo-lysosomal system (ELS) of mussel digestive cells. The activities of 5'-nucleotidase (AMPase), arylsulphatase (ASase) and acid phosphatase (AcPase), which are transported towards acidic compartments as membrane proteins, were localised by enzyme cytochemistry. AcPase activity was found within large heterolysosomes and residual bodies. ASase was located in endosomes, endolysosomes and heterolysosomes. AcPase and ASase activities were recorded within small vesicles and cisterns of the trans-Golgi network. Conversely, AMPase activity was primarily found in microvilli and apical vesicles and, less conspicuously, in lysosomes and the cis-side of the Golgi and the cis-Golgi network (CGN). In order to understand the processes of synthesis and maturation of these lysosomal enzymes, selected glycoconjugates were localised after lectin cytochemistry. N-acetylglucosamine, mannose and fucose residues were almost ubiquitous in the ELS, as were galactose residues, which were apparently less abundant. N-acetylglucosamine residues occurred in the inner membrane co-localised with mannose residues within the lysosomal and pre-lysosomal acidic compartments. Based on these results, glycosylation and sorting pathways are proposed for both soluble and membrane enzymes. Unlike in mammalian cells, O-glycosylation is fully completed in the CGN, mannose addition in N-glycosylation extends beyond the CGN and galactose addition is fully achieved at the intermediate side. Sorting of soluble lysosomal enzymes, as in crustaceans, is mediated by the indirect transport of membrane-linked proteins with GlcNAc1-P6Man residues that are removed in endolysosomes and heterolysosomes.