Dictyostelium lysosomal proteins with different sugar modifications sort to functionally distinct compartments

Many Dictyostelium lysosomal enzymes contain mannose-6-phosphate (Man-6-P) in their N-linked oligosaccharide chains. We have now characterized a new group of lysosomal proteins that contain N-acetylglucosamine-1-phosphate (GlcNAc-1-P) linked to serine residues. GlcNAc-1-P-containing proteins, which include papain-like cysteine proteinases, cofractionate with the lysosomal markers and are in functional vesicles of the endosomal/lysosomal pathway. Immunoblots probed with reagents specific for each carbohydrate modification indicate that the lysosomal proteins are modified either by Man-6-P or GlcNAc-1-P, but not by both. Confocal microscopy shows that the two sets of proteins reside in physically and functionally distinct compartments. Vesicles with GlcNAc-1-P fuse with nascent bacteria-loaded phagosomes less than 3 minutes after ingestion, while those with Man-6-P do not participate in bacterial digestion until about 15 minutes after phagocytosis. Even though both types of vesicles fuse with phagosomes, GlcNAc-1-P- and Man-6-P-bearing proteins rarely colocalize. Since both lysosomal enzymes and their bound carbohydrate modifications are stable in lysosomes, a targeting or retrieval mechanism based on these carbohydrate modifications probably establishes and/or maintains segregation.