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 characteriz ed a new group of lysosomal proteins that contain N-acetylglucosamine- 1-phosphate (GlcNAc-1-P) linked to serine residues. GlcNAc-1-P-contain ing proteins, which include papain-like cysteine proteinases, cofracti onate with the lysosomal markers and are in functional vesicles of the endosomal/lysosomal pathway, Immunoblots probed with reagents specifi c for each carbohydrate modification indicate that the lysosomal prote ins are modified either by Man-6-P or GlcNAc-1-P, but not by both, Con focal microscopy shows that the two sets of proteins reside in physica lly and functionally distinct compartments, Vesicles with GlcNAc-1-P f use with nascent bacteria-loaded phagosomes less than 3 minutes after ingestion, while those with Man-6-P do not participate in bacterial di gestion until about 15 minutes after phagocytosis, Even though both ty pes of vesicles fuse with phagosomes, GlcNAc-1-P- and Man-6-P-bearing proteins rarely colocalize, Since both lysosomal enzymes and their bou nd carbohydrate modifications are stable in lysosomes, a targeting or retrieval mechanism based on these carbohydrate modifications probably establishes and/or maintains segregation.