FIGURE 1.

Modification of N-glycans on lysosomal enzymes. Phosphorylation of mannose residues on lysosomal enzymes' N-linked oligosaccharides is carried out by the GlcNAc phosphotransferase, which transfers GlcNAc-1-phosphate from UDP-GlcNAc to the C-6 hydroxyl group of mannose to form a Man-P-GlcNAc phosphodiester. The N-glycans can be further modified by the uncovering enzyme, which removes the GlcNAc moiety to expose the Man-6-P phosphomonoester. The five potential sites of mannose phosphorylation are indicated in gray. The three arms of the N-glycan are labeled and their mannose residues designated A–I are shown.