Practice question

Soluble acid hydrolases that degrade macromolecules inside lysosomes must be diverted from default secretory flow that would otherwise release them extracellularly. In cis-Golgi, UDP-GlcNAc: lysosomal enzyme N-acetylglucosamine-1-phosphotransferase, itself activated by Site-1 protease cleavage, adds GlcNAc-1-phosphate to carbon-6 hydroxyl of selected mannose residues on N-linked high-mannose oligosaccharides of hydrolase precursors arriving from ER. A second enzyme, uncovering enzyme, removes GlcNAc leaving mannose-6-phosphate monoester exposed. In trans-Golgi network, two mannose-6-phosphate receptors, cation-dependent MPR46 and cation-independent MPR300, recognize clustered phosphomannose with high avidity using mannose-6-phosphate homology domains, concentrating cargo into clathrin-coated vesicles assembled by GGA and AP-1 adaptors that bind dileucine and acidic motifs in receptor tails. Vesicles fuse with early and late endosomes where acidic pH near 5.5 triggers ligand dissociation, receptors recycle to Golgi for reuse, and hydrolases continue to lysosomes where phosphatase removes tag. Inherited deficiency in phosphotransferase causes I-cell disease with hypersecretion, proving essentiality of Golgi-based mannose-6-phosphate code for lysosomal biogenesis and intracellular digestion. Additional coordination with cellular stress pathways ensures fidelity, prevents aggregation, and links trafficking to growth control and proteostasis maintenance across diverse cell types and developmental stages.