Practice question

Lysosomal enzymes traverse secretory pathway originating from rough ER translocation, folding and initial N-glycosylation, then Golgi where specific modification marks them for diversion from default secretion. Cis-Golgi N-acetylglucosamine phosphotransferase recognizes lysine-containing conformational patch present only on soluble hydrolases, transferring GlcNAc-one-phosphate onto terminal mannose residues of high mannose oligosaccharides, forming phosphodiester. Uncovering enzyme in trans-Golgi removes GlcNAc leaving mannose-six-phosphate monoester. Trans-Golgi network clathrin adaptors AP1 and GGA recruit mannose-six-phosphate receptors that bind M6P tag with high affinity, clustering into vesicles delivering to early endosomes where acidic pH five point five dissociates cargo, receptor recycles. Lysosomes mature through Rab conversion. Mitochondria generate ATP via respiration, peroxisomes oxidize lipids, ribosomes synthesize proteins, therefore not source. Brefeldin A disrupts Golgi causing missorting to extracellular space, and I-cell disease lacking phosphotransferase results in severe psychomotor retardation due to empty lysosomes, confirming Golgi origin and M6P-dependent sorting. Integration with cell cycle kinases, calcium signaling and mechanical cues ensures coordinated remodeling during growth, migration and differentiation.