Practice question

Dissociation of lysosomal enzymes from mannose-6-phosphate receptors relies on progressive acidification along endocytic pathway driven by V-ATPase multisubunit rotary pump. In trans-Golgi network pH near 6.7 binding between M6P monoester and receptor pocket containing arginine histidine is high affinity nanomolar permitting packaging into vesicles. Upon arrival in late endosomes pH drops via pumping to 5.5-6.0 protonating histidines reducing electrostatic complementarity affinity over hundredfold causing cargo release into lumen while receptors remain membrane bound for retromer VPS26-29-35 recycling to TGN. If late endosomal pH increased to 6.6 by weak base ammonium chloride, chloroquine, or selective V-ATPase inhibitors concanamycin and bafilomycin A1, receptor-ligand complex fails to dissociate, enzymes stay tightly bound to receptor cannot be freed for delivery to lysosomes and instead recycle back with receptor to TGN depleting late endosome of newly delivered hydrolases causing insufficiency phenotype. Clathrin budding at TGN uses ARF1-GTP pH independent, SNARE stability largely pH independent around 6.6; pH-sensitive switch ensures unidirectional delivery essential.