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#mannose-6-phosphate

3 public questions tagged with this topic.

Cell Organelles

Proteins destined for lysosomes are tagged with:

Lysosomal hydrolases require segregation from secretory proteins to avoid uncontrolled extracellular degradation. Selectivity conferred by addition of mannose-six-phosphate recognition marker in Golgi apparatus. In cis-Golgi UDP-GlcNAc phosphotransferase complex recognizes common conformational surface formed by lysine residues on hydrolases absent from secretory proteins, transferring N-acetylglucosamine-one-phosphate to C6 of mannose residues on high mannose N-glycans creating phosphodiester intermediate. Second enzyme GlcNAc-one-phosphodiester alpha-N-acetylglucosaminidase in trans Golgi removes GlcNAc leaving exposed M6P monoester. Trans-Golgi network contains two M6P receptors cation-dependent and cation-independent that bind M6P at neutral pH, recruit clathrin adaptor GGA and AP1 forming vesicles destined to late endosomes where acidic pH causes dissociation, hydrolase released. Receptors recycle. Ubiquitin signals proteasome, SUMO nuclear transport, glucose-six-phosphate metabolism. Deficiency produces I-cell disease with missorted enzymes in plasma, confirming essential tag role. Integration with cell cycle kinases, calcium signaling and mechanical cues ensures coordinated remodeling during growth, migration and differentiation. Cross-talk via plectin, desmoplakin and plus-end tracking proteins coordinates cytoskeletal networks for efficient force distribution and organelle positioning.

Ref: Kornfeld & Mellman; M6P receptors TGN sorts hydrolases to endosomes, I-cell disease marker.

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Protein Sorting and Trafficking

What type of signal directs proteins from the Golgi to lysosomes?

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.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 13: Mannose-6-Phosphate Tagging for Lysosomal Targeting.

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