Targeting Proteins to Endosomes and Lysosomes

Targeting Proteins to Endosomes and Lysosomes

  • Endosomes:

    • Vesicles budding off the TGN (Trans-Golgi Network).

    • Can eventually mature into lysosomes.

  • Lysosomes:

    • Function as the cell's garbage disposal, degrading proteins, RNAs, and other cellular components.

    • Contain various enzymes capable of degrading different macromolecules.

Mechanism for Targeting Lysosomal Enzymes

  1. ER to Golgi Transport: Enzymes destined for the lysosome are synthesized in the ER and transported to the Golgi.

  2. Initial Glycosylation: In the ER, these enzymes undergo initial glycosylation, ensuring proper folding.

  3. CGN Modification (Addition of GlcNAc-1-Phosphate):

    • Upon arrival in the CGN (cis-Golgi network), an enzyme adds GlcNAc-1-phosphate to the carbon six of mannose residues on the protein.

    • Specifically, the modification occurs on the sixth carbon of mannose.

  4. MGN Modification (Removal of GlcNAc):

    • The protein then moves to the MGN (medial-Golgi network).

    • Another enzyme removes the GlcNAc, leaving only the phosphate on the carbon six of mannose.

  5. Mannose-6-Phosphate Tag: The presence of phosphate on the sixth carbon of mannose acts as the targeting signal for lysosomes.

  6. TGN Binding and Packaging:

    • In the TGN (trans-Golgi network), receptors bind to the mannose-6-phosphate tag.

    • These receptors package the tagged proteins into vesicles.

  7. Endosome Formation: The vesicles containing the mannose-6-phosphate tagged enzymes bud off from the TGN, forming endosomes.

  8. Lysosome Maturation: The endosomes then mature into lysosomes, delivering the enzymes to their final destination.