Explain the targeting of soluble lysosomal proteins to endosomes and lysosomes

The targeting of soluble lysosomal proteins, such as hydrolytic enzymes, to endosomes and lysosomes is a well-organised and highly regulated process. This mechanism ensures that such enzymes reach the lysosomes accurately, where they are needed for intracellular digestion, and are not secreted outside the cell. This entire process takes place through a series of interconnected steps, involving specific molecular signals, vesicle formation and organelle targeting, which begins in the rough endoplasmic reticulum (RER) and ends in the lysosome.

Step 1: Synthesis in the Rough Endoplasmic Reticulum (RER)

Soluble lysosomal enzymes are synthesised by ribosomes attached to the rough ER. As the mRNA is translated, the growing polypeptide is inserted into the ER lumen. Inside the ER, these proteins are properly folded and undergo N-linked glycosylation, which prepares them for further modifications in the Golgi.

Step 2: Transfer to the Golgi Apparatus

From the ER, the glycosylated proteins are transported via COPII-coated vesicles to the cis-Golgi. As they move from the cis to the trans side, they undergo additional sugar modifications, preparing them for the critical mannose-6-phosphate tagging.

Step 3: Addition of Mannose-6-Phosphate (M6P) Tag

In the medial-Golgi, a two-step enzymatic reaction occurs:

1. GlcNAc phosphotransferase adds a phospho-GlcNAc group to specific mannose residues.
2. A second enzyme removes the GlcNAc, leaving behind the mannose-6-phosphate (M6P) group.

This M6P acts as a sorting signal to ensure correct lysosomal targeting.

Step 4: Recognition and Packaging in Trans-Golgi Network (TGN)

In the trans-Golgi network, the M6P-tagged enzymes are recognised by mannose-6-phosphate receptors (M6PRs). These receptor-ligand complexes are packed into clathrin-coated vesicles with the help of adaptor proteins (like AP1), which link clathrin to the vesicle membrane and select the cargo.

Step 5: Delivery to Early Endosomes

These clathrin-coated vesicles bud off from the TGN and fuse with early endosomes. The acidic pH of the endosome causes the enzyme to dissociate from the M6P receptor. The receptor is then recycled back to the TGN for reuse.

Step 6: Final Transport to Lysosomes

From the early endosomes, the enzymes are delivered to late endosomes, and finally to lysosomes. In the lysosomes, the enzymes become fully active in the acidic environment and help in breaking down macromolecules like proteins, lipids, and carbohydrates.