Questions: Trans-Golgi Network and Protein Sorting

Question 1

A researcher uses gene editing to disable the enzyme that adds mannose-6-phosphate tags to lysosomal hydrolases. What would she observe about the hydrolases in these cells?

• A. Hydrolases would accumulate in the ER because they cannot complete N-glycosylation without the M6P tag
• B. Hydrolases would be secreted out of the cell via the constitutive secretory pathway
• C. Hydrolases would be degraded in the Golgi because the TGN cannot package untagged cargo
• D. Hydrolases would accumulate in the trans-Golgi network, blocking all vesicle trafficking

Answer: (B)

Without the M6P tag, lysosomal hydrolases cannot be recognized by M6P receptors at the TGN and are not sorted into the lysosome-directed vesicles. Instead, they enter the default constitutive secretory pathway and are released into the extracellular space. This is exactly what occurs in I-cell disease (mucolipidosis II): lysosomal enzymes are found in the blood and urine rather than in lysosomes, and undigested material accumulates inside cells with swollen, dysfunctional lysosomes.

Question 2

An ER-resident chaperone is accidentally swept forward into the Golgi by anterograde transport. Which molecular mechanism ensures its retrieval?

• A. The protein's large size prevents it from being packaged into small Golgi vesicles, so it passively diffuses back
• B. KDEL receptors in the Golgi recognize the C-terminal KDEL sequence and package the protein into COPI-coated retrograde vesicles
• C. Mannose-6-phosphate receptors in the TGN detect the chaperone's glycans and route it back to the ER
• D. The ER's translocon machinery actively reaches into the Golgi to pull mislocalized proteins back

Answer: (B)

ER-resident soluble proteins carry a C-terminal KDEL tetrapeptide (Lys-Asp-Glu-Leu). Golgi membranes contain KDEL receptors that recognize this sequence and capture escaped ER proteins, packaging them into COPI-coated vesicles for retrograde transport back to the ER. Once the protein returns to the ER's neutral pH, the KDEL receptor releases it (KDEL binding is pH-dependent, with higher affinity at the Golgi's slightly acidic pH). This retrieval system maintains the distinct molecular identity of each secretory compartment.

Question 3

Constitutive secretion is a specialized pathway used primarily by professional secretory cells (like pancreatic acinar cells) that continuously produce large amounts of a specific exported protein.

• T. True
• F. False

Answer: False

Constitutive secretion is the *default* pathway in all cells — proteins without any special sorting signal are continuously packaged and delivered to the plasma membrane or released extracellularly. It requires no special signal, no triggering event, and no storage step. Regulated secretion is the specialized pathway: it is restricted to cells with secretory granules (neurons, endocrine cells, exocrine glands) and requires an external signal (such as calcium influx) to trigger fusion and release.

Question 4

Mannose-6-phosphate receptors release their lysosomal cargo in the acidic environment of late endosomes and are then recycled back to the TGN for reuse.

• T. True
• F. False

Answer: True

The M6P sorting cycle is pH-dependent. M6P receptors bind hydrolases in the TGN at near-neutral pH and release them in the acidic late endosome (pH ~5.5), where the phosphate group is also removed from the mannose. The receptor is then recycled in retrograde vesicles back to the TGN. This recycling allows a relatively small number of receptors to sort large amounts of hydrolase over time.

Question 5

Why must lysosomal hydrolases be sequestered within lysosomes rather than released into the cytoplasm, and what disease illustrates what happens when the targeting mechanism fails?

Model answer: Lysosomal hydrolases are digestive enzymes — proteases, lipases, nucleases, and glycosidases — that break down all major classes of biological molecules. Releasing them into the cytoplasm would destroy cellular structures and trigger cell death. I-cell disease (mucolipidosis II) results from deficiency of the enzyme that adds the M6P tag: hydrolases are routed to the default secretory pathway and released extracellularly instead of reaching lysosomes. Without functional hydrolases, undigested macromolecules accumulate in swollen lysosomes, causing severe lysosomal storage disease.

The M6P system is a landmark example of how cells use molecular tags to solve the sorting problem: how does a protein know where to go in a cell with dozens of distinct compartments? The answer is a combinatorial code of signals (KDEL, M6P, dilysine, signal anchors) recognized by specific receptors at the appropriate sorting station. Disrupting any one tag reveals its essential function by misrouting the tagged cargo.