Sorting a Cargo Receptor at the Golgi
A cell biologist studies a transmembrane cargo receptor that cycles between the trans-Golgi network (TGN) and endosomes. At 20 degrees C, cargo accumulates in the TGN and fails to reach endosomes; shifting to 37 degrees C releases the block. Depleting clathrin heavy chain reduces, but does not abolish, cargo exit from the TGN. Mutating a cytoplasmic dileucine motif in the receptor's tail reduces endosomal delivery by about 80% without changing surface expression. A dynamin mutant (K44A) traps cargo in long tubules emanating from the TGN. The receptor's lumenal domain is unchanged in all conditions.
1. Interpret the 20 degrees C block and the clathrin-depletion result together. What does each tell you about the trafficking step?
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The 20 degrees C block is a classic thermally reversible exit/trafficking arrest: at 20 degrees C, cargo accumulates at the TGN because membrane budding and/or fusion is slowed, and rewarming to 37 degrees C restores forward traffic. It localizes the rate-limiting step to exit from the TGN, not to synthesis or lumenal folding. Clathrin depletion reducing but not abolishing exit means clathrin-coated vesicles are a major but not exclusive route. There must be a clathrin-independent exit pathway (e.g., a distinct coat or bulk-flow mechanism) that carries some cargo. So the correct conclusion is that TGN-to-endosome traffic is partially clathrin-dependent, and the residual flux after depletion is not an artifact but evidence of route redundancy.
2. The dileucine mutation and the dynamin K44A mutant both impair endosomal delivery. Explain how these two defects act at different mechanistic levels.
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The dileucine motif is a cytoplasmic sorting signal recognized by adaptor complexes (e.g., AP-1/AP-2 or related clathrin adaptors) that recruit cargo into a forming bud. Mutating it reduces cargo capture/selection-the receptor is less efficiently packaged, so delivery falls ~80% even though the machinery itself is intact. The dynamin K44A defect acts later, at vesicle scission: dynamin's GTPase-driven constriction is required to release the bud from the donor membrane. Blocking it does not prevent cargo selection or bud formation; instead it traps cargo in long tubules still connected to the TGN, which is exactly what is observed. So the dileucine mutation is a cargo-sorting lesion, and the dynamin mutant is a membrane-fission lesion; they impair the same pathway at different steps. The unchanged lumenal domain and surface expression argue against a folding or gross-mislocalization explanation.