The ubiquitin–proteasome pathway
A misfolded cytosolic protein is tagged with a chain of ubiquitin molecules. Where will it be degraded?
Answer D: explanation for every choice
A
Mannose-6-phosphate targets newly made enzymes to lysosomes. It is not a degradation signal for cytosolic proteins.
B
The nucleolus assembles ribosomal subunits.
C
The rough ER synthesizes secretory and membrane proteins.
D
Correct. A polyubiquitin chain marks a protein for degradation by the 26S proteasome.
TakeawayPolyubiquitin → proteasome.
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Reviewed 2026-11-05 by a medical student. Spot an error? Report it with question ID bb107.