A defect in the mannose-6-phosphate (M6P) receptor protein would most directly impair the proper targeting of which cellular component?
Correct: C
The mannose-6-phosphate (M6P) tag is a crucial signal on proteins synthesized in the ER and processed in the Golgi apparatus. This tag specifically marks lysosomal hydrolases. The M6P receptor in the trans-Golgi network binds these tagged enzymes and packages them into vesicles that are subsequently delivered to lysosomes. A defect in this receptor would prevent these vital digestive enzymes from reaching the lysosomes, leading to their misdirection (often secreted) and a buildup of undigested material within the cell, characteristic of lysosomal storage diseases. Other options involve different targeting pathways.