A child appears to be suffering symptoms of I-cell disease. Using Western blot analysis of his skin fibroblast proteins and antibodies against N-acetylglucosamine phosphotransferase, the child shows the usual levels of this enzyme. Furthermore, isolated N-acetylglucosamine phosphotransferase showed normal activity in an enzymatic assay. Which of the following could explain why the child presents symptoms of I-cell disease?
A.The N-acetylglucosamine phosphotransferase has a mutation in its active site that impairs enzymatic activity.
B.The N-acetylglucosamine phosphotransferase is secreted due to a mutation that prevents its packaging into COP I coated vesicles.
C.A mutation in the p54 subunit of the signal recognition particle prevents it from binding GTP.
D.The N-acetylglucosamine phosphotransferase accumulates in the ER due to a mutation that prevents its packaging into COP II coated vesicles