A KDEL sequence is added at the C-terminus of a secreted glycoprotein X (500 amino acid residues) having no site for N -linked glycosylation and expressed from an inducible promoter. Following 10, 20 and 60 minutes of induction, ER are purified and probed for newly synthesized glycoprotein X-KDEL. The immunoblot obtained is shown below.
Which one of the following statements is the most likely explanation for presence of the higher molecular weight bands in lanes 20 and 60 minutes?
1
The signal sequence is not removed from some of the glycoprotein X-KDEL molecules.
2
Glycoprotein X-KDEL becomes modified in the endoplasmic reticulum after protein synthesis is completed.
3
The glycoprotein X-KDEL molecules are modified in the Golgi prior to ER retrieval.
4
The quality control mechanism in the ER recognizes a pool of glycoprotein X-KDEL as being aberrant and targets it for degradation.