The KDEL signal sequence is required for retaining proteins within the ER lumen. Assume there is a protein named XY. XY is intended to function in the ER lumen. How does protein XY acquire the KDEL signal sequence? That is, where does the KDEL sequence come from?

A. It is added to the protein XY during protein synthesis.
B. It is added to the protein XY in the ER lumen.
C. It is part of protein XY's AA sequence.
D. The KDEL sequence exists in a different polypeptide that interacts and binds to protein XY.