What is true of protein glycosylation in the ER?
A. Only proteins phosphorylated on an asparagine residue become glycosylated.
B. Sugar residues are added one at a time by a series of enzymes attached to the ER membrane. C. A block of sugar residues is added to the N-terminal signal sequence, creating a common, N-linked oligosaccharide.
D. Only proteins bearing a dolichol residue become glycosylated.
E. Oligosaccharides are added by an enzyme that has its active site on the lumenal side of the ER membrane.