Protein Glycosylation: Implications for In Vivo Functions and Therapeutic Applications - Advances in Biochemical Engineering Biotechnology - page 161

The consensus sequence Asn-X-Ser/Thr is a necessary, but not sufficient,
requirement for addition of Asn-linked glycans. Less than half of the known
tripeptide sequences in secreted proteins are only glycosylated, and this may be
due to differences in accessibility of a sequon in a protein and to OST and the
folding of the nascent protein chain [38]. The conformation and rate of trans-
lation of nascent polypeptide too influence the frequency of sequon utilization.
Thus, preventing cotranslational disulfide bond formation in ER in presence of
dithiothreitol (DTT) leads to complete glycosylation of a sequon in tPA that
otherwise undergoes variable glycosylation in untreated cells. This shows that

Endoplasmic Reticulum

Cis-Golgi

Median-Golgi

Trans-Golgi

Secretion

Fig. 3. Representative pathway for the biosynthesis of Asn-linked oligosaccharide. Biosynthe-
sis starts in the rough endoplasmic reticulum and proceeds through the cis -, medial-, and
trans -Golgi apparatus, before glycoprotein is secreted. Some of the enzymes involved are:
oligosaccharyl transferase (OST);

a -glucosidase I & II ( a G I & II); ER-mannosidase ( a M);

M I & II); N -acetylglucosaminyl transferase I & II (GnT I & II);
fucosyl transferase (FucT); galactosyl transferase (GalT); sialyl transferase (ST).

a -mannosidase I & II ( a

-Gn;

-Man; -Glc; -Fuc; -Gal; -SA (adopted from [40])