10. N-linked glycosylation in proteins of eukaryotes is initiated in the:
a. lumen of the endoplasmic reticulum
b. lumen of the Golgi apparatus
c. inside the nucleus
d. within endosomes
N-linked glycosylation in eukaryotic proteins initiates in the lumen of the endoplasmic reticulum. This co-translational process adds a pre-assembled oligosaccharide to asparagine residues.
Correct Answer
a. lumen of the endoplasmic reticulum. The oligosaccharyltransferase enzyme transfers the glycan from dolichol pyrophosphate to nascent polypeptides during translation in the ER lumen.
Option Analysis
-
a. Lumen of the endoplasmic reticulum: Correct site for initiation; glycan assembly starts on the cytosolic ER side then flips to the lumen for transfer.
-
b. Lumen of the Golgi apparatus: Incorrect; Golgi handles maturation and trimming after ER initiation.
-
c. Inside the nucleus: Incorrect; nucleus manages transcription/DNA, not glycosylation.
-
d. Within endosomes: Incorrect; endosomes process internalized materials, not protein glycosylation initiation.
N-linked glycosylation initiation in eukaryotes occurs in the endoplasmic reticulum lumen, a key process for protein folding and quality control in secretory pathways. This modification attaches complex glycans to asparagine residues, essential for eukaryotic cell function.
Process Overview
Glycan precursor Glc3Man9GlcNAc2 assembles on dolichol-P at the ER membrane, then transfers via oligosaccharyltransferase (OST) to Asn-X-Ser/Thr motifs in the ER lumen during translation. Initial glucose trimming follows in the ER, with full maturation in Golgi.
-
Cytosolic steps build the base oligosaccharide.
-
Lumenal transfer ensures co-translational modification.
-
Calnexin/calreticulin binding aids folding.
Common Misconceptions
Golgi processes N-glycans but does not initiate; ER handles the first attachment. Nucleus and endosomes lack glycosylation machinery.
