Q.31 N-linked glycosylation in proteins of eukaryotes is initiated in the:
- Lumen of the endoplasmic reticulum
- Lumen of the Golgi apparatus
- Inside the nucleus
- Within endosomes
Correct Answer: Lumen of the endoplasmic reticulum.
N-linked glycosylation begins co-translationally in the ER lumen when oligosaccharyltransferase adds a pre-assembled GlcNAc₂Man₉Glc₃ oligosaccharide from dolichol-PP to asparagine in the Asn-X-Ser/Thr motif.
Option Analysis
Lumen of the endoplasmic reticulum
Correct. The process starts here: lipid-linked oligosaccharide (LLO) flips into the ER lumen, where OST transfers it to nascent polypeptides during translation.Lumen of the Golgi apparatus
Wrong. Golgi modifies N-glycans (trimming, adding sugars) but initiation occurs earlier in ER; O-linked glycosylation starts in Golgi.Inside the nucleus
Incorrect. Nucleus handles transcription/RNA processing, not protein glycosylation which requires ER membrane machinery.Within endosomes
False. Endosomes manage endocytosis/trafficking/recycling; no role in N-glycan initiation.Introduction to N-Linked Glycosylation Eukaryotes ER Lumen
N-linked glycosylation in eukaryotes ER lumen marks the start of a vital protein modification pathway. In this GATE Life Sciences question, the correct site is the endoplasmic reticulum lumen, essential for glycoprotein folding and quality control.
Initiation Process in ER Lumen
A 14-sugar LLO assembles on dolichol-PP (cytosol then flips to lumen). OST recognizes N-X-S/T sequons on ribosome-emerging chains, transferring the glycan co-translationally.
Stage Location Key Event LLO assembly ER membrane (cytosol → lumen) GlcNAc₂Man₉Glc₃ built Transfer ER lumen OST adds to Asn Processing ER/Golgi Glucose trimming, mannose removal Why Not Other Compartments?
Golgi handles maturation; nucleus lacks glycosylation enzymes; endosomes recycle proteins post-modification.
GATE Relevance
Tests ER protein processing pathway. Memorize: N-linked = ER initiation; O-linked = Golgi start.
