# DBT JRF 2022 — Question 17

Exam: DBT JRF 2022 · Section: B · Topic: Cell Biology › Protein Glycosylation & ER Processing · Difficulty: Medium

## Question

Proteins are glycosylated in the endoplasmic reticulum. In the final structures, the glycan moieties are typically present:

## Options

- **A.** on the surface of these proteins.
- **B.** in the core of these proteins.
- **C.** on the surface as well as in the core of these proteins.
- **D.** on the membrane inserted segments of these proteins.

## Answer

Option A

## Explanation

Location of glycan moieties on glycoproteins: Glycosylation occurs in the ER and Golgi. The key principle is topological: • Proteins enter the ER lumen (equivalent to the extracellular space topologically). • Glycosylation enzymes add sugars to the LUMENAL face of the ER/Golgi membrane. • As vesicles bud and fuse, the lumenal face always faces outward or into the lumen. • For secreted and plasma membrane proteins, glycans end up on the EXTRACELLULAR/OUTER surface. N-linked glycosylation: added to Asn in Asn-X-Ser/Thr motifs in the ER lumen. O-linked glycosylation: added to Ser/Thr in the Golgi lumen. Conclusion: Glycan moieties are present on the SURFACE (extracellular-facing side) of glycoproteins, never in the hydrophobic core or transmembrane segments. Answer: On the surface of proteins (Option 1).

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