GATE 2023 BT – Question 21

Genetics, Cellular and Molecular Biology · Cell Biology: Post-translational modifications, protein trafficking, cell death and autophagy, extracellular matrix · 1 mark · Multiple choice

Intracellular proteins are targeted for proteolytic degradation in proteasomes upon conjugation with

ubiquitin
integrin
peptidase
calreticulin

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Correct answer: (A) ubiquitin

Explanation

Cytosolic and nuclear proteins that are damaged, misfolded or short-lived are marked for destruction by the **ubiquitin-proteasome system**. Ubiquitin, a small 76-amino-acid protein, is attached to a lysine of the target by E1, E2 and E3 enzymes. A chain of four or more ubiquitins (a polyubiquitin tag) is recognised by the **26S proteasome**, which unfolds and degrades the protein.

**Integrin:** a cell-adhesion receptor.
**Peptidase:** an enzyme that cleaves peptide bonds, not a tag.
**Calreticulin:** a chaperone in the endoplasmic reticulum.

Answer **ubiquitin** (A).