Intracellular proteins are targeted for proteolytic degradation in proteasomes upon
conjugation with
Options:
(A) ubiquitin
(B) integrin
(C) peptidase
(D) calreticulin
Official Answer Key:
A
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2023 Q.21)**
**1. Governing Concept**:
- **Topic**: Cell Biology & Ubiquitin-Proteasome System (UPS)
- Targeted intracellular protein degradation occurs via the 26S proteasome. Target proteins are marked for destruction through covalent attachment of polyubiquitin chains via an enzymatic cascade involving ubiquitin-activating enzyme (E1), conjugating enzyme (E2), and ubiquitin ligase (E3).
- **Polyubiquitin** chains linked via Lysine-48 (K48) serve as the canonical destruction signal recognized by the 19S regulatory particle of the proteasome.
**2. Option Analysis**:
- **Option (A) [ubiquitin] is CORRECT**: Covalent modifier targeting substrates to the 26S proteasome.
- **Option (B) [integrin] is INCORRECT**: Transmembrane adhesion receptor linking cells to the extracellular matrix.
- **Option (C) [peptidase] is INCORRECT**: Proteolytic enzyme, not a covalent targeting tag.
- **Option (D) [calreticulin] is INCORRECT**: Endoplasmic reticulum chaperone assisting glycoprotein folding.
**3. Final Answer**:
- **Correct Choice**: **A**
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