Subtopic: Fundamentals of Downstream processing (chromatography, filtration)
Question Statement:
Co-translational translocation of proteins is observed in ________.
Options:
(A) endoplasmic reticulum
(B) Golgi complex
(C) mitochondria
(D) peroxisomes
Official Answer Key:
A
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2024 Q.17)**
**1. Governing Concept**:
- **Topic**: Cell Biology & Protein Sorting / Translocation
- **Co-translational translocation** occurs when a nascent polypeptide chain carrying an N-terminal hydrophobic signal sequence is recognized by the Signal Recognition Particle (SRP), arresting elongation until the ribosome binds the SRP receptor on the **endoplasmic reticulum (ER)** membrane, where translation and translocon passage occur simultaneously.
- Mitochondria, chloroplasts, and peroxisomes import proteins primarily post-translationally.
**2. Option Analysis**:
- **Option (A) [endoplasmic reticulum] is CORRECT**: The classic organellar site of co-translational protein translocation.
- **Option (B) [Golgi complex] is INCORRECT**: Receives proteins from the ER via vesicular transport, not direct co-translational translocon import.
- **Option (C) [mitochondria] is INCORRECT**: Utilizes post-translational translocation via TOM/TIM translocase complexes.
- **Option (D) [peroxisomes] is INCORRECT**: Employs post-translational import of fully folded proteins via Pex receptors.
**3. Final Answer**:
- **Correct Choice**: **A**
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