GATE 2024 BT Q.17: solution and answer

Official answer: ASource: Official answer key, GATE 2024 BT (IISc Bengaluru)

1. Co-translational translocation means the polypeptide is threaded into or across a membrane while it is still being synthesized on the ribosome, not after release into the cytosol.

2. Option A (ER): The signal sequence emerging from the ribosome is recognized by the signal recognition particle (SRP), which pauses translation and targets the ribosome-nascent chain complex to the SRP receptor on the rough ER membrane. The chain is then threaded through the Sec61 translocon as translation resumes - the defining example of co-translational translocation. Option A is correct.

3. Option B (Golgi complex): The Golgi receives proteins by vesicular transport from the ER only after they have already been translated and translocated into the ER lumen; no ribosome-coupled translocation occurs at the Golgi. Option B is wrong.

4. Option C (mitochondria): Mitochondrial matrix and inner-membrane proteins are synthesized on free cytosolic ribosomes, released as complete or nearly complete chains, kept unfolded by cytosolic chaperones such as Hsp70, and imported afterward through the TOM/TIM complexes - this is post-translational import, not co-translational. Option C is wrong.

5. Option D (peroxisomes): Peroxisomal matrix proteins are likewise fully synthesized and folded in the cytosol, then imported post-translationally via PTS1/PTS2 signals recognized by the Pex5/Pex7 receptors (peroxisomes can even import folded, oligomeric proteins). Option D is wrong.

6. Only the ER pathway couples translation and translocation in real time, so the answer is (A) endoplasmic reticulum.