What is the role of signal recognition particle (SRP) during ER translation?

• A. It binds to the signal peptide to target the ribosome to the ER
• B. It catalyzes peptide bond formation
• C. It cleaves the signal peptide
• D. It degrades misfolded proteins

Answer: (A)

The key idea is that SRP serves as a targeting and timing device during ER translation. As the nascent polypeptide emerges from the ribosome and presents a hydrophobic signal sequence, SRP binds to that signal and to the ribosome, pausing translation briefly. This SRP-ribosome-nascent chain complex then docks at the SRP receptor on the rough ER membrane, guiding the ribosome to the Sec61 translocon. Once anchored, translation resumes and the growing chain is threaded into the ER lumen or inserted into the membrane through the translocon. After docking, SRP is released and recycled, while the signal peptide is eventually cleaved by signal peptidase inside the ER.

So, SRP’s role is to recognize the signal peptide and target the ribosome–nascent chain to the ER, enabling co-translational translocation. The other processes—peptide bond formation, signal peptide cleavage, and degradation of misfolded proteins—are carried out by different components (the ribosome for elongation, signal peptidase for cleavage, and ER quality control/ERAD for misfolded proteins).