During synthesis of a secretory protein, a hydrophobic N-terminal signal peptide emerges from the translating ribosome. Which event occurs immediately next?

• A Signal recognition particle binds the signal peptide and pauses translation ✓
• B The completed protein is released free into the cytosol and later imported into the ER
• C Peptidyl transferase cleaves the signal peptide within the ribosomal tunnel
• D The 80S ribosome dissociates into subunits and reinitiates at an internal AUG

Explanation

The signal recognition particle (SRP), a ribonucleoprotein containing 7SL RNA, binds the emerging signal peptide, halts elongation, and docks the whole complex onto the SRP receptor on the rough ER membrane. Elongation then resumes through the translocon. This co-translational targeting distinguishes secretory and lysosomal proteins from mitochondrial and nuclear proteins, which are made free on cytosolic ribosomes and imported post-translationally.

Reference: Lippincott's Illustrated Reviews: Biochemistry, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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