Which sequence directs a ribosome toward the endoplasmic reticulum for secretion or membrane insertion?

• A. A signal peptide.
• B. A nuclear localization signal.
• C. A mitochondrial targeting sequence.
• D. A peroxisomal targeting sequence.

Answer: (A)

Targeting to the endoplasmic reticulum for proteins that will be secreted or inserted into membranes is directed by a signal peptide at the nascent protein’s N-terminus. This short, hydrophobic sequence is recognized early by the signal recognition particle, which pauses translation and escorts the ribosome–nascent chain to the ER membrane. Once there, the ribosome binds the Sec61 translocon and translation resumes, threading the growing polypeptide into the ER lumen or embedding it in the membrane. The signal peptide is typically cleaved off as translation continues, allowing the protein to enter the secretory pathway. Other targeting signals direct proteins to different organelles (nucleus, mitochondria, peroxisomes), so they don’t guide ribosomes to the ER.