What is a signal peptide and how does it relate to the secretory pathway?

• A. It marks the catalytic site of a protein
• B. It is an N-terminal signal directing nascent peptide to the ER; enters secretory pathway or becomes membrane protein
• C. It is a nuclear localization signal
• D. It is a stop-transfer sequence

Answer: (B)

The main idea is that a signal peptide is an N-terminal tag that directs how a protein is handled by the cell. An N-terminal signal peptide guides the growing polypeptide to the endoplasmic reticulum, where the ribosome docks to the ER membrane and translation continues through the translocon. This routes the protein into the secretory pathway, so it can be secreted outside the cell or integrated into a membrane. Usually, the signal peptide is cleaved off in the ER, and the protein assumes its final location after further processing in the secretory system. This is different from other targeting signals: it isn’t a marker of catalytic activity, it isn’t a nuclear localization signal, and a stop-transfer sequence is a separate feature that halts translocation to anchor a protein in the membrane.