What is a signal peptide and why is it important for protein targeting?

• A. A signal peptide is a sequence that signals mRNA transcription.
• B. A signal peptide directs a nascent protein to the secretory pathway (ER) or other compartments.
• C. A signal peptide binds to Golgi only after folding.
• D. A signal peptide directs ribosome to nucleus.

Answer: (B)

Signal peptides are short sequences at the start of many newly made proteins that act as destination labels, directing where the protein should go inside the cell. As the protein is synthesized, the signal peptide is recognized by the signal recognition particle, which pauses translation and brings the ribosome to the endoplasmic reticulum. Translation then resumes with the growing protein threaded into the ER through a channel, so the protein enters the ER lumen or embeds in the ER membrane. The signal peptide is typically removed by a signal peptidase, and the protein can then travel through the secretory pathway to be secreted, reside in the ER, Golgi, lysosomes, or become part of a membrane. This targeting is crucial because secreted and membrane proteins often require specific folding, disulfide bonds, and glycosylation that occur in the ER and Golgi, and misrouting can disrupt function. Other options aren’t correct because signal peptides are not about signaling transcription, they don’t specifically target proteins to the Golgi after folding, and they aren’t responsible for directing ribosomes to the nucleus—the latter is done by nuclear localization signals.