What is a "signal peptide"?

A signal peptide is indeed a short peptide sequence that plays a critical role in directing proteins to the endoplasmic reticulum (ER) for secretion or membrane insertion. This sequence is typically located at the N-terminus of a nascent polypeptide chain and is crucial for the proper trafficking and localization of the protein within the cell.

When ribosomes synthesize a protein that contains a signal peptide, this sequence is recognized by a signal recognition particle (SRP). The SRP halts translation temporarily and directs the ribosome to the ER membrane. Once the ribosome is correctly positioned on the ER, translation resumes, and the signal peptide facilitates the translocation of the growing polypeptide into the ER lumen. Once inside, the signal peptide is usually cleaved off by signal peptidases, allowing the remaining protein to fold correctly and be further processed for secretion or integration into the membrane.

The other choices do not accurately describe the function of a signal peptide. While there are peptides that stabilize proteins, tags for degradation, and peptides that assist in protein folding, these functions are not related to the fundamental role of signal peptides in directing protein localization within the cell.