What signal directs proteins to the endoplasmic reticulum (ER)?

Proteins destined for the endoplasmic reticulum (ER) are directed there by a specific amino acid sequence known as the signal peptide or ER signal sequence. This sequence is typically located at the N-terminus of the nascent polypeptide chain. When the ribosome begins translating the mRNA, the presence of this signal peptide causes the ribosome to be targeted to the ER membrane. As the protein is synthesized, the signal peptide is recognized by the signal recognition particle (SRP), which then guides the ribosome to the ER membrane where translation can continue, allowing the growing polypeptide to be co-translationally translocated into the lumen of the ER.

This mechanism is vital for the correct localization of proteins that are either secreted from the cell or embedded in the membranes of the endoplasmic reticulum and other organelles. The specificity of this signal is critical for maintaining cellular function, as proteins need to be precisely directed to their respective sites to perform their roles effectively.