How are proteins directed to the endoplasmic reticulum during protein synthesis?

Question: Explain the process of protein targeting to the endoplasmic reticulum during protein synthesis. Answer: During protein synthesis, proteins are targeted to the endoplasmic reticulum through a series of steps. First, protein synthesis begins in the cytoplasm, where the ribosome translates mRNA into polypeptide chains containing a signal sequence at the N-terminus. This signal sequence is recognized by a signal recognition particle (SRP), which binds and pauses translation. The ribosome-SRP complex is then targeted to the endoplasmic reticulum membrane with the assistance of an SRP receptor. The ribosome docks onto a translocon, and translation resumes as the polypeptide chain is threaded through the translocon channel into the endoplasmic reticulum. The signal sequence is cleaved off, and the protein undergoes folding and post-translational modifications in the endoplasmic reticulum. Finally, the mature protein is transported to its final destination within or outside the endoplasmic reticulum.