Final answer:
When SRP binds to SRP receptor in the ER, the SRP is released, allowing the protein to be threaded into a translocation channel in the RER lumen where it undergoes modifications.
Step-by-step explanation:
When an SRP complex binds to the SRP in the endoplasmic reticulum (ER), the SRP is then released into the lumen, which then results in the protein being threaded into a protein translocation channel. This process begins when an SRP (signal recognition particle) binds to the hydrophobic signal peptide of a nascent polypeptide on a ribosome. Translation elongation halts temporarily until the ribosome-SRP complex finds and binds to an SRP receptor on the membrane of the rough endoplasmic reticulum (RER).
Once docked on the RER membrane, the SRP detaches from the growing polypeptide chain and is recycled for future use. Translation then resumes, and the growing polypeptide is co-translationally translocated into the lumen of the RER. After entering the RER lumen, the protein undergoes various modifications, such as folding, the acquisition of side chains, or the addition of sugar molecules. Further sorting and modification occur in the Golgi apparatus before the proteins reach their final cellular destination.