This happens because the proteins for the ER (proteins for secretion) are translocated into the ER during their synthesis (translation) on membrane-bound ribosomes and this is called cotranslational translocation. Proteins that are destined for the ER are marked with the signal sequence (signal peptide) at the amino terminus of the growing polypeptide chain. The signal is a series of hydrophobic amino acids, usually found near the N-terminus of the protein. and it’s recognized by a protein complex called signal-recognition particle (SRP), which takes the ribosome to the ER.
