Final answer:
The molecular tag required to keep proteins in the RER is a signal sequence or signal peptide, which is recognized by a signal recognition particle, and later cleaved off by a signal peptidase.
Step-by-step explanation:
The molecular tag required to retain or retrieve proteins in the rough endoplasmic reticulum (RER) is a signal sequence or signal peptide. This signal sequence directs the ribosome to the RER membrane, where synthesis of the secretory protein continues into the lumen of the RER. The signal recognition particle (SRP) binds to this hydrophobic signal peptide during the protein synthesis. In the RER, proteins undergo modifications like core glycosylation, folding, and if needed, are equipped with signal sequences for proper sorting and transport to their final destinations. The signaling sequence is then cleaved off by a signal peptidase in the RER, demonstrating the presence of processing activity within the RER.