Figure 6. Different fates of polypeptides synthesized in ribosomes bound to the endoplasmic reticulum membrane.

Proteins synthesized in membrane-bound ribosomes are inserted into the ER membranes or translocated into the ER lumen by a process that begins during their synthesis. After their synthesis is completed, these proteins either remain in the ER or are transferred to the membranes or luminal cavities of other organelles within the endomembrane system.