Watch this video (from Ricochet Science) to refresh your understanding of the endomembrane system.
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Recall from the tutorial on eukaryotic cells that proteins fall into one of two basic categories. The first are proteins that will stay inside the cell and are mainly produced by free ribosomes. The second are proteins that are intended to leave the cell; they are mainly produced by ribosomes bound to the rough endoplasmic reticulum (RER).
Proteins that are intended to leave the cell contain signal sequences that facilitate their sorting within the cell. Newly synthesized proteins produced in the rough ER have a unique signal sequence that directs them to this organelle. The figure below illustrates the sorting process. If a translational complex (a free ribosome with an associated mRNA molecule) (1) has a growing protein with this sequence at its N-terminus (the end that is synthesized first), then a signal recognition particle (SRP) binds to the growing polypeptide (2). The SRP facilitates association with the rough ER, and the translational complex docks to the rough ER (the once free ribosome is now a bound ribosome) at a specific receptor site located within rough ER pore complexes (3). The growing polypeptide is then inserted through the pore and translation continues; note: the polypeptide is now being inserted into the lumen of the rough ER as it is synthesized (4). Specific peptidases inside the lumen recognize the signal sequence and this short stretch is removed (5). Once translation is complete, the translational complex dissociates, leaving the newly synthesized protein inside the rough ER.
From the rough endoplasmic reticulum, the protein moves to the Golgi where it is further modified and labelled for export. From the Golgi, the protein is transferred to the plasma membrane (via a transport vesicle) where it is exported outside of the cell.
The scientist who discovered signal sequences and their role in protein trafficking was awarded the Nobel Prize in Medicine for his work. You can read more about this at the Nobel Prize Web site .