Overview of cellular shipping routes
- Proteins are fed into the ER during translation if they have an amino sequence called a signal peptide. In general, proteins bound for organelles in the endomembrane system (such as the ER, Golgi apparatus, and lysosome) or for the exterior of the cell must enter the ER at this stage.
- Proteins that do not have a signal peptide stay in the cytosol for the rest of translation. If they lack other "address labels," they'll stay in the cytosol permanently. However, if they have the right labels, they can be sent to the mitochondria, chloroplasts, peroxisomes, or nucleus after translation.
The endomembrane system and secretory pathway
- Signal recognition particle (SRP) binds to the signal peptide as it emerges from the ribosome.
- SRP brings the ribosome to the ER by binding to a receptor on the ER surface. The receptor is associated with other proteins that make a pore.
- The ribosome resumes translating, feeding the polypeptide through the pore and into the ER lumen (interior).
- An enzyme associated with the pore snips off the signal peptide.
- Translation continues, and the growing amino acid chain slides into the ER lumen.
- The completed polypeptide is released into the ER lumen, where it floats freely.