Fate of Proteins After Translation
Proteins undergo sorting, folding, modification, and degradation after translation.
Protein Sorting
Newly made proteins are directed to specific cellular destinations via sorting signals—short peptide sequences in the protein. There are three main sorting mechanisms:
- Gated Transport (to nucleus): Proteins with a nuclear localization signal (NLS) pass through the nuclear pore complex. NLS is not removed after import, enabling reuse after cell division.
- Transmembrane Transport (to mitochondria): Cytosolic proteins with an N-terminal signal peptide are guided by receptors and translocators into mitochondria in an unfolded state. The signal is cleaved inside.
- Vesicular Transport (to ER/Golgi/lysosomes/secretion): Proteins with an N-terminal ER signal are recognized by signal recognition particle (SRP) and translated directly into the ER lumen. They are then transported by vesicles to the Golgi, and from there to:
- Cell membrane (default secretion)
- Secretory vesicles (regulated, clathrin-coated)
- Lysosomes (via mannose-6-phosphate signal)
- Vesicle Coating:
- COPII: forms vesicles for transport from the ER to the Golgi
- COPI: forms vesicles for transport from the Golgi to the ER
I-cell Disease: Failure to add mannose-6-phosphate leads to misrouting of lysosomal enzymes, causing inclusion bodies and fatal childhood disease.