Final answer:
In the ER, misfolded proteins are marked by ubiquitin for degradation. The ubiquitin-proteasome system ensures these proteins are digested into amino acids. This system is critical for protein quality control and regulating protein lifespan.
Step-by-step explanation:
The process in which misfolded proteins in the ER are identified and subsequently returned to the cytoplasm for degradation involves the ubiquitin-proteasome system. Ubiquitin, a 76-amino acid polypeptide, tags proteins for destruction by attaching to them. This "tagging" serves as a signal for the proteasome, a large complex of polypeptides, to recognize and digest the targeted proteins.
During the degradation process, the targeted protein is digested into short peptide fragments by proteolytic enzymes within the interior of the proteasome core. These fragments are released and further broken down into free amino acids in the cytoplasm. The sequence begins with the misfolded proteins being recognized and ubiquitinated, a process implying the end of the protein's lifespan and marking it for degradation.
Ubiquitin acts not only as a marker for protein degradation but also plays a role in controlling gene expression by determining the longevity of proteins. To ensure that proteins reach their final destinations in their correct form, they are processed through the endomembrane system, involving the RER and the Golgi apparatus, before degradation in case they are flawed.