- Endoplasmic reticulum‑associated degradation (ERAD) is a core component of the cellular protein quality control system that ensures only properly folded proteins proceed through the secretory pathway. Because the ER is responsible for folding, modifying and trafficking one‑third of the proteome, even small disturbances in its folding environment can lead to the accumulation of misfolded or aberrant proteins. ERAD identifies these defective proteins, retrotranslocates them to the cytosol, ubiquitinates them and directs them to the ubiquitin–proteasome system for degradation. This prevents proteotoxic stress, maintains ER homeostasis and supports organismal health.
- ERAD begins with substrate recognition. Misfolded proteins expose hydrophobic regions, unpaired cysteines or abnormal glycan structures that are detected by ER‑resident chaperones and lectins such as BiP/GRP78, calnexin, calreticulin and EDEM. These factors distinguish folding intermediates from terminally misfolded proteins, ensuring that ERAD targets only those substrates that cannot be rescued by the proteostasis network.
- Once identified, ERAD substrates are delivered to membrane‑embedded complexes containing E3 ubiquitin ligases such as HRD1, gp78 and RNF5. These ligases polyubiquitinate misfolded proteins, marking them for degradation. Ubiquitination is tightly coordinated with retrotranslocation, a process in which substrates are extracted from the ER lumen or membrane and moved into the cytosol. Retrotranslocation requires ATP‑dependent pulling forces generated by the AAA‑ATPase p97/VCP, which interacts with ubiquitinated substrates and delivers them to the proteasome.
- The proteasome unfolds and degrades ERAD substrates into peptides, completing the quality‑control cycle. This rapid turnover prevents toxic accumulation of misfolded proteins and maintains ER function under physiological and stress conditions. ERAD is essential for clearing misfolded glycoproteins, mutant membrane proteins, stalled folding intermediates and proteins damaged by oxidative or metabolic stress.
- ERAD is tightly integrated with the unfolded protein response (UPR). During ER stress, ATF6 and XBP1s upregulate ERAD components, increasing degradation capacity. PERK‑mediated translational attenuation reduces the influx of new proteins, giving ERAD time to clear accumulated substrates. Together, ERAD and the UPR form a multilayered defence system that restores ER homeostasis.
- ERAD also interfaces with autophagy. When misfolded proteins form aggregates too large for proteasomal degradation, selective autophagy pathways such as ER‑phagy remove damaged ER regions. This cooperation ensures efficient clearance of proteotoxic material under severe stress.
- Dysregulation of ERAD contributes to numerous diseases. In neurodegenerative disorders, impaired ERAD leads to accumulation of misfolded proteins that disrupt neuronal function. In metabolic diseases such as diabetes, ERAD influences insulin production and β‑cell survival. In cancer, tumour cells often exploit ERAD to tolerate high secretory demand and proteotoxic stress, making ERAD components attractive therapeutic targets.
- In summary, ERAD is a central ER quality‑control pathway that identifies misfolded proteins, retrotranslocates them to the cytosol, ubiquitinates them and directs them to the proteasome for degradation. Through coordinated action with chaperones, UPR signalling and autophagy, ERAD preserves proteostasis and protects cells from stress‑induced damage. Its essential role in homeostasis, metabolism and disease makes ERAD a major focus of modern cell‑stress biology.
