Export of Misfolded Proteins out of the ER

The ER-associated degradation (ERAD) pathway exports misfolded proteins from the ER lumen to the cytosol for degradation. After protein folding quality check in the ER, correctly folded proteins proceed to their destinations, but terminally misfolded proteins that cannot be refolded by ER chaperones must be removed. ERAD prevents accumulation of defective proteins that could harm cellular function.

Hrd1 is a dual-function protein that acts as both a ubiquitin ligase and a retrotranslocation channel. It forms the core of the retrotranslocation complex and creates a channel through which misfolded proteins pass from the ER lumen to the cytosol. As proteins emerge on the cytosolic side, Hrd1 ubiquitinates them, marking them for proteasomal degradation.

Lectin chaperones like OS-9 recognize misfolded proteins by binding to trimmed glycans on their surface. The enzyme α-mannosidase I first trims mannose residues from the glycans attached to the misfolded protein. OS-9, bound to the luminal domain of the retrotranslocation complex, then identifies these trimmed glycans as a signal that the protein is misfolded and requires export.

The Cdc48 AAA-ATPase complex is recruited by ubiquitinated protein segments and uses ATP hydrolysis to generate mechanical force. This energy pulls the remaining misfolded protein through the Hrd1 channel, completing the retrotranslocation process. All ERAD pathways ultimately depend on this cytoplasmic AAA-ATPase to provide the force necessary for extracting proteins across the ER membrane.

Once fully retrotranslated into the cytosol, ubiquitinated misfolded proteins are transferred by chaperones like Bag6 to the proteasome for degradation. The N-glycanase removes any remaining attached glycans during or after retrotranslocation. The proteasome recognizes the ubiquitin tags and degrades the protein into small peptides.

The ER uses a lectin-independent system for non-glycosylated proteins involving the BiP chaperone and ERdj cofactors. Under ER stress, HERP, an ubiquitin-like membrane protein, assembles with Hrd1 and AAA-ATPase to form a complex. BiP recruits non-glycosylated misfolded proteins to this HERP complex for retrotranslocation and ubiquitination, with HERP delivering them to the proteasome.

Different ERAD pathways exist because misfolded proteins have different topologies and locations within the ER. ERAD-L targets luminal proteins, ERAD-M targets membrane proteins, and ERAD-C targets cytoplasmic domains using different ubiquitin ligases. Some misfolded proteins require machinery from multiple pathways to exit the ER, demonstrating that pathway diversity accommodates the structural variety of ER proteins.