Name: how the er clears misfolded proteins
Uploaded: 2023-04-30T10:16:17.000Z
Duration: 1 min 32 s
Channel: JoVE

Misfolded proteins in the ER lumen are recognized and transported back to the cytosol for degradation by the ER-associated degradation or ERAD pathway.
The pathway starts in the ER lumen with the enzyme &#945;-mannosidase I, interacting with the terminally misfolded protein to trim mannose residues from the attached glycans.
OS-9, a lectin chaperone bound to the luminal domain of the protein retrotranslocation complex, recognizes the trimmed glycan on the misfolded protein.
Hrd1 protein is the crucial member of this complex that acts as a channel as well as a ubiquitin ligase.
Other constituent proteins of the retrotranslocation complex push the misfolded protein into the Hrd1 channel.
As the protein emerges on the cytosolic side, it is ubiquitinated by the Hrd1 protein, while the N-glycanase removes the attached glycans.
The ubiquitinated protein segment then recruits the newly assembled Cdc48 AAA-ATPase complex, which uses ATP hydrolysis to pull the remaining protein through the channel.
After the complete retrotranslocation into the cytosol, chaperones like Bag6 pass on the ubiquitinated protein into the proteasome for degradation.

export of misfolded proteins out of the er

Export of Misfolded Proteins out of the ER

After folding, the ER assesses the quality of secretory and membrane proteins. The correctly folded proteins are cleared by the calnexin cycle for transport to their final destination, while misfolded proteins are held back in the ER lumen. The ER chaperones attempt to unfold and refold the misfolded proteins but sometimes fail to achieve the correct native conformation. Such terminally misfolded proteins are then exported to the cytosol by ER-associated degradation or ERAD pathway for degradation.
Different modes of ERAD pathways
Hrd1 is a ubiquitin ligase and a retro-translocation channel for misfolded polypeptides. It complexes with another membrane protein, Hrd3, to form the core ERAD machinery. The other players associated with these core proteins depend upon the topology of the misfolded protein. The retrotranslocation complex depends on the luminal lectin chaperones to identify misfolded proteins. Studies in the yeast system suggest the presence of ERAD-C, ERAD-L, and ERAD-M pathways to export misfolded proteins from various parts of the ER. The target proteins of the ERAD-L and ERAD-M pathways use the Hrd1 channel to exit the ER lumen and membrane, respectively, while the ERAD-C pathway utilizes the Doa10p ubiquitin ligase complex to degrade the cytoplasmic domains of the ER membrane proteins. Sometimes, these pathways can overlap as some misfolded proteins need translocation machinery from two different ERAD pathways to exit the ER. The mammalian ERAD machinery is more complex than yeast and has multiple proteins that export and mark proteins for proteasomal degradation. Eventually, all ERAD pathways converge due to their dependence on a cytoplasmic AAA-ATPase to provide the mechanical force to extract proteins through the membrane.
Degradation of non-glycosylated proteins
The ER has another lectin-independent system that monitors and clears the non-glycosylated proteins from ER. It consists of the resident ER chaperone, BiP protein, and the ER-localized DnaJ family members (ERdjs). ER stress induces the production of HERP, a ubiquitin-like membrane protein, which assembles with other ERAD proteins like the Hrd1 and AAA-ATPase, forming a complex. BiP uses some ERdj proteins as cofactors to recruit non-glycosylated misfolded proteins to the HERP complex for retrotranslocation and ubiquitination of the misfolded protein. HERP is associated with the proteasome on the cytosolic side, where it delivers the misfolded proteins for degradation.

After folding, the ER assesses the quality of secretory and membrane proteins. The correctly folded proteins are cleared by the calnexin cycle for ...

export-of-misfolded-proteins-out-of-the-er

Education

JoVE Core

Cell Biology

Unit 1

Transmembrane Transport in Endoplasmic Reticulum and Peroxisomes

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KEY TERMS AND CONCEPTS

how the er clears misfolded proteins

The endoplasmic reticulum (ER) checks secretory and membrane proteins after they fold. Correctly folded proteins move through the calnexin cycle and continue to their final destination. Misfolded proteins are kept in the ER lumen, where ER chaperones try to unfold and refold them.

When refolding fails, the protein becomes terminally misfolded. The ER then sends it to the cytosol through ER-associated degradation, or ERAD, for destruction. In this pathway, Hrd1 acts as both a ubiquitin ligase and a retrotranslocation channel, and it works with Hrd3 to form the core ERAD machinery.

Different ERAD routes handle proteins based on where the defect is located. In yeast, ERAD-L removes misfolded proteins from the ER lumen, ERAD-M removes proteins with defects in the membrane, and ERAD-C handles proteins with faulty cytoplasmic regions. ERAD-L and ERAD-M use the Hrd1 channel, while ERAD-C uses the Doa10p ubiquitin ligase complex. These routes can overlap, because some misfolded proteins need machinery from two ERAD pathways to leave the ER.

Lectin chaperones in the ER lumen help identify misfolded proteins before retrotranslocation. Mammalian ERAD is more complex than yeast ERAD and uses multiple proteins to export proteins and mark them for proteasomal degradation. Even with these differences, the pathways converge on a cytoplasmic AAA-ATPase, which provides the force needed to pull proteins through the membrane.

The ER also has a lectin-independent system for clearing non-glycosylated proteins. This system uses the chaperone BiP and ER-localized DnaJ family members, called ERdjs. During ER stress, HERP, a ubiquitin-like membrane protein, is produced and joins ERAD proteins such as Hrd1 and the AAA-ATPase to form a complex.

BiP uses some ERdj proteins as cofactors to recruit non-glycosylated misfolded proteins to the HERP complex. The proteins are then retrotranslocated, ubiquitinated, and delivered to the proteasome on the cytosolic side for degradation. HERP helps connect this delivery step to the proteasome so the misfolded protein can be cleared.

How the ER Clears Misfolded Proteins

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The endoplasmic reticulum (ER) checks secretory and membrane proteins after they fold. Correctly folded proteins move through the calnexin cycle and continue to their final destination. Misfolded proteins are kept in the ER lumen, where ER chaperones try to unfold and refold them. When refolding fails, the protein becomes terminally misfolded. The ER then sends it to the cytosol through ER-associated degradation, or ERAD, for destruction. In this pathway, Hrd1 acts as both a ubiquitin ligase...

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key terms and concepts

Learn how the ER sorts folded proteins, sends misfolded ones through ERAD, and clears non-glycosylated proteins using BiP, HERP, and the proteasome.

How the ER Clears Misfolded Proteins | JoVE K12

cell biology

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How the ER Clears Misfolded Proteins

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