Degradation of Misfolded Secretory and Membrane Proteins and Associated Diseases


Protein homeostasis is maintained through a balance among protein synthesis, folding, assembly and degradation. The latter is crucial also to prevent accumulation of misfolded products in the cell. The conjugation to ubiquitin marks proteins for degradation by the proteasome. Secretory and membrane proteins are monitored for proper folding and oligomerisation in the endoplasmic reticulum (ER). In this compartment, defective proteins are recognised and targeted to the proteasome in a process called ER‐associated protein degradation or ERAD. A first step of retrotranslocation across the ER membrane to the cytosol is required. Ubiquitylation is carried out by ER enzymes and is also functionally intertwined with retrotranslocation. Malfunctioning of ERAD machinery or accumulation of folding‐defective proteins in the ER is associated with various human diseases ranging from neurodegenerative disorders to cancer. The design of drugs that meliorate ERAD or promote protein folding could provide new therapeutic strategies against these diseases.

Key Concepts

  • The ER is the organelle where secretory or membrane proteins are folded and assembled with the aid of chaperones and oxidoreductases before they exit the ER.
  • A quality control inside the ER monitors protein folding, and terminally misfolded or unassembled proteins are selectively recognised and targeted to ERAD.
  • Trimmed N‐glycans of terminally misfolded luminal glycoproteins are signals for recognition as a substrate for degradation.
  • ERAD substrates are sorted to different degradation pathways based on the location of the misfolded region and the topology of the protein.
  • ERAD involves several factors that are organised in modules and cooperate in the processes of substrate recognition, retrotranslocation, ubiquitylation and targeting of substrates to the proteasome where cleavage occurs.
  • The retrotranslocation of the substrates from the ER to the cytosol is mediated by still undefined channel proteins.
  • Retrotranslocated substrates are ubiquitylated on the cytosolic side of the ER by membrane‐associated E3 ubiquitin ligases.
  • In ERAD, ubiquitylation is dynamically interconnected with substrate extraction from the ER membrane, delivery to the proteasome, removal of the N‐glycan chains from glycosylated polypeptides and processing of the substrates for final disposal.
  • Malfunctioning of ERAD components or accumulation of misfolded substrates causes various human diseases.

Keywords: ER; ERAD; proteolysis; ubiquitin; ubiquitylation; proteasome; protein folding; protein trafficking; secretory proteins; membrane proteins; glycosylation

Figure 1. ERAD pathways in yeast. Hrd1p and Doa10p complexes and their known partners are represented. ERAD‐M and ERAD‐L degrade misfolded membrane proteins with misfolded TMDs or luminal soluble proteins via the Hrd1p complex. ERAD‐C degrades integral membrane proteins with a misfolded domain in the cytoplasm via the Doa10p complex. All ERAD pathways require cytosolic Cdc48p for protein translocation and extraction from the ER membranes. The substrate is ubiquitylated during or after translocation and is targeted to the proteasome (PRT) for degradation. Ub, ubiquitin. The classification for the different ERAD pathways also exists in mammalian cells but is less stringent.Reproduced from Lemus and Goder (2014) © U.S. National Library of Medicine, National Institutes of Health.
Figure 2. The CNX/CRT cycle and the generation of the N‐glycan signal for glycoprotein degradation. The main steps are: (1) addition by oligosaccharyltransferase (OST) of an N‐linked oligosaccharide (GlcNac2Man9Glu3) to the nascent polypeptide chain; (2) removal of the first glucose by GluI; association of the deglucosylated polypeptides with malectin, a lectin‐like protein; (3) Removal of the second glucose by GluII; (4) Interaction of monoglucosylated polypeptides with CNX or CRT; (5) Cleavage of the last glucose by GluII; (6) Recognition of the misfolded proteins and reglucosylation of their N‐linked chains by UGT1. The polypeptide enters another round of interaction with CNX or CRT; (7) Export of properly folded proteins through vesicles budded from the ER exit site; (8) Processing of the N‐linked oligosaccharides of terminally misfolded polypeptides by ER mannosidases and (9) targeting of the polypeptides to the proteasome for final disposal. : , ‐acetylglucosamine (GlcNac); , Mannose (Man) and , Glucose (Glu). Reproduced with permission from Tannous et al. (2015) © Elsevier.
Figure 3. Organisation of mammalian ERAD. The factors that mediate ERAD are organised into modules. In each module, factors tend to form stable interactions, whereas factors from different modules in general bind in a more dynamic manner. Within the same module factors can also form parallel pathways to mediate the retrotranslocation of different substrates. Specific recognition and recruitment of luminal misfolded proteins (ERAD‐L) and their selective targeting for insertion in the ER membrane occur in modules 1 and 2. Subsequently, ubiquitin conjugation is performed by the ubiquitylation modules 3 and 4. This is followed by extraction from the ER membrane via the AAA+–ATPase p97 complex (module 4). Dislocated products are shuttled to the proteasome, and removal of N‐linked chains by NGly1 facilitates the degradation of glycoprotein substrates (modules 5 and 6). Proteasomes are thought to be brought in proximity of the retrotranslocation site through interaction with ER membranes. The recognition and degradation of ERAD‐M and ERAD‐C substrates do not require the recognition and initiation modules in the ER. The dashed arrows indicate the flow of substrates. Reproduced with permission from Christianson and Ye (2014). © Nature Publishing Group.


Aebi M, Bernasconi R, Clerc S and Molinari M (2010) N‐glycan structures: recognition and processing in the ER. Trends in Biochemical Sciences 35: 74–82.

Allen MD, Buchberger A and Bycroft M (2006) The PUB domain functions as a p97 binding module in human peptide N‐glycanase. Journal of Biological Chemistry 281: 25502–25508.

Ballar P, Ors AU, Yang H and Fang S (2010) Differential regulation of CFTRDeltaF508 degradation by ubiquitin ligases gp78 and Hrd1. The International Journal of Biochemistry & Cell Biology 42: 167–173.

Benyair R, Ogen‐Shtern N and Lederkremer GZ (2015) Glycan regulation of ER‐associated degradation through compartmentalization. Seminars in Cell & Developmental Biology 41: 99–109.

Bergbold N and Lemberg MK (2013a) Emerging role of rhomboid family proteins in mammalian biology and disease. Biochimica et Biophysica Acta 1828: 2840–2848.

Braakman I and Hebert DN (2013) Protein folding in the endoplasmic reticulum. Cold Spring Harbor Perspectives in Biology 5: a013201.

Brodsky JL (2007) The protective and destructive roles played by molecular chaperones during ERAD (endoplasmic‐reticulum‐associated degradation). Biochemistry Journal 404: 353–363.

Caglayan AO, Comu S, Baranoski JF, et al. (2015) NGLY1 mutation causes neuromotor impairment, intellectual disability, and neuropathy. European Journal of Medical Genetics 58: 39–43.

Christianson JC, Olzmann JA, Shaler TA, et al. (2012) Defining human ERAD networks through an integrative mapping strategy. Nature Cell Biology 14: 93–105.

Christianson JC and Ye Y (2014) Cleaning up in the endoplasmic reticulum: ubiquitin in charge. Nature Structural & Molecular Biology 21: 325–335.

Elsasser S, Gali RR, Schwickart M, et al. (2002) Proteasome subunit Rpn1 binds ubiquitin‐like protein domains. Nature Cell Biology 4: 725–730.

Enns GM, Shashi V, Bainbridge M, et al. (2014) Mutations in NGLY1 cause an inherited disorder of the endoplasmic reticulum‐associated degradation pathway. Genetics in Medicine: Official Journal of the American College of Medical Genetics 16: 751–758.

Finley D (2009) Recognition and processing of ubiquitin‐protein conjugates by the proteasome. Annual Review of Biochemistry 78: 477–513.

Fleig L, Bergbold N, Sahasrabudhe P, et al. (2012) Ubiquitin‐dependent intramembrane rhomboid protease promotes ERAD of membrane proteins. Molecular Cell 47: 558–569.

Greenblatt EJ, Olzmann JA and Kopito RR (2011) Derlin‐1 is a rhomboid pseudoprotease required for the dislocation of mutant alpha‐1 antitrypsin from the endoplasmic reticulum. Nature Structural & Molecular Biology 18: 1147–1152.

Guerriero CJ and Brodsky JL (2012) The delicate balance between secreted protein folding and endoplasmic reticulum‐associated degradation in human physiology. Physiological Reviews 92: 537–576.

Hampton RY and Sommer T (2012) Finding the will and the way of ERAD substrate retrotranslocation. Current Opinion in Cell Biology 24: 460–466.

Hochstrasser M (2006) Lingering mysteries of ubiquitin‐chain assembly. Cell 124: 27–34.

Huang C, Harada Y, Hosomi A, et al. (2015) Endo‐beta‐N‐acetylglucosaminidase forms N‐GlcNAc protein aggregates during ER‐associated degradation in Ngly1‐defective cells. Proceedings of the National Academy of Sciences of the United States of America 112: 1398–1403.

Kerscher O, Felberbaum R and Hochstrasser M (2006) Modification of proteins by ubiquitin and ubiquitin‐like proteins. Annual Review of Cell and Developmental Biology 22: 159–180.

Kravtsova‐Ivantsiv Y, Sommer T and Ciechanover A (2013) The lysine48‐based polyubiquitin chain proteasomal signal: not a single child anymore. Angewandte Chemie 52: 192–198.

Leitman J, Shenkman M, Gofman Y, et al. (2014) Herp coordinates compartmentalization and recruitment of HRD1 and misfolded proteins for ERAD. Molecular Biology of the Cell 25: 1050–1060.

Lemus L and Goder V (2014) Regulation of endoplasmic reticulum‐associated protein degradation (ERAD) by ubiquitin. Cells 3: 824–847.

Li W, Bengtson MH, Ulbrich A, et al. (2008) Genome‐wide and functional annotation of human E3 ubiquitin ligases identifies MULAN, a mitochondrial E3 that regulates the organelle's dynamics and signaling. PLoS One 3: e1487.

Madura K (2004) Rad23 and Rpn10: perennial wallflowers join the melee. Trends in Biochemical Sciences 29: 637–640.

Malhotra JD and Kaufman RJ (2007) The endoplasmic reticulum and the unfolded protein response. Seminars in Cell & Developmental Biology 18: 716–731.

Metzger MB, Hristova VA and Weissman AM (2012) HECT and RING finger families of E3 ubiquitin ligases at a glance. Journal of Cell Science 125: 531–537.

Morito D and Nagata K (2015) Pathogenic hijacking of ER‐associated degradation: is ERAD flexible? Molecular Cell 59: 335–344.

Nakatsukasa K, Kamura T and Brodsky JL (2014) Recent technical developments in the study of ER‐associated degradation. Current Opinion in Cell Biology 29: 82–91.

Neutzner A, Neutzner M, Benischke AS, et al. (2011) A systematic search for endoplasmic reticulum (ER) membrane‐associated RING finger proteins identifies Nixin/ZNRF4 as a regulator of calnexin stability and ER homeostasis. Journal of Biological Chemistry 286: 8633–8643.

Nishitoh H, Matsuzawa A, Tobiume K, et al. (2002) ASK1 is essential for endoplasmic reticulum stress‐induced neuronal cell death triggered by expanded polyglutamine repeats. Genes and Development 16: 1345–1355.

Olzmann JA, Kopito RR and Christianson JC (2013) The mammalian endoplasmic reticulum‐associated degradation system. Cold Spring Harbor Perspectives in Biology 5 (9: pii: a01318).

Pickart CM (2001) Mechanisms underlying ubiquitination. Annual Review of Biochemistry 70: 503–533.

Pickart CM and Cohen RE (2004) Proteasomes and their kin: proteases in the machine age. Nature Reviews. Molecular Cell Biology 5: 177–187.

Sato T, Sako Y, Sho M, et al. (2012) STT3B‐dependent posttranslational N‐glycosylation as a surveillance system for secretory protein. Molecular Cell 47: 99–110.

Shimizu Y and Hendershot LM (2007) Organization of the functions and components of the endoplasmic reticulum. Advances in Experimental Medicine and Biology 594: 37–46.

Shimizu Y, Okuda‐Shimizu Y and Hendershot LM (2010) Ubiquitylation of an ERAD substrate occurs on multiple types of amino acids. Molecular Cell 40: 917–926.

Tannous A, Pisoni GB, Hebert DN and Molinari M (2015) N‐linked sugar‐regulated protein folding and quality control in the ER. Seminars in Cell & Developmental Biology 41: 79–89.

Thibault G and Ng DT (2012) The endoplasmic reticulum‐associated degradation pathways of budding yeast. Cold Spring Harbor Perspectives in Biology 4 (12: pii: a013193).

Verma R, Aravind L, Oania R, et al. (2002) Role of Rpn11 metalloprotease in deubiquitination and degradation by the 26S proteasome. Science 298: 611–615.

Wang Q, Groenendyk J and Michalak M (2015) Glycoprotein quality control and endoplasmic reticulum stress. Molecules 20: 13689–13704.

Weissman AM (2001) Themes and variations on ubiquitylation. Nature Reviews. Molecular Cell Biology 2: 169–178.

Zattas D and Hochstrasser M (2015) Ubiquitin‐dependent protein degradation at the yeast endoplasmic reticulum and nuclear envelope. Critical Reviews in Biochemistry and Molecular Biology 50: 1–17.

Further Reading

Bergbold N and Lemberg MK (2013b) Emerging role of rhomboid family proteins in mammalian biology and disease. Biochimica et Biophysica Acta 1828: 2840–2848.

Morimoto RI, Selkoe DJ and Kelly JW (eds) (2011) Protein Homeostasis. Cold Spring Harbor, New York: Cold Spring Harbor Laboratory Press.

Suzuki T (2015) The cytoplasmic peptide: N‐glycanase (Ngly1) ‐ basic science encounters a human genetic disorder. Journal of Biochemistry 157: 23–34.

Contact Editor close
Submit a note to the editor about this article by filling in the form below.

* Required Field

How to Cite close
Kaufman, Randal J, and Popolo, Laura(Jun 2016) Degradation of Misfolded Secretory and Membrane Proteins and Associated Diseases. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0022577.pub2]