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N-glycan processing in ER quality control.

Lloyd W Ruddock1, Maurizio Molinari

  • 1Biocenter Oulu and Department of Biochemistry, University of Oulu, FIN-90014 Oulu, Finland.

Journal of Cell Science
|November 1, 2006
PubMed
Summary

Protein glycosylation in the endoplasmic reticulum (ER) is vital for protein folding and quality control. Modifications to N-glycans by ER enzymes regulate protein processing and prevent ER stress.

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Area of Science:

  • Biochemistry
  • Molecular Biology
  • Cell Biology

Background:

  • Asparagine (Asn) glycosylation is a common protein modification in the endoplasmic reticulum (ER) lumen.
  • Core glycans enhance polypeptide hydrophilicity, preventing aggregation of nascent proteins.
  • ER-resident enzymes modify N-glycans, influencing protein folding, stability, and fate.

Purpose of the Study:

  • To elucidate the role of N-glycan processing in protein quality control within the ER.
  • To understand how N-glycan modifications regulate protein interaction with chaperones like calnexin.
  • To investigate the mechanisms linking N-glycan dismantling to ER-associated degradation (ERAD) under ER stress.

Main Methods:

  • Characterization of N-glycan processing pathways involving glucosidase I (GI), glucosidase II (GII), UDP-glucose:glycoprotein glucosyltransferase (UGT), and mannosidases.
  • Analysis of newly synthesized proteins' access to the calnexin chaperone system.
  • Study of de-/re-glucosylation cycles regulating substrate retention in the calnexin system.
  • Investigation of EDEM variants' role in accelerating N-glycan dismantling and promoting ERAD.
  • Examination of protein folding and quality control mechanisms in cells lacking specific ER chaperones.

Main Results:

  • Removal of terminal glucose residues from N-glycans facilitates entry into the calnexin chaperone system.
  • Cycles of de-/re-glucosylation by GII and UGT1 control protein retention within the calnexin system.
  • Accelerated N-glycan dismantling, induced by EDEM variants, promotes ER-associated degradation (ERAD) during ER stress.
  • Studies in chaperone-deficient cells provided insights into protein folding and quality control regulation.

Conclusions:

  • Tight regulation of N-glycan modifications is essential for maintaining ER protein quality control.
  • Proper N-glycan processing ensures the synthesis of functional polypeptides.
  • Dysfunctional N-glycan modification can lead to ER accumulation of misfolded proteins, contributing to ER constipation.

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