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Protein Folding Quality Check in the RER01:29

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ER is the primary site for the maturation and folding of soluble and transmembrane secretory proteins. The calnexin cycle is a specific chaperone system that folds and assesses the confirmation of N-glycosylated proteins before they can exit the ER lumen. The primary players of this quality check pipeline are the lectins, ER-resident chaperones, and a glucosyl transferase enzyme. In case the calnexin system in the lumen fails to salvage a misfolded protein, it is transported to the cytoplasm...
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Endoplasmic Reticulum01:39

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The Endoplasmic Reticulum (ER) in eukaryotic cells is a substantial network of interconnected membranes with diverse functions, from calcium storage to biomolecule synthesis. A primary component of the endomembrane system, the ER manufactures phospholipids critical for membrane function throughout the cell. Additionally, the two distinct regions of the ER specialize in the manufacture of specific lipids and proteins.
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The endoplasmic reticulum or ER makes up for more than half of the membranes in a cell and accounts for 10% of total cell volume. It is also the primary protein and lipid synthesis factory for most cell organelles, such as the Golgi apparatus, lysosomes, secretory vesicles, and the plasma membrane. Despite being the most extensive and functionally complex subcellular organelle, ER was the last to be discovered. After years of deliberation, Keith Porter and George Palade in the year 1954,...
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Export of Misfolded Proteins out of the ER01:32

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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...
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The organelle-specific signaling sequences direct proteins synthesized in the cytosol to their final destination like ER, mitochondria, peroxisomes, etc. Some of the proteins directed to ER are then trafficked via vesicles to other organelles within the cell or the extracellular environment through the Golgi complex. For example, the rough ER synthesizes soluble proteins for transportation to the lysosomes or secretion out of the cell. It can also synthesize transmembrane proteins that can...
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Protein quality control at the endoplasmic reticulum.

Kathleen McCaffrey1, Ineke Braakman2

  • 1Cellular Protein Chemistry, Utrecht University, 3584CH Padualaan 8, 3584 CH Utrecht, The Netherlands k.mccaffrey@uu.nl.

Essays in Biochemistry
|October 17, 2016
PubMed
Summary

The endoplasmic reticulum (ER) is crucial for protein folding and quality control. Its protein quality control (PQC) system ensures cellular health and prevents diseases linked to protein misfolding.

Keywords:
BiPCOPII vesiclesER transloconER-associated degradation (ERAD)ER-exit sitesERManIGolgiGrp94PDIcalnexin/calreticulindisulfide bond formationendoplasmic reticulum (ER)glycosylationsignal peptideunfolded protein response (UPR)

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Area of Science:

  • Cell Biology
  • Molecular Biology
  • Biochemistry

Background:

  • The endoplasmic reticulum (ER) is central to the secretory pathway, responsible for folding and maturing proteins.
  • Secretory proteins require modifications like glycosylation and disulfide bonds within the ER for proper function.
  • The ER acts as a critical checkpoint for misfolded or aggregating proteins, preventing cellular toxicity.

Purpose of the Study:

  • To review key features of the ER's protein quality control (PQC) system.
  • To highlight the ER's role in maintaining the health of the cellular secretome.
  • To discuss the implications of ER PQC failure in human diseases.

Main Methods:

  • Literature review of ER protein folding and quality control mechanisms.
  • Discussion of specialized ER modifications (e.g., glycosylation, disulfide bond formation).
  • Analysis of the ER's role as a checkpoint for misfolded proteins.

Main Results:

  • The ER possesses a sophisticated PQC system essential for secretory protein maturation.
  • This PQC system identifies and manages misfolded or aggregation-prone proteins.
  • Dysfunction in ER PQC is linked to aging and various human diseases.

Conclusions:

  • The ER's protein quality control machinery is vital for cellular secretome health.
  • Understanding ER PQC mechanisms is crucial for addressing diseases associated with protein misfolding.
  • Maintaining ER PQC function is key to cellular homeostasis and preventing disease.