Name: rough er protein folding changes
Uploaded: 2023-04-30T10:07:37.000Z
Duration: 1 min 26 s
Channel: JoVE

N-linked glycosylation and disulfide bond formation are two important protein modifications that occur in the ER.
During glycosylation, the oligosaccharyltransferase complex adds branched oligosaccharide molecules to some select asparagine residues of an incoming polypeptide chain.
In contrast, disulfide bond formation occurs between two closely spaced cysteine residues on the same or different polypeptide chains.
It involves two key players &#8212; protein disulfide isomerase, or PDI, and ER oxidoreductase I, or Ero1.
The PDI molecule resembles a horseshoe with disulfide bonds in its active site.
Oxidized PDI has an open conformation and binds unfolded polypeptides. First, the reduced cysteine residue of the polypeptide forms a disulfide link with the enzyme&#39;s active site.
This intermediate then interacts with another closely spaced cysteine on the polypeptide chain, resulting in a disulfide bond between the two residues.
Subsequently, the PDI changes to a closed conformation, releasing the polypeptide.
Ero1 converts the reduced PDI back to its oxidized state, preparing it for another round of reaction.

protein modifications in the rer

Protein Modifications in the RER

Modification of secretory and transmembrane proteins entering the rough ER begins in the ER lumen. These modifications aid in protein folding and stabilize the acquired tertiary structure. Protein modifications in the rough ER co-occur at different stages of protein folding.
Broadly, these modifications can be categorized into four main categories &#8212; glycosylation, formation of disulfide bonds, assembly of protein subunits, and specific proteolytic cleavages like removal of signal sequences.
N-linked glycosylation of proteins
Almost half of the membrane and soluble proteins that transit via the rough ER are converted to a glycoprotein by the covalent addition of carbohydrate moieties, making it the most common ER modification. The oligosaccharide is attached to the asparagine residues of the tripeptide sequences Asn-X-Ser and Asn-X-Thr, where X can be any amino acid, except proline. The oligosaccharyltransferase membrane complex can catalyze glycosylation during cotranslational as well as post-translational protein translocation.
Glycosylation alters inherent physical properties of the protein. For instance, N-linked glycoproteins have improved thermodynamic kinetics and fold better as compared to their non-glycosylated counterparts. Glycosylation also increases protein stability by masking cleavage sites and hydrophobic stretches. Additionally, resident ER chaperones like BiP and lectins use the polypeptide glycosylation status to assess the correctness of protein folding before clearing it for exit from the ER.
Disulfide bonding of polypeptide chains
Disulfide bond formation is favored in an oxidizing environment and is formed predominantly in the rough ER lumen. However, a small fraction of disulfide bonds can form in the mitochondrial intermembrane space.
Protein disulfide isomerase (PDI) is the most abundant and best characterized oxidoreductase in the ER lumen. While the oxidized PDI forms disulfide linkage between cysteine residues, the reduced PDI acts as a proofreader, correcting inappropriately paired cysteine residues by rearranging the disulfide linkage. ER oxidoreductase 1, or Ero1, utilizes a significant fraction of molecular oxygen available in the cell to recycle oxidized PDI and generate hydrogen peroxide. Both PDI and Ero1 are responsible for the oxidative folding of proteins and maintaining redox homeostasis inside the ER.

Modification of secretory and transmembrane proteins entering the rough ER begins in the ER lumen. These modifications aid in protein folding and ...

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Cell Biology

Unit 1

Transmembrane Transport in Endoplasmic Reticulum and Peroxisomes

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KEY TERMS AND CONCEPTS

rough er protein folding changes

Rough ER protein folding changes help secretory and transmembrane proteins mature inside the ER lumen. These changes support folding and help stabilize the protein’s final three-dimensional shape. They happen at different stages as the protein folds.

The main rough ER modifications fall into four groups: glycosylation, disulfide bond formation, assembly of protein subunits, and specific proteolytic cleavages, such as removal of signal sequences. N-linked glycosylation is the most common. In this step, a carbohydrate chain is added to asparagine in the sequence Asn-X-Ser or Asn-X-Thr, where X can be any amino acid except proline.

This glycosylation can occur while the protein is being moved into the ER or after translocation is complete. The oligosaccharyltransferase membrane complex carries out the reaction. N-linked glycoproteins often fold better than non-glycosylated proteins. Glycosylation also improves stability by hiding cleavage sites and hydrophobic stretches.

ER chaperones use glycosylation status to check whether folding is correct before a protein leaves the ER. BiP and lectins are examples of these resident ER chaperones. They help judge whether the protein is ready for export.

Disulfide bonds also form mainly in the rough ER lumen. This bond making is favored in an oxidizing environment. A small fraction of disulfide bonds can also form in the mitochondrial intermembrane space.

Protein disulfide isomerase, or PDI, is the best known and most abundant oxidoreductase in the ER lumen. Oxidized PDI forms disulfide links between cysteine residues. Reduced PDI acts as a proofreader and can rearrange incorrectly paired cysteines.

ER oxidoreductase 1, or Ero1, helps recycle oxidized PDI. It uses a significant amount of cellular oxygen and produces hydrogen peroxide. Together, PDI and Ero1 support oxidative folding and help maintain redox balance in the ER.

Rough ER Protein Folding Changes

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Rough ER protein folding changes help secretory and transmembrane proteins mature inside the ER lumen. These changes support folding and help stabilize the protein’s final three-dimensional shape. They happen at different stages as the protein folds. The main rough ER modifications fall into four groups: glycosylation, disulfide bond formation, assembly of protein subunits, and specific proteolytic cleavages, such as removal of signal sequences. N-linked glycosylation is the most common. In...

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key terms and concepts

Explore how proteins are modified in the rough ER through glycosylation and disulfide bond formation, plus the enzymes that help folding and stability.

Rough ER Protein Folding Changes | JoVE K12

cell biology

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Rough ER Protein Folding Changes

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