mRNA Proteasome ,>Protein fragments Ribosome 8 Exit UDP UDP-Glc Correctly folded
ID: 98975 • Letter: M
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mRNA Proteasome ,>Protein fragments Ribosome 8 Exit UDP UDP-Glc Correctly folded Glucosidase Unfolded and UGGT 4 Glucosidase ll 2 5000 ER lumen Calnexin ARR Cytosol FIGURE 8.17 Quality control: ensuring that misfolded proteins do not proceed forward. Based on this proposed mechanism, misfolded proteins are recognized by a glucosyltrans- ferase (UGGT) which adds a glucose to the end of the oligosaccharide chains. Glycoproteins containing monoglucosylated oligosaccharides are recognized by the membrane-bound chaperone calnexin and given an opportunity to achieve their correctly folded (native) state. If that does not occur after repeated attempts, the protein is dislocated to the cytosol and destroyed. The steps are described in the text. A soluble chaperone (calreticulin) participates in this same quality-control pathway. SouRCE: L. Ellgaard et al., Science 286:984, 1999; copyright 1999, reprinted with permission from AAASExplanation / Answer
Ans.) At the time of protein synthesis, the newly formed polypeptides begin to presume their final three-dimensional conformation by their folding intermediates that depend on their primary sequence. One of the major aspects of glycoprotein quality control in the endoplasmic reticulum (ER) is their proper folding which involves in variety of mechanisms i.e. secondary structure formation, disulfide bond formation and quaternary structure formation by oligomerization.
UGGT determines whether the glycoprotein folded properly not. IT involves in reglucosylation of the glycan if proper folding has not carried out. Calnexin is basically a type of chaperones found in the ER. It entails calcium ions for their activity and bind to monoglucosylated, thus retaining the protein in the ER until proper folding carried out. -Glucosidase I evacuates the terminal 1–2-connected glucose and glucosidase II at that point expels both 1-3-connected glucose units. In spite of the fact that glucosidase II may expel the second glucose buildup from glycoprotein with a solitary glycan chain, the trimming of the second glucose happens all the more proficiently when there is a moment N-glycan chain on a similar protein. Depend upon the cell sort, a variable number of mannose deposits likewise might be expelled while glycoprotein stay in the ER.
Labeling with description of each of the step given in the mentioned figure -
1.) Here, this step is N-glycosylation of protein entering to the ER.
2.) Removal of glucose residue by the action of -glucosidases GI, GII and generation of monoglucosylated oligosaccharides.
3.) Detection of monoglucosylated saccharides.
4.) Dissociation of misfolded glycoprotein after the removal of the last glucose residue.
5.) Hydrolysis of the residual glucose residue by GII and release of the glycoprotein from the lectin attachment.
6.) Transfer to the Golgi complex of unrecognized glycoprotein.
7.) Retro - translocation of misfolded glycoprotein to the cytoplasm.
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