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. 2017 Aug 7;12(10):1695–1698. doi: 10.2215/CJN.02830317

Figure 2.

Figure 2.

Protein folding and quality control process in the endoplasmic reticulum. (1) Nascent polypeptide (polycystin 1) enters the endoplasmic reticulum through the SEC61 complex, which is composed of three subunits of SEC61 and further associated with SEC62 and SEC63p. (2) In the endoplasmic reticulum, the glycan is transferred to the nascent polypeptide by oligosaccharyltransferase. (3) Glucosidase I cleaves the first glucose residue. (4) Glucosidase II composed of α- and β-subunits cleaves the second glucose residue. This allows the nascent peptide to enter the protein folding calnexin and calreticulin cycle. (5) During this step, glucosidase II removes the innermost glucose from the glycan, thus releasing the protein from the chaperones. (6) If the protein is properly folded, it is transported to Golgi. (7) The misfolded proteins are reglycosylated by UDP-glucose glycoprotein glucosyltransferase (UGGT), allowing more time in the folding calnexin and calreticulin cycle. (8) Proteins that fail to fold will eventually be transported back to the cytoplasm for degradation through the SEC61 complex.