15.13 : Protein Folding Quality Check in the RER

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 for degradation.

Lectin Chaperone System: The Glycoprotein Binding Proteins

The lectin chaperones — calnexin, and calreticulin are distinguished from other ER chaperones due to their ability to bind N-linked glycans. The N-linked glycans are abundantly present on most polypeptide chains entering the ER lumen, and the lectins utilize this modification for proofreading the accuracy of protein folding. Calnexin and calreticulin, both have very similar functional domains. They primarily bind mono-glucosylated N-linked glycans using their glycan-binding lectin domain, and use the P-domain on their flexible arm to interact with other ER chaperones.

Lectin Binding ER Chaperones: The Accessory Helpers

Three functionally distinct ER chaperones — Erp57, cyclophilin B, and Erp29, assist the calnexin/calreticulin proteins. ERp57 is a protein disulfide isomerase that catalyzes the oxidation and isomerization of disulfide bonds. While cyclophilin B rearranges and corrects the disulfide bonds, ERp29 is a general chaperone that aids peptide folding.

UDP-glucose:glycoprotein Glucosyltransferase 1: The Proofreader

UDP-glucose:glycoprotein glucosyltransferase 1, or UGGT1, a protein folding sensor, binds polypeptide and acts as the single glycoprotein folding quality control checkpoint. It marks incorrectly folded glycoproteins to be retained in the ER by selective reglucosylation. The N-terminal of UGGT acts as the misfold sensor, whereas the C-terminal harbors the reglucosylating glucosyltransferase.