Protein Folding Quality Check in the RER

N-linked glycans serve as recognition markers for lectin chaperones in the ER lumen. Precursor glycans with three glucose residues are trimmed sequentially, leaving a monoglucosylated form that binds calnexin and calreticulin. These lectins recruit additional chaperones to assist folding. When glucose is removed completely, UGGT1 assesses folding accuracy and can reglucosylate misfolded proteins for another folding attempt.

Calnexin and calreticulin are membrane-bound and soluble lectin chaperones that specifically recognize monoglucosylated N-linked glycans on unfolded polypeptides. They bind these glycans through their lectin domain and use a flexible P-domain to interact with accessory ER chaperones. Together, they facilitate proper protein folding by recruiting helper proteins like ERp57, cyclophilin B, and ERp29 to assist the folding process.

UGGT1 acts as the single quality control checkpoint for glycoprotein folding in the ER. Its N-terminal domain senses misfolded polypeptides, while its C-terminal domain catalyzes reglucosylation by adding glucose from UDP-glucose back to non-glucosylated glycans. This marks incorrectly folded proteins for retention and recycling through the calnexin/calreticulin system until proper folding is achieved.

Three functionally distinct accessory chaperones support calnexin and calreticulin. ERp57 is a protein disulfide isomerase that catalyzes oxidation and isomerization of disulfide bonds. Cyclophilin B rearranges and corrects disulfide bonds, while ERp29 provides general chaperone assistance for peptide folding. Together, these proteins enhance the efficiency of proper glycoprotein maturation in the ER lumen.

The calnexin cycle uses sequential glucose trimming and reglucosylation to trap misfolded proteins in the ER. When UGGT1 detects incorrect folding, it reglucosylates the glycan, allowing calnexin and calreticulin to rebind and recycle the polypeptide for another folding attempt. Only proteins achieving correct structure escape reglucosylation and are permitted to exit the ER for further processing.

Unfolded polypeptides enter the ER lumen with precursor N-linked glycans containing three terminal glucose residues. Glucosidase I removes one glucose, followed by glucosidase II removing a second glucose, leaving a monoglucosylated trimmed glycan. This monoglucosylated form is recognized by calnexin and calreticulin lectins, initiating the quality control cycle for proper protein folding.

The calnexin cycle ensures only correctly folded glycoproteins exit the ER, maintaining protein quality in the secretory pathway. By coupling glycan trimming with folding assessment through UGGT1, the system prevents misfolded proteins from being secreted. This quality control mechanism protects cellular function and prevents accumulation of defective proteins that could cause cellular dysfunction or disease.