Kontrola jakości fałdowania białka w RER

In the ER lumen, an unfolded polypeptide has multiple precursor N-linked glycans attached to it.

These precursor glycans are branched oligosaccharides, each with three terminal glucose residues.

Two of these glucose residues are sequentially removed by the action of glucosidase I and glucosidase II, leaving behind a trimmed N-linked oligosaccharide with a single glucose.

The membrane-bound calnexin and soluble calreticulin are lectin chaperones, which specifically bind such monoglucosylated trimmed glycans.

The lectins then recruit other chaperones to assist in polypeptide folding.

Soon after, glucosidase II trims out the final glucose residue, creating non-glucosylated glycans on the polypeptide.

The UDP-glucose:glycoprotein glucosyltransferase 1, or UGGT1 enzyme, then assesses the protein for its folding accuracy.

If the folding is correct, the protein is allowed to exit the ER.  Otherwise, UGGT1 adds a glucose molecule from UDP-glucose back to the glycan attached to the polypeptide.

The reglucosylation recycles the polypeptide for another round of folding by calnexin/calreticulin chaperones until the correct protein structure is achieved.