The main function of the calnexin/calreticulin cycle is to:
Calnexin and calreticulin together with oxidoreductase ERp57 and glucosyltransferase UGGT constitute specialized folding cycle dedicated to N-glycosylated proteins comprising more than eighty percent of secretory proteome. After glucose trimming to monoglucosylated form by glucosidase I and II, nascent proteins bind lectin site of membrane-bound calnexin or soluble calreticulin, which prevents aggregation, retains them in ER lumen and recruits ERp57 via extended proline-rich P-domain arm to catalyze disulfide bond formation and isomerization. Folding sensor UGGT inspects surface hydrophobicity and molten globule character; if non-native patches exposed, it transfers single glucose from UDP-glucose to high-mannose glycan regenerating monoglucosylated ligand permitting re-entry into lectin cycle, providing multiple folding chances without commitment to degradation. Properly folded proteins are not substrate for UGGT, lose final glucose via glucosidase II and exit toward Golgi via cargo receptors. After several futile cycles, mannose trimming by EDEM family terminates cycling and hands off to ERAD lectins OS-9 and XTP3-B. Cycle thus integrates glycan code with oxidative and hydrophobic inspection ensuring high fidelity maturation beyond simple forward transport and preventing aggregation.
Ref: Caramelo & Parodi, J Biol Chem 283: 2008, Calnexin-Calreticulin Cycle in Folding.