Calnexin (CNX) is a 67kDa integral protein of the endoplasmic reticulum (ER). It consists of a large (50 kDa) N-terminal calcium-binding lumenal domain, a single transmembrane helix and a short (90 residues), acidic cytoplasmic tail. Calnexin is a chaperone, characterized by assisting protein folding and quality control, ensuring that only properly folded and assembled proteins proceed further along the secretory pathway. It specifically acts to retain unfolded or unassembled N-linked glycoproteins in the ER. Calnexin binds only those N-glycoproteins that have GlcNAc2Man9Glc1 oligosaccharides. Calnexin associates with the protein folding enzyme ERp57 to catalyze glycoprotein specific disulfide bond formation and also functions as a chaperone for the folding of MHC class I α-chain in the membrane of the ER. A prolonged association of calnexin with mutant misfolded PMP22 known to cause Charcot-Marie-Tooth Disease leads to the sequestration, degradation and inability of PMP22 to traffic to the Schwann cell surface for myelination.
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