Key words: protein folding, molecular chaperones, calnexin, quality control, endoplasmic reticulum. Its unique ability to capture unfolded glycoproteins through their large oligosaccharide moieties may have evolved as a means to overcome accessibility problems imposed by being constrained within a lipid bilayer. Unlike other molecular chaperones that are soluble proteins, calnexin is an intrinsic component of the ER membrane. Available evidence suggests that calnexin utilizes its lectin property during initial capture of a newly synthesized glycoprotein and that subsequent association (and chaperone function) is mediated through polypeptide interactions. Calnexin also has the capacity to bind to polypeptide segments of unfolded glycoproteins. The preference for glycoproteins resides in calnexin's ability to function as a lectin with specificity for the GlC 1Man 9GlcNAc 2 oligosaccharide, an early intermediate in the processing of Asn-linked oligosaccharides. Most proteins that are translocated into the ER are glycosylated at Asn residues, and calnexin's interactions are almost exclusively restricted to proteins that possess this posttranslational modification. It is also a component of the quality control system that prevents proteins from progressing along the secretory pathway until they have acquired proper tertiary or quaternary structure. Recent studies have demonstrated that calnexin functions as a molecular chaperone to facilitate the folding and assembly of proteins in the ER. It also exhibits prolonged binding to misfolded or incompletely folded proteins. Calnexin is an integral membrane protein of the endoplasmic reticulum (ER) that binds transiently to a wide array of newly synthesized membrane and secretory proteins.
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