The role of the lectin calnexin in conformation independent binding to N-linked glycoproteins and quality control.

It has often been speculated that one of the selective forces for the evolution of Nlinked glycosylation is that of productive protein folding. Strong experimental support has now been put forward as a consequence of studies with the endoplasmic reticulum resident membrane protein calnexin and its luminal homologue calreticulin. These resident proteins, at least one of which (calnexin) is universal to all eukaryotes, act principally as lectins which recognize monoglucosylated intermediates of high mannose containing N-linked glycoproteins. A molecular chaperone apparatus consisting of the coupled actions of uridine diphosphate glucose: glycoprotein glucosyl transferase, calnexin/calreticulin and glucosidase II has been reconstituted using fully defined constituents in vitro. That this apparatus also functions as one of the regulatory mechanisms in the identification and triaging of misfolded glycoproteins (quality control) has also shown recent experimental support. From the perspective of calnexin N-linked glycosylation and more specifically monoglucosylation allows the temporal coupling of glycoprotein folding within the spatial confines of the endoplasmic reticulum and provides a mechanism to direct misfolded glycoproteins away from productive folding intermediates.

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