Calnexin can interact with N-linked glycans located close to the endoplasmic reticulum membrane

Calnexin is a central component of the 'quality control' system in the endoplasmic reticulum (ER). Calnexin binds to monoglycosylated oligosaccharides on incompletely folded soluble and membrane proteins in the lumen of the ER and prevents exit from the organelle. We have previously found that the oligosaccharide transferase enzyme can add glycosyl moieties to a membrane protein when the acceptor site is as close as 12-13 residues away from the nearest transmembrane segment (J. Biol. Chem. 268, 5798). We now show that calnexin can bind to oligosaccharides located this close to the membrane, suggesting that its binding site is held at a similar distance from the membrane as is the active site of the oligosaccharide transferase. We further show that calnexin can bind efficiently to glycosylated but not to non-glycosylated forms of a bacterial inner membrane protein, suggesting that it does not have a general affinity for non-glycosylated proteins.