Relationship between calnexin and BiP in suppressing aggregation and promoting refolding of protein and glycoprotein substrates

Calnexin (CNX) is a membrane protein of the endoplasmic reticulum that has been defined primarily as a lectin, yet is capable of functioning as a molecular chaperone with non-glycosylated proteins in vitro. Here, we assess the relative contributions of the oligosaccharide- and polypeptide-binding sites of CNX to its in vitro chaperone functions by comparing it with the Hsp70 chaperone of the endoplasmic reticulum, BiP. Both proteins were equally effective in preventing the aggregation of non-glycosylated citrate synthase, indicating that the polypeptide-binding site of CNX is capable of functioning at a level similar to that of Hsp70. However, when confronted with glycoprotein substrates, the lectin site of CNX provided a significant advantage over BiP in suppressing aggregation. CNX also cooperated with BiP and the J domain of Sec63p in the ATP-dependent refolding of glycoprotein and non-glycosylated substrates. The lectin site of CNX was essential for refolding of the glycoprotein. These findings reinforce the function of CNX as a bona fide chaperone and illustrate how its lectin site confers advantages relative to other chaperones when confronted with glycoprotein substrates.