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Calreticulin (CRT) is thought to be a molecular chaperone that interacts with glycoproteins exclusively through a lectin site specific for monoglucosylated oligosaccharides. However, this chaperone function has never been directly demonstrated nor is it clear how lectin–oligosaccharide interactions facilitate glycoprotein folding. Using purified components, we show that CRT suppresses the aggregation not only of a glycoprotein bearing monoglucosylated oligosaccharides but also that of non-glycosylated proteins. Furthermore, CRT forms stable complexes with unfolded, non-glycosylated substrates but does not associate with native proteins. ATP and Zn 2+ enhance CRT's ability to suppress aggregation of non-glycoproteins, whereas engagement of its lectin site with purified oligosaccharide attenuates this function. CRT also confers protection against thermal inactivation and maintains substrates in a folding …