Calreticulin functions in vitro as a molecular chaperone for both glycosylated and non-glycosylated proteins |
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| Authors: | Saito Y Ihara Y Leach M R Cohen-Doyle M F Williams D B |
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| Affiliation: | Department of Biochemistry, Medical Sciences Building, University of Toronto, Toronto, Ontario, Canada M5S 1A8. |
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| Abstract: | 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-competent state. We conclude that in addition to being a lectin CRT possesses a polypeptide binding capacity capable of discriminating between protein conformational states and that it functions in vitro as a classical molecular chaperone. |
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