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ERp57 is a thiol oxidoreductase that catalyzes disulfide formation in heavy chains of class I histocompatibility molecules. It also forms a mixed disulfide with tapasin within the class I peptide loading complex, stabilizing the complex and promoting efficient binding of peptides to class I molecules. Since ERp57 associates with the lectin chaperones calnexin and calreticulin, it is thought that ERp57 requires these chaperones to gain access to its substrates. To test this idea, we examined class I biogenesis in cells lacking calnexin or calreticulin or that express an ERp57 mutant that fails to bind to these chaperones. Remarkably, heavy chain disulfides formed at the same rate in these cells as in wild type cells. Moreover, ERp57 formed a mixed disulfide with tapasin and promoted efficient peptide loading in the absence of interactions with calnexin and calreticulin. These findings suggest that ERp57 has the capacity to recognize its substrates directly in addition to being recruited through lectin chaperones. We also found that calreticulin could be recruited into the peptide loading complex in the absence of interactions with both ERp57 and substrate oligosaccharides, demonstrating the importance of its polypeptide binding site in substrate recognition. Finally, by inactivating the redox-active sites of ERp57, we demonstrate that its enzymatic activity is dispensable in stabilizing the peptide loading complex and in supporting efficient peptide loading. Thus, ERp57 appears to play a structural rather than catalytic role within the peptide loading complex.

Original publication

DOI

10.1074/jbc.m808356200

Type

Journal article

Journal

The Journal of biological chemistry

Publication Date

04/2009

Volume

284

Pages

10160 - 10173

Addresses

Departments of Biochemistry and Immunology, University of Toronto, Toronto M5S 1A8, Canada.

Keywords

Cell Line, Fibroblasts, Animals, Mice, Oxidoreductases, Peptides, Calnexin, Calreticulin, Molecular Chaperones, Histocompatibility Antigens Class I, Magnetic Resonance Spectroscopy, Molecular Conformation, Oxidation-Reduction, Catalysis, Protein Disulfide-Isomerases