The function of the oxidoreductase ERp57 in the major histocompatibility complex (MHC) class I peptide-loading complex has remained elusive. Here we show that in the absence of tapasin, the alpha(2) disulfide bond in the MHC class I peptide-binding groove was rapidly reduced. Covalent sequestration of ERp57 by tapasin was needed to protect the alpha(2) disulfide bond against reduction and thus to maintain the binding groove in a peptide-receptive state. Allelic variations in MHC class I tapasin dependency reflected their susceptibility to reduction of the alpha(2) disulfide bond. In the absence of sequestration, ERp57 acted directly on the alpha(2) disulfide bond. Our work provides insight into how the immune system customizes 'quality control' in the endoplasmic reticulum to fit the needs of antigen presentation.
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