ERp57 is an oxidoreductase that, in conjunction with calnexin and calreticulin, assists disulfide bond formation in folding glycoproteins. ERp57 also forms a mixed disulfide with the MHC class I-specific chaperone tapasin, and this dimeric conjugate edits the peptide repertoire bound by MHC class I molecules. In cells unable to form the conjugate, because of tapasin mutation in human studies or ERp57 deletion in mouse studies, peptide loading is impeded. Subtle differences between the mouse and human systems have been observed. Here, we address these differences and expand the analysis to investigate the role of ERp57 redox functions in MHC class I peptide loading. We show in human cells that in the absence of conjugate formation MHC class I recruitment and/or stabilization in the MHC class I peptide-loading complex is impaired, similar to observations in mouse cells. However, we found no role for the enzymatic activities of either the a or a domain redox sites of ERp57 in peptide loading. Our data argue that the function of ERp57 in peptide loading is likely caused by other ERp57 functional domains or a combinatorial feature of the tapasin-ERp57 conjugate.antigen presentation ͉ antigen processing ͉ human ͉ protein folding ͉ quality control S table loading of peptides onto MHC class I/ 2 -microglobulin ( 2 m) dimers requires coordinated action within the peptide-loading complex (PLC), which consists of TAP1, TAP2, tapasin, ERp57, calreticulin (CRT), MHC class I heavy chain (HC), and  2 m (1). Tapasin is a critical component of the PLC, but the in vivo role of ERp57 in peptide loading and editing is not entirely resolved. ERp57 is an oxidoreductase that promotes proper disulfide bond formation in folding glycoproteins through its association(s) with calnexin (CNX) and/or CRT (2). Like protein disulfide isomerase, ERp57 is composed of four domains with the a and aЈ domains containing redox active CXXC motifs. During the biosynthetic folding of MHC class I HC, it appears to act in a manner consistent with models of glycoprotein quality control (3). However, within the PLC, Cys-57 of ERp57 forms a disulfide bond with tapasin Cys-95, and tapasin inactivates the substrate dissociation step, or ''escape pathway,'' of ERp57, making this interaction very stable (4).We examined MHC class I assembly in human B lymphoblastoid cells expressing HLA-B*4402 and a tapasin construct in which Cys-95 was mutated to Ala (C95A) to prevent conjugate formation (5). PLC formation was qualitatively normal except for the absence of ERp57, but the stability of peptide-MHC class I complexes assembled in these cells was decreased, consistent with association with a pool of lower-affinity peptides. In mouse B cells lacking ERp57, surface expression of H2-K b molecules was reduced by Ϸ50%, and turnover was faster than in ERp57-expressing cells. H2-K b recruitment into the PLC was affected, and its trafficking through the Golgi was accelerated. Furthermore, presentation of an H2-K b -restricted epitope derived from ovalbumin was reduced in E...