AbstractTapasin is disulfide linked to ERp57 within the peptide loading complex. In cell‐free assays, a soluble variant of the tapasin/ERp57 dimer recruits MHC class I molecules and promotes peptide binding to them, whereas soluble tapasin alone does not. Here we show that within cells, tapasin conjugation with ERp57 is as critical as its integration into the membrane for efficient MHC class I assembly, surface expression, and Ag presentation to CD8+ T cells. Elimination of both of these properties severely compromises tapasin function, in keeping with predictions from in vitro studies.