In contrast to the fairly well-characterized mechanism of assembly of MHC class I-peptide complexes, the disassembly mechanism by which peptide-loaded MHC class I molecules are released from the peptide-loading complex and exit the endoplasmic reticulum (ER) is poorly understood. Optimal peptide binding by MHC class I molecules is assumed to be sufficient for triggering exit of peptide-filled MHC class I molecules from the ER. We now show that protein disulfide isomerase (PDI) controls MHC class I disassembly by regulating dissociation of the tapasin-ERp57 disulfide conjugate. PDI acts as a peptide-dependent molecular switch; in the peptide-bound state, it binds to tapasin and ERp57 and induces dissociation of the tapasin-ERp57 conjugate. In the peptide-free state, PDI is incompetent to bind to tapasin or ERp57 and fails to dissociate the tapasin-ERp57 conjugates, resulting in ER retention of MHC class I molecules. Thus, our results indicate that even after optimal peptide loading, MHC class I disassembly does not occur by default but, rather, is a regulated process involving PDI-mediated interactions within the peptide-loading complex.
INTRODUCTIONAssembly of major histocompatibility complex (MHC) class I molecules within the endoplasmic reticulum (ER) is essential for bringing antigenic peptides to CD8 ϩ T-cells, which scan and destroy the infected and transformed cells. MHC class I quality control ensures both the ER retention of suboptimally loaded MHC class I molecules and the release of those with a higher affinity to the cell surface. Therefore, the peptide-editing process for optimal peptide loading is critical for increasing the recognition by CD8 ϩ T-cells (Van Kaer, 2002;Cresswell, 2005).MHC class I assembly is a highly regulated process mediated by several ER-resident chaperones and accessory molecules (Cresswell, 2000;Williams et al., 2002a). After initial interaction with the lectin-like chaperone calnexin, MHC class I heavy chain- 2 -microglobulin ( 2 m) heterodimers are incorporated into the peptide-loading complex (PLC), a multimolecular unit of ER proteins that promotes optimal peptide loading into MHC class I molecules (Hammerling et al., 1998;Pamer and Cresswell, 1998;Elliott and Williams, 2005). The PLC contains calreticulin, tapasin, transporters associated with antigen processing (TAP) and two thioldependent oxidoreductases, ERp57 and protein disulfide isomerase (PDI) (Garbi et al., 2005;Park et al., 2006;Santos et al., 2007).Several functions have been proposed for tapasin in facilitating optimal peptide loading and optimization of the peptide cargo (Grandea and Van Kaer, 2001;Purcell et al., 2001;Momburg and Tan, 2002). Tapasin bridges heavy chain- 2 m heterodimers to TAP (Sadasivan et al., 1996) and thus provides physical proximity between MHC class I molecules and TAP. Tapasin also retains MHC class I molecules in the ER (Schoenhals et al., 1999;Barnden et al., 2000;Grandea et al., 2000), which might enhance peptide loading. In addition, tapasin optimizes the repertoire of bound pep...