Upon interaction of Beta-2-microglobin (B2M) with MHC class I Heavy Chain (HC), calnexin is fully replaced by its soluble ortholog calreticulin (CRT) and this complex is incorporated into the peptide loading complex (PLC). PLC is a multiprotein complex that includes CRT, ERp57 and the additional components tapasin, transporter associated with antigen processing (TAP) and Bap31. The stoichometry of components in PLC remains unclear. The PLC loads antigenic peptides onto MHC class I molecules; components of the PLC cooperate to stabilize the MHC class I complex and optimally load peptides. Tapasin is a type I transmembrane protein that interacts directly with TAP and tethers the MHC complex to it. TAP facilitates the transport of peptides from the cytosol to the ER lumen. B cell receptor–associated protein (Bap31), a putative cargo receptor, associates with HC and acts as a retrograde transporter, carrying peptide-loaded class I MHC molecules.