Competition for Access to the Rat Major Histocompatibility Complex Class I Peptide-loading Complex Reveals Optimization of Peptide Cargo in the Absence of Transporter Associated with Antigen Processing (TAP) Association

Major histocompatibility complex (MHC) class I molecules load peptides in the endoplasmic reticulum in a process during which the peptide cargo is normally optimized in favor of stable MHC-peptide interactions. A dynamic multimolecular assembly termed the peptide-loading complex (PLC) participates in this process and is composed of MHC class I molecules, calreticulin, ERp57, and tapasin bound to the transporter associated with antigen processing (TAP) peptide transporter. We have exploited the observation that the rat MHC class I allele RT1-A^a, when expressed in the rat C58 thymoma cell line, effectively competes and prevents the endogenous RT1-A^u molecule from associating with TAP. However, stable RT1-A^u molecules are assembled efficiently in competition with RT1-A^a, demonstrating that cargo optimization can occur in the absence of TAP association. Defined mutants of RT1-A^a, which do not allow formation of the PLC, fail to become thermostable in C58 cells. Wild-type RT1-A^a, which does allow PLC formation, also fails to become thermostable in this cell line, which carries the rat TAPB transporter that supplies peptides incompatible for RT1-A ^a binding. Full optimization of RT1-A^a requires the presence of the TAP2A allele, which is capable of supplying suitable peptides. Thus, formation of the PLC alone is not sufficient for optimization of the MHC class I peptide cargo.