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-Aa, when expressed in the rat C58 thymoma cell line, effectively competes and prevents the endogenous RT1-Au molecule from associating with TAP. However, stable RT1-Au molecules are assembled efficiently in competition with RT1-Aa, demonstrating that cargo optimization can occur in the absence of TAP association. Defined mutants of RT1-Aa, which do not allow formation of the PLC, fail to become thermostable in C58 cells. Wild-type RT1-Aa, 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-Aa binding. Full optimization of RT1-Aa 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.
Ford
Stuart
S
Division of Cell Biology and Immunology, School of Life Sciences, University of Dundee, Dundee DD1 5EH, United Kingdom.
Antoniou
Antony
A
Butcher
Geoffrey W
GW
Powis
Simon J
SJ
D000818:Animals; D017951:Antigen Presentation; D001665:Binding Sites; D002460:Cell Line; D006649:Histocompatibility Antigens / Q000276:immunology / Q000378:metabolism*; D015395:Histocompatibility Antigens Class I / Q000276:immunology / Q000378:metabolism; D026901:Membrane Transport Proteins / Q000276:immunology / Q000378:metabolism*; D011485:Protein Binding; D051381:Rats
D006649:Histocompatibility Antigens; D015395:Histocompatibility Antigens Class I; D026901:Membrane Transport Proteins; C038353:histocompatibility antigens RT, rat