Point mutations in the alpha 2 domain of HLA-A2.1 define a functionally relevant interaction with TAP.
BACKGROUND
Glycoproteins encoded by the major histocompatibility complex class I region (MHC class I) present peptide antigens to cytotoxic T cells (CTLs). Peptides are delivered to the site of MHC class I assembly by the transporter associated with antigen processing (TAP), and cell lines that lack this transporter are unable to present endogenous antigens to CTLs. Although it has been shown that a fraction of newly synthesized class I molecules are in physical association with TAP, it is not known whether this interaction is functionally relevant, or where on the class I molecule the TAP binding site might be.


RESULTS
C1R cells transfected with a mutant HLA-A2.1 heavy chain (HC), where threonine at position 134 in the alpha 2 domain is changed to lysine (T134K), are unable to present endogenous antigens to CTLs. We have studied the biochemistry of this mutant in C1R cells, and found that a large pool of unstable empty class I HC-beta 2m (beta-2 microglobulin) heterodimers exist that are rapidly transported to the cell surface. The T134K mutant seemed to bind peptide antigens and assemble with beta 2m as efficiently as wild-type HLA-A2.1. However, we show here that the inefficiency with which T134K presents intracellular antigen is associated with its inability to interact with the TAP heterodimer.


CONCLUSIONS
These experiments establish that the class I-TAP interaction is obligatory for the presentation of peptide epitopes delivered to the endoplasmic reticulum (ER) by TAP. Wild-type HLA-A2.1 molecules in TAP-deficient cells are retained in the ER, whereas T134K is rapidly released to the cell surface, but is unstable, suggesting a role for the TAP complex as an intracellular checkpoint that only affects the release of class I molecules with stably bound peptide ligands.
Lewis
J W
JW
Nuffield Department of Clinical Medicine, University of Oxford, John Radcliffe Hospital, UK.
Neisig
A
A
Neefjes
J
J
Elliott
T
T
D000071181:ATP Binding Cassette Transporter, Subfamily B, Member 2; D000071450:ATP Binding Cassette Transporter, Subfamily B, Member 3; D018528:ATP-Binding Cassette Transporters / Q000276:immunology*; D000818:Animals; D001665:Binding Sites; D001692:Biological Transport; D002460:Cell Line; D002462:Cell Membrane; D015789:HLA-A2 Antigen / Q000235:genetics / Q000276:immunology*; D015395:Histocompatibility Antigens Class I / Q000276:immunology; D006801:Humans; D008285:Major Histocompatibility Complex / Q000276:immunology*; D010641:Phenotype; D017354:Point Mutation; D011817:Rabbits
D000071181:ATP Binding Cassette Transporter, Subfamily B, Member 2; D000071450:ATP Binding Cassette Transporter, Subfamily B, Member 3; D018528:ATP-Binding Cassette Transporters; D015789:HLA-A2 Antigen; D015395:Histocompatibility Antigens Class I; C073119:TAP1 protein, human; C071692:TAP2 protein, human