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 α2 domain is changed to lysine (T134K), are unable to present endogenous antigens to CTLs. We have studied the biochemistry of this mutant in ClR cells, and found that a large pool of unstable empty class I HC-β2m (β-2 microglobulin) heterodimers exist that are rapidly transported to the cell surface. The T134K mutant seemed to bind peptide antigens and assemble with β2m 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., Neisig, A., Neefjes, J., & Elliott, T. (1996). Point mutations in the α2 domain of HLA-A2.1 define a functionally relevant Interaction with TAP. Current Biology, 6(7), 873–883. https://doi.org/10.1016/S0960-9822(02)00611-5