Selection and binding of peptides to human transporters associated with antigen processing and rat cim-a and -b.
Cytotoxic T lymphocytes recognize antigenic peptides presented by MHC class I molecules. The peptides are generated in the cytosol by proteasomes, and probably also other proteases, and are then translocated into the endoplasmic reticulum (ER) lumen. The transporters associated with Ag processing (TAP) are key molecules for transporting peptides from the cytosol to the lumen of the ER. Using semipermeabilized cells, TAP-dependent peptide translocation was demonstrated, and the selectivity of peptide translocation was based on the carboxyl-terminal amino acid of peptides. We have examined peptide binding proteins in the ER membrane and the selection of peptides for binding to TAP by using a panel of peptides of different sequences and carboxyl-termini as well as peptides containing D amino acids. Peptides bound to TAP molecules in the absence of ATP. The presence of ATP induced binding of peptides to two additional membrane proteins (58 and 43 kDa). The selection of peptides by TAP molecules was based on peptide sequence and the carboxyl-terminal amino acid. Peptides containing D amino acid did not bind to TAP molecules. Rat cim-a and -b selected peptides differently, and selection was not only dependent on the carboxyl-terminal residue of the peptide, but included an influence of the peptide sequence. The different off-rates after peptide binding to TAP, indicated a dual binding step of peptide to TAP. ATP regulated the off-rate of peptides at a high affinity binding step. Our results demonstrate that the binding of peptides to TAP molecules is specific and most likely involves a multiple step pathway.