Joost Roelse

Prior to their association with major histocompatibility complex (MHC) class I molecules, peptides generated from cytosolic antigens need to be translocated by the MHC-encoded peptide transporter (TAP) into the lumen of the endoplasmic reticulum (ER). While class I molecules possess well-known binding characteristics for peptides, the fine specificity of TAP for its peptide substrates has not been analyzed in detail. Previously, we have studied the effect of amino acid variations at the N-terminal, the C-terminal, and the penultimate residue on the efficiency of peptide translocation. Using permeabilized cells, we have shown that TAP pre-selects peptides in an allele- and species-specific manner, for which only the C-terminal residue is crucial. This finding is confirmed in the present study by using microsomes containing different TAP. The influence of amino acid substitutions at positions 2 to 7 of 9-residue model peptides on TAP-dependent peptide translocation is systematically examined. Only a few amino acid substitutions at these positions affect the efficiency of peptide translocation significantly, e.g. Pro at position 2 or 3 negatively influences transport whereas Glu at positions 6 and 7 enhances transport. The differences in translocation by the rat TAP alleles a or u, mouse TAP and human TAP are, however, minor for the peptide with internal substitutions used in this study. These results show that the C-terminal residue essentially governs the species-specific substrate specificity of TAP.

Abstract A crucial step in antigen presentation by MHC class I molecules is the translocation of peptide fragments of protein antigens from the cytosol to the lumen of the endoplasmic reticulum (ER). Here, peptides are bound with high affinity by freshly assembled MHC class I H-chain/l32m heterodimers and are then intracellularly transported to the cell surface and presented by class I molecules to cog+ T cells (1, 2). An ER-located peptide pump termed TAP (Transporter associated with Antigen Presentation) has been genetically defined and demonstrated to translocate peptides from the cytosol to the lumen of the ER (3, 4). Essential to the characterization of TAP were mutant cell lines with inactivated genes for one or both of the two subunits of TAP. In the absence of TAP activity, MHC class I molecules are not loaded with peptides. This results in retention of the class I heterodimer in the ER and decreased cell surface expression of class I molecules.