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Proteolytic processing of peptides in the lumen of the endoplasmic reticulum for antigen presentation by major histocompatibility class I

We have tested the hypothesis that MHC class I molecules are actively involved as protease in the production of natural MHC class I ligands. First, the structure of a class I molecule was analyzed for homology with catalytic sites of known proteases. While several clusters of amino acids in the restriction element resembled protease active sites, structural discrepancies and the influence of nearby residues suggest that these sites are unlikely to have protease activity. Second, we have tested the presentation of viral cytotoxic T cell determinants with affinity for the same restriction element (H‐2Kd or Kk), when targeted as tandem peptides into the endoplasmic reticulum. Peptide transporter‐defective cells were used to exclude cleavage of the tandem peptides by cytosolic proteases. Cleavage by signal peptidase of the tandem peptides was ascertained. The C‐terminal peptides in the tandem arrays were almost exclusively presented, suggesting that an aminopeptidase in the endoplasmic reticulum degraded the N‐terminally positioned peptides. This result is inconsistent with an MHC class I‐catalyzed cleavage following binding of longer peptides in the cleft of the restriction elements. Finally, we conclusively show that an aminopeptidase in the endoplasmic reticulum is also involved in antigen presentation in cells with a functional peptide transporter.