FIGURE 23-25 Class I MHC pathway of antigen processing and presentation. Step 1: Acquisition of antigen is synonymous with the production of proteins with errors (premature termination, misincorporation). Step 2: Dysfunctional proteins are targeted for degradation by ubiquitinylation. Step 3: Proteolysis is carried out by the proteasome. In cells exposed to interferon γ, the catalytically active β subunits of the proteasome are replaced by interferon-induced immune-specific β subunits. Step 4: Peptides are delivered to the interior of the endoplasmic reticulum (ER) via the dimeric TAP peptide transporter. Step 5: Peptide is loaded onto newly made class I MHC molecules within the peptide-loading complex. Step 6: The fully assembled class I MHC–peptide complex is transported to the cell surface via the secretory pathway. See text for details.