MHC class I complex synthesis and assembly. MHC class I heavy chains are cotranslationally inserted into the ER. Proper folding, association with the β₂m light chain, and peptide loading are assisted by chaperones such as calnexin, calreticulin, tapasin, and ERp57. E1, E2, and E3 ubiquitin-conjugating enzymes tag proteins in the cytosol for destruction by the proteasome. Peptides are pumped into the ER by the TAP transporters. Properly folded, peptide-loaded MHC class I complexes progress through the secretory pathway for display on the cell surface. TAP, transporter associated with antigen processing.