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.