the folding and assembly of MHC class 1 molecules takes place in the lumen of the endoplasmic reticulum the initial folding of the class 1 heavy or alpha chain is aided by the chaperone calnexin the partially folded chain is transferred to a second chaperone calreticulin which aids the further folding of the chain and the association of beta 2 microglobulin other proteins herb 57 and tap hacen associate with the nascent class 1 molecule which binds to the TAT transporter viet a person to form a peptide loading complex the peptides that bind to the MHC class 1 molecule are generated by a large protein complex the proteasome which is found in the cytoplasm the proteasome degrades proteins within the cytosol of the cell to produce short peptides which have been transported through the endoplasmic reticulum membrane by the tap transporter in the ER these peptides are modified by the action of an enzyme known as the endoplasmic reticulum amino peptidase associated with antigen processing or ear app ear app trims the amino terminus of many peptides and allows peptides that are initially too long to bind to the MHC molecules some peptides do not bind to the MHC molecule at all others can bind but are unstable these are released from the MHC molecule a process called peptide editing finally a peptide binds to the MHC molecule with high affinity to make a stable complex this causes the final step in the folding of the class one molecule to take place and the dissociation of the peptide loading complex the peptide loaded MHC class 1 molecule is now free to exit the endoplasmic reticulum and be transported via the Golgi apparatus to the cell surface where it can be recognized by the antigen receptors of cd8 t-cells you