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Major Histocompatibility Complex (MHC) class I molecules are synthesized and assembled in the endoplasmic reticulum (ER). Their assembly involves binding to a multicomponent complex, containing the transporter associated with antigen processing (TAP) and the ER-resident proteins, calnexin, calreticulin, tapasin and Erp57.

The main role of this complex is to supply nascent MHC class I molecules with peptides that are generated in the cytosol by proteasomes. Loading of MHC class I molecules with peptides results in their dissociation from the TAP complex and transport from the ER to the Golgi on route to the plasma membrane.

ER export of membrane and secretory proteins (cargo proteins) requires sorting from ER resident proteins and packaging into COPII-coated vesicles.