Tapasin: an ER chaperone that controls MHC class I assembly with peptide

The stable assembly of MHC class I molecules with peptides in the endoplasm ic reticulum (ER) involves several accessory molecules. One of these access ory molecules is tapasin, a transmembrane protein that tethers empty class I molecules to the peptide transporter associated with antigen processing ( TAP). Here, evidence is presented that tapasin retains class I molecules in the ER until they acquire high-affinity peptides.