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

The stable assembly of MHC class I molecules with peptides in the endoplasmic reticulum (ER) involves several accessory molecules. One of these accessory 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.