New genes in the MHC that encode proteins for antigen processing

Most cells process proteins into short peptides that are displayed on the cell surface bound to class I or class II proteins encoded by the major histocompatibility complex (MHC). These protein-peptide complexes can then be recognized by the circulating lymphocytes of the immune system. Several genes found recently in the MHC encode proteins with possible roles in the supply of peptides to class I molecules. The results imply that the peptides are produced in the cytoplasm by proteasomes and are translocated into the endoplasmic reticulum by 'peptide transporters' related to the multidrug resistance proteins. While there is little biochemical evidence to validate these ideas, Robert DeMars and Thomas Spies discuss here the arguments supporting this view. New data indicate that there may also be factors for class II peptide-processing hidden in the MHC.