Peptides produced in the cytosol are protected from complete degradation by cellular chaperones such as the TCP-1 ring complex (TRiC), but many of these peptides are longer than can be bound by MHC class I molecules. Evidence indicates that the carboxy terminus of peptide antigens is produced by cleavage in the proteasome. However, the amino terminus of peptides that are too long to bind MHC class I molecules can be trimmed by an enzyme called the endoplasmic reticulum aminopeptidase associated with antigen processing (ERAAP). Like other components of the antigen-processing pathway, expression of ERAAP is increased by IFN-γ stimulation. Mice lacking the enzyme ERAAP have an altered repertoire of peptides loaded onto MHC class I molecules. Although the loading of some peptides is not affected by the absence of ERAAP, other peptides fail to load normally, and many unstable and immunogenic peptides not normally present are found bound to MHC molecules on the cell surface. This causes cells from ERAAP-deficient mice to be immunogenic for T cells from wild-type mice, demonstrating that ERAAP is an important editor of the normal peptide:MHC repertoire.
6-4 Newly synthesized MHC class I molecules are retained in the endoplasmic reticulum until they bind a peptide.
Binding a peptide is an important step in the assembly of a stable MHC class I molecule. When the supply of peptides into the endoplasmic reticulum is disrupted, as in TAP-mutant cells, newly synthesized MHC class I molecules
Fig. 6.7 TAP1 and TAP2 form a peptide transporter in the endoplasmic reticulum membrane. Upper panel: TAP1 and TAP2 are individual polypeptide chains, each with one hydrophobic and one ATP-binding domain. The two chains assemble into a heterodimer to form a four-domain transporter typical of the ATP-binding cassette (ABC) family. The hydrophobic transmembrane domains have multiple transmembrane regions (not shown here). The ATP-binding domains lie within the cytosol, whereas the hydrophobic domains project through the membrane into the lumen of the endoplasmic reticulum (ER) to form a channel through which peptides can pass. Lower panel: electron microscopic reconstruction of the structure of the TAP1:TAP2 heterodimer. Panel a shows the surface of the TAP transporter as seen from the lumen of the ER, looking down onto the top of the transmembrane domains, while panel b shows a lateral view of the TAP heterodimer in the plane of the membrane. The ATP-binding domains form two lobes beneath the transmembrane domains; the bottom edges of these lobes are just visible at the back of the lateral view. TAP structures courtesy of G. Velarde.
are held in the endoplasmic reticulum in a partly folded state. This explains why the rare human patients who have been identified with immunodeficiency due to defects in TAP1 and TAP2 have few MHC class I molecules on their cell surfaces, a condition known as MHC class I deficiency. The folding and assembly of a complete MHC class I molecule (see Fig. 4.19) depends on the association of the MHC class I α chain first with β2-microglobulin and then with peptide, and this process involves a number of accessory proteins with chaperone-like functions. Only after peptide has bound is the MHC class I molecule released from the endoplasmic reticulum and transported to the cell surface.