Nature Immunology
3, 1121 - 1122 (2002)
doi:10.1038/ni1202-1121
The final cut: how ERAP1 trims MHC ligands to sizeKirsten Falk
& Olaf RötzschkeMax-Delbrück-Center for Molecular Medicine, Robert-Rössle-Str. 10, 13125 Berlin, Germany. falk@mdc-berlin.de
or roetzsch@mdc-berlin.de Proteolytic cleavage generates the peptide repertoire displayed by MHC class I. Now, an interferon- −inducible aminopeptidase in the endoplasmic reticulum has been identified as the final player in this complex process.Major histocompatibility complex (MHC) class I molecules are present in almost all nucleated cells. They function as receptors of short peptide fragments, which they transport to the cell surface to present for surveillance by the immune system. More than 10,000 different peptides are displayed on each cell. These peptides are eight or nine amino acids long, and the composition of the pool mirrors the intracellular protein content. Infections by pathogens or transformations of the cell result in the appearance of foreign or "abnormal" peptides, which mark the cell for destruction by cytotoxic T cells. It has been known for some time that the ligands of MHC class I molecules are generated by proteolytic degradation of endogenous proteins. Analysis of the amino acid sequences of these ligands suggested that two consecutive proteolytic steps are needed to produce their two ends. The proteasome, identified ten years ago as a key element in the processing pathway, is responsible for the generation of the COOH terminus, but the protease producing the NH2 terminus of the peptide ligands has remained elusive. Two studies published in this issue of Nature Immunology, by Saric et al.1 and York et al.2, and another report by Serwold et al.3 in a recent issue of Nature now identify as the enzyme in question the endoplasmic reticulum aminopeptidase 1 (ERAP1), which trims the NH2 terminus of peptide ligands to their proper length.
What is the evidence that ERAP1 is the sought-after protease? First, location. In the past years several other peptidases had been introduced as potential candidates4,
5,
6. But unlike these putative proteases, ERAP1 is expressed in the lumen of the endoplasmic reticulum (ER)1,
3, the compartment where the loading of MHC class I molecules takes place. Second, specificity. A unique feature of ERAP1 is that its proteolytic activity is controlled by the size of the peptide substrate2. ERAP1 rapidly degrades longer peptides, but its activity is substantially reduced once the peptide is trimmed down to a size of eight or nine amino acids, the optimal length for binding MHC class I molecules. Third, regulation by interferon- (IFN-). IFN- influences the expression of several important elements of the antigen processing pathway7. This includes the two subunits of the "transporter associated with antigen processing" (TAP) as well as the MHC class I molecule itself, all of which are upregulated by IFN-. The proteasome is converted into an "immuno-proteasome" with enhanced efficiency for the generation of potential precursor peptides by the integration of IFN--inducible subunits, and gene expression of ERAP1 is strongly upregulated by IFN-.
In addition to this circumstantial evidence, the three studies provide some direct in vivo proof. In a series of elegant experiments in living cells, Serwold et al.3 and York et al.2 demonstrate the influence of ERAP1 expression on the presentation of several peptide antigens by using inhibiting small interfering RNA (siRNA). Corresponding results were also obtained in similar experimental systems in which ERAP1 or a functionally inactivated mutant form was overexpressed1,
2.
With this information, we can now fit the parts of the MHC class I processing pathway together like the pieces of a jigsaw puzzle (Fig. 1). As postulated8, the proteolytic generation of peptide ligands occurs in two successive steps. In the first step, proteins (preferentially those marked by ubiquitination) are degraded in the cytosol by the proteasome, also known as the cellular "shredder." Through coordinated double cuts, the proteasome generates protein fragments that contain the hydrophobic or positively charged COOH-terminal anchor residue required for MHC class I binding. Although in some instances optimal octamers or nonamers might be produced directly in the cytosol, most fragments seem to represent NH2-terminal-extended precursor peptides. These peptides are then channeled into the lumen of the ER. Peptide translocation is mediated by TAP, an ATP-driven transporter that seems to be particularly well adapted for the transfer of these precursors9. In the second step, the NH2 termini of precursor peptides are trimmed in the lumen of the ER by ERAP11,
2,
3. ERAP1 trimming slows down when lengths of eight or nine amino acids are achieved, such that MHC class I molecules can finally select the ligands that meet their allele-specific binding requirements10.
 | | |
Thus, ERAP1 seems to be the "missing link" that completes the MHC class I processing pathway, but it remains to be seen to what extent other proteases, such as the previously characterized cytosolic aminopeptidases4,
5,
6, participate in this process. The substrate specificities of what are apparently the four key elements—the proteasome, TAP, ERAP1 and MHC class I molecules—seem to result from a remarkable process of co-evolution, and it will be especially useful to resolve the molecular mechanism by which ERAP1 adapts its activity to the size of the peptide. Another open question is whether ERAP1 binds directly to the components involved in peptide loading. A recent study suggests a close interaction between the aminopeptidase and MHC class I molecules, because efficient trimming in the ER apparently requires the presence of appropriate MHC molecules11. Peptide loading takes place in a highly coordinated complex consisting of the MHC class I molecule, the TAP transporter and several other components, including the chaperones (calnexin and calreticulin), the oxidoreductase Erp57 and tapasin, a protein which links the MHC molecule to the peptide transporter12. So far, no proteolytic activity has been detected in the loading complex, but it seems almost mandatory that ERAP1 be integrated in this complex.
What are the practical applications of this discovery? Because ERAP1 seems to be an integral part of the antigen processing pathway, it should be potential therapeutic target when undesired or self-directed immune responses have to be contained. Inhibitors of proteasomal degradation have already drawn some interest, most recently as potential anticancer agents13. Their effect, however, relates to their regulatory function in the degradation of proteins controlling cell growth and apoptosis, and not to antigen processing. In principal, inhibition of ERAP1 should make it possible to suppress more specifically the presentation of "harmful" peptide antigens—which would be relevant, for instance, in the context of autoimmune disease. Yet all three studies1,
2,
3 showed that blockade of ERAP1 activity for some epitopes had just the opposite effect. Apparently, ERAP1 not only generates antigenic epitopes but also destroys some peptide antigens. Blockade of ERAP1 will therefore not necessarily result in an abrogation of antigen presentation but rather could influence the composition of the peptide repertoire in a fashion that is currently unpredictable. Future trials with specific inhibitors or ERAP1-deficient mice should reveal more detailed insights that ultimately should allow evaluation of its potential as a drug target.
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