ERAP1 shapes just part of the immunopeptidome.

ERAP1 is an aminopeptidase involved in trimming long peptides to the lengths required for presentation by MHC class I. ERAP1 substrate preference is for peptides with hydrophobic or aliphatic N-terminal amino acids, with lower efficacy with charged and small hydrophilic amino acids and almost complete inefficiency with proline. Since ERAP1 efficiently trims peptides to eight amino acids or even shorter, and many MHC-I allotypes can only bind peptides that are eight or nine amino acids or longer, ERAP1 both produces and destroys potential ligands of these alleles. The observation that ERAP1 modulates the levels of presentation for only a subset of the immunopeptidome conflicts with the common assumption that most MHC-I peptides are derived from longer peptides that are produced by the proteasome, transported into the endoplasmic reticulum (ER) by the Transporter Associated Peptide Presentation (TAP) and then trimmed by ERAP1. A more likely mechanism is that cellular protein degradation produces surplus amounts of peptides that fit perfectly and are rapidly loaded onto the MHC, with only a minority of peptides requiring trimming within the ER before loading. Alternatively, ERAP1 may not be present in all ER compartments or vesicles where peptide processing and loading take place and thus affects just a subset of the immunopeptidome.