Aminopeptidases in the ER (endoplasmic reticulum) of the cell perform the last stage of trimming of antigenic peptide precursors before loading the mature antigenic peptides onto MHC class I molecules.
ERAP1 (ER aminopeptidase 1) trims antigenic peptide precursors from their N-terminus, one residue at a time, stopping at the correct peptide length required for MHC loading:
We are currently investigating the mechanism that ERAP1 and its homologous enzymes ERAP2 and IRAP utilise to generate antigenic peptides, using a variety of molecular biology approaches, hopping to unravel the exact role of the enzyme in antigenic peptide selection (for examples read this paper and this paper).