ER aminopeptidases generate a unique pool of peptides for MHC class I molecules

We define here the specificity and significance of proteases in the endopla smic reticulum (ER) that generate peptides for presentation by major histoc ompatibility complex (MHC) class molecules. We show that aminopeptidases ef ficiently trimmed all residues except proline that flank the NH2-termini of antigenic precursors in the ER and caused an accumulation of X-P-X-n pepti des. An aminopeptidase inhibitor blocked peptide trimming in the ER and, co nsequently, the generation of peptide-loaded MHC molecules. Peptide trimmin g in the ER is therefore a key step in the MHC class I antigen-processing p athway and also explains the paradox of why many MHC class molecules displa y peptides with the X-P-X. motif despite the inability of the transporter a ssociated with antigen processing to transport such peptides from the cytop lasm.