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Nat Immunol. 2001 Jul;2(7):644-51.

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

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Division of Immunology, Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720-3200, USA.


We define here the specificity and significance of proteases in the endoplasmic reticulum (ER) that generate peptides for presentation by major histocompatibility complex (MHC) class I molecules. We show that aminopeptidases efficiently trimmed all residues except proline that flank the NH2-termini of antigenic precursors in the ER and caused an accumulation of X-P-Xn peptides. An aminopeptidase inhibitor blocked peptide trimming in the ER and, consequently, the generation of peptide-loaded MHC molecules. Peptide trimming in the ER is therefore a key step in the MHC class I antigen-processing pathway and also explains the paradox of why many MHC class I molecules display peptides with the X-P-Xn motif despite the inability of the transporter associated with antigen processing to transport such peptides from the cytoplasm.

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