Large scale mass spectrometric profiling of peptides eluted from HLA molecules reveals N-terminal-extended peptide motifs.

The majority of >2000 HLA class I molecules can be clustered according to overlapping peptide binding specificities or motifs recognized by CD8(+) T cells. HLA class I motifs are classified based on the specificity of residues located in the P2 and the C-terminal positions of the peptide. However, it has been suggested that other positions might be relevant for peptide binding to HLA class I molecules and therefore be used for further characterization of HLA class I motifs. In this study we performed large-scale sequencing of endogenous peptides eluted from K562 cells (HLA class I null) made to express a single HLA molecule from HLA-B3501, -B3502, -B3503, -B3504, -B3506, or -B3508. Using sequence data from >1,000 peptides, we characterized novel peptide motifs that include dominant anchor residues extending to all positions in the peptide. The length distribution of HLA-B35-bound peptides included peptides of up to 15 residues. Remarkably, we determined that some peptides longer than 11 residues represented N-terminal-extended peptides containing an appropriate HLA-B35 peptide motif. These results provide evidence for the occurrence of endogenous N-terminal-extended peptide-HLA class I configurations. In addition, these results expand the knowledge about the identity of anchor positions in HLA class I-associated peptides that can be used for characterization of HLA class I motifs.