Sequence signals for generation of antigenic peptides by the proteasome: implications for proteasomal cleavage mechanism.
Proteasomal cleavage of proteins is the first step in the processing of most antigenic peptides that are presented to cytotoxic T cells. Still, its specificity and mechanism are not fully understood. To identify preferred sequence signals that are used for generation of antigenic peptides by the proteasome, we performed a rigorous analysis of the residues at the termini and flanking regions of naturally processed peptides eluted from MHC class I molecules. Our results show that both the C terminus (position P1 of the cleavage site) and its immediate flanking position (P1') possess significant signals. The N termini of the peptides show these signals only weakly, consistent with previous findings that antigenic peptides may be cleaved by the proteasome with N-terminal extensions. Nevertheless, we succeed to demonstrate indirectly that the N-terminal cleavage sites contain the same preferred signals at position P1'. This reinforces previous findings regarding the role of the P1' position of a cleavage site in determining the cleavage specificity, in addition to the well-known contribution of position P1. Our results apply to the generation of antigenic peptides and bare direct implications for the mechanism of proteasomal cleavage. We propose a model for proteasomal cleavage mechanism by which both ends of cleaved fragments are determined by the same cleavage signals, involving preferred residues at both P1 and P1' positions of a cleavage site. The compatibility of this model with experimental data on protein degradation products and generation of antigenic peptides is demonstrated.
Altuvia
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Y
Department of Molecular Genetics, The Hebrew University - Hadassah Medical School, Jerusalem, 91120, Israel.
Margalit
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H
D000818:Animals; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D019992:Databases as Topic; D015395:Histocompatibility Antigens Class I / Q000737:chemistry / Q000378:metabolism*; D006801:Humans; D051379:Mice; D009097:Multienzyme Complexes / Q000378:metabolism*; D010446:Peptide Fragments / Q000737:chemistry* / Q000378:metabolism*; D019151:Peptide Library; D046988:Proteasome Endopeptidase Complex; D013602:T-Lymphocytes, Cytotoxic / Q000276:immunology
D015395:Histocompatibility Antigens Class I; D009097:Multienzyme Complexes; D010446:Peptide Fragments; D019151:Peptide Library; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex