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A post-translational modification of nuclear proteins, N(G),N(G)-dimethyl-Arg, found in a natural HLA class I peptide ligand
Authors:Yagüe J  Vázquez J  López de Castro J A
Institution:Centro de Biología Molecular Severo Ochoa, Consejo Superior de Investigaciones Científicas and Universidad Autónoma de Madrid, Facultad de Ciencias, Cantoblanco, Spain.
Abstract:Presentation of peptides derived from endogenous proteins by class I major histocompatibility complex molecules is essential both for immunological self-tolerance and induction of cytotoxic T-cell responses against intracellular parasites. Despite frequent and diverse post-translational modification of eukaryotic cell proteins, very few class I-bound peptides with post-translationally modified residues are known. Here we describe a natural dodecamer ligand of HLA-B39 (B*3910) derived from an RNA-binding nucleoprotein that carried N(G),N(G)-dimethyl-Arg. Although common among RNA-binding proteins, this modification was not previously known among natural class I ligands. The sequence of this peptide was determined by Edman degradation and electrospray ion trap mass spectrometry. The fragmentation pattern of the dimethyl-Arg side chain observed with this latter technique allowed us to unambiguously assign the isomeric form of the modified residue. The post-translationally modified ligand was a prominent component (1-2%) of the B*3910-bound peptide repertoire. The dimethyl-Arg residue was located in a central position of the peptide, amenable to interacting with T-cell receptors, and most other residues in the middle region of the peptide were Gly. These structural features strongly suggest that the post-translationally modified residue may have a major influence on the antigenic properties of this natural ligand.
Keywords:HLA-B39  post-translational modifications  MHC-bound peptides  peptide sequencing  mass spectrometry
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