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Department of Pathology and Center for Immunology, Washington University School of Medicine, and
Department of Chemistry, Washington University, St. Louis, MO 63110
We report here the identification and quantitation of a minor epitope from hen egg white lysozyme (HEL) isolated from the class II MHC molecule I-Ak of APCs. We isolated and concentrated the peptides from the I-Ak extracts by a peptide-specific mAba, followed by their examination by electrospray mass spectrometry. This initial step improved the isolation, recovery, and quantitation and allowed us to identify 13 different minor peptides using the Ab specific for the HEL tryptic fragment 3445. The HEL peptides varied on both the amino and carboxy termini. The shortest peptide was a 13-mer (residues 3345), and the longest peptide was a 19-mer (residues 3149). The two most abundant were 3147 (1.3 pmol) and 3146 (1 pmol), while the least abundant were 3145 (40 fmol) and 3245 (4 fmol). Only 0.3% of the total class II molecules were occupied by this family of HEL peptides. The amount of the 3147 peptide, the predominant member of this series, was 22 times lower than that of 4862, the major epitope of HEL. The 3147 peptide bound about 20-fold weaker to I-Ak compared with the dominant 4862 peptide. Thus, the lower abundance of the minor epitope correlated with its weaker binding strength.
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