Effect of solution conformation on antibody recognition of a protein core epitope from gastrointestinal mucin (MUC2)

Antibody recognition of the tandem repeat unit of MUC2 glycoprotein was investigated. To clarify the role of secondary structure, the immunoreactivity and conformation of overlapping and truncated peptides were investigated. For this several MUC2 peptides have been synthesized and their secondary structure has been analyzed by circular dichroism and Fourier transform infrared spectroscopical methods. For the binding studies a MUC2 mucin protein core-specific monoclonal antibody was used in competition RIA experiments. The minimal size peptide functioning as epitope was peptide 18PTGTQ22. Within the immunodominant 13TPTPTPTGTQTPTT26 region we found that all peptides recognized by the 996 monoclonal antibody adopted beta-turns secondary structure. Peptides 15TPTPTGTQ22 and 16PTPTGTQ22, containing the most prominent beta-turn(s), had the strongest immunoreactivity. It was also observed that peptides with Pro on their N-termini (16PTPTGTQ22, 18PTGTQ22) adopt a different type of beta-turn in TFE than peptides with Thr at their N-terminal. Based on the antibody binding, molecular dynamics calculations, and secondary structure analysis, we propose a model for the epitope structure of the MUC2 mucin tandem repeat.