Identification and solution conformation of multiple epitopes recognized by a MUC2 mucin-specific monoclonal antibody

We have identified the optimal epitope, (21)TQTPT(25), in the tandem repeat of mucin 2 (MUC2) glycoprotein by using glycoprotein-specific monoclonal antibody, MAb 994, and synthetic, overlapping and truncated oligopeptides corresponding to the sequence (13)TPTPTPTGTQTPTT(26). We found that peptides containing the (21)TQTPT(25) sequence were able to inhibit the 994 antibody binding and also peptides (21)TQTPT(25) and (17)TPTGTQTPT(25) were the most inhibitory compounds with the lowest IC(50) value (IC(50) = 4 and 3 muM, respectively) tested. Interestingly, (21)TQTPT(25) peptide adopts an unordered structure even in TFE, a solvent that promotes an ordered conformation, as detected by circular dichroism and Fourier-transform infrared spectroscopy. However, Thr at position 26 or amidation of Thr(25) at the C-terminus results in a much weaker (3 orders of magnitude) MAb interaction, which can be due to the presence of a turn conformation in peptides with a T(26) or an amide C-terminus. We have also observed that MAb 994 recognized two other pentapeptides with the TX(1)TX(2)T motif, like (13)TPTPT(17) (IC(50) = 180 muM) and (19)TGTQP(23) (IC(50) = 65 muM), whose sequences are present in the native glycoprotein. These findings might suggest that in the MUC2 tandem repeat unit there are multiple antigenic sites available for recognition in underglycosylated tumor tissue and also explain the heteroclitic nature of MAb 994. (C) 2002 Elsevier Science (USA). All rights reserved.