Binding of synthetic peptides by a human monoclonal IgM with an unusual combining site structure

Using X-ray crystallography, a human monoclonal IgM cryoglobulin (Mez) was found to have an unusual combining site topography. Analysis of the unligan ded Fv at 2.6 Angstrom resolution revealed that the HCDR3 had partitioned t he active site into two compartments [Ramsland PA et al 2000. Mol Immunol. 37: 295-310]. The two cavities had dimensions and chemical properties that were compatible with the binding of peptides. In this study, libraries of p eptides were prepared using solid-phase synthesis. Binding of the intact Me z IgM to these peptides was tested by enzyme-linked immunoassays. Screening of 400 dipeptides revealed that binding was markedly skewed toward amino a cids with aromatic side-chains (Phe and Trp), especially when located in th e second position. Preferential recognition of aromatic side-chains by Mez IgM was confirmed with larger peptides of three to five residues, but C-ter minal positioning was not favored in these peptides. Mez IgM also showed bi nding propensities for acidic residues (Asp and Glu) as well as several oth er sidechains with different chemical properties, including His, Pro, Asn a nd Gln. Mez IgM recognized sets of overlapping octapeptides representing th e sequences of the constant domains of human IgG1 heavy chains. These pepti des represented similar stretches of polypeptide on the three-dimensional s tructures of all three constant domains (CHI, CH2 and CH3). Thus, Mez IgM m ay recognize structurally homologous regions of immunoglobulin domains, whi ch were conserved during the evolution of the immune system. Copyright, (C) 2001 John Wiley & Sons, Ltd.