Characterisation of epitopes on human p53 using phage-displayed peptide libraries: insights into antibody-peptide interactions.

We previously described the use of a phage-displayed library of random hexapeptides to define and localise the epitope on the human tumor suppressor protein p53 recognised by the monoclonal antibody PAb240. Here we have extended these results to a further eight anti-p53 monoclonal antibodies and to two further libraries, which display 12-mer and 20-mer peptides, respectively. First, we showed that selection of PAb240 binding clones from the 12-mer and 20-mer libraries gives essentially identical results to those obtained by screening the 6-mer library. Second, we used the 6-mer and 12-mer libraries to define the detailed specificity profiles of six antibodies (DO-1, DO-2, DO-7, Bp53-11, Bp53-12 and Bp53-19), which recognise the same short, highly immunogenic N-terminal segment of p53. Finally, we employed all three libraries to reveal the distinct mechanisms by which PAb421 and PAb122, two monoclonal antibodies that allosterically activate sequence-specific DNA binding by p53, react specifically with the same positively-charged C-terminal segment. In each case the epitope locations inferred from the selected sequences were confirmed by probing an array of overlapping synthetic peptides representing the primary sequence of p53. The results emphasise the consequences for epitope mapping of screening random, as opposed to antigen-derived, peptide libraries; specifically (1) that comparison of selected sequences reveals the contribution of individual residues to binding energy and specificity; (2) that heteroclitic reactions can lead to definition of a consensus that is related to but distinct from the immunising epitope and (3) that isolation of non-immunogen-homologous "mimotope" sequences reveals discrete, alternative ligand structures. The results with PAb421 and PAb122 provide examples where, while selection from the 12-mer and 20-mer libraries leads to isolation of immunogen-homologous sequences, selection from the 6-mer library results in the isolation either of no binding clones (PAb122) or solely of "mimotope" sequences with no discernible homology to the original antigen (PAb421). In addition the results with PAb421 reveal that linear epitopes can be longer than previously thought and can be formally discontinuous, consisting of independent contact motifs, which show promiscuous relative positioning.