New binding specificities derived from Min-23, a small cystine-stabilized peptidic scaffold.

The randomization of both internal and surface residues in small protein domains followed by selection from a display library is emerging as a powerful strategy to obtain novel binding specificities. Small and stable scaffold motifs observed in disulfide-rich proteins are attractive because they are small, stable, and accessible to chemical synthesis. The elementary structural motif found in the squash trypsin inhibitor EETI-II (Ecballium elaterium trypsin inhibitor) is the cystine stabilized beta-sheet (CSB) motif, found in nearly 50% of all known small disulfide-rich protein families. We have used Min-23, a short 23-residue peptide containing the CSB motif and shown to be a stable autonomous folding unit and one of the smallest scaffolds described to date, as a scaffold for selection of new binding ligands. We demonstrate that the core CSB motif in Min-23 is permissive to loop insertion, using peptide epitopes from hemagglutinin (HA) and Gla-protein (E). A phage library of more than 10(8) different clones has been constructed by insertion of a randomized sequence on a beta-turn of the Min-23 peptide. The selection of this library on a variety of 7 different targets allowed the isolation of 21 new specific binders, confirming the potential of Min-23 as a scaffold for the development of new ligands. The derived library is able to provide a wide range of novel compounds with possible applications in various biological and pharmaceutical areas.