An autonomously folding beta-hairpin derived from the human YAP65 WW domain: attempts to define a minimum ligand-binding motif.

WW domains are broadly distributed among natural proteins; these modules play a role in bringing specific proteins together. The ligands recognized by WW domains are short segments rich in proline residues. We have tried to identify the minimum substructure within a WW domain that is required for ligand binding. WW domains typically comprise ca. 40 residues and fold to a three-stranded beta-sheet. Structural data for several WW domain/ligand complexes suggest that most or all of the intermolecular contacts involve beta-strands 2 and 3. We have developed a 16-residue peptide that folds to a beta-hairpin conformation that appears to mimic beta-strands 2 and 3 of the human YAP65 WW domain, but this peptide does not bind to known ligands. Thus, the minimum binding domain is larger than the latter two strands of the WW domain beta-sheet.