Solution structure of an atypical WW domain in a novel β-clam-like dimeric form

The WW domain is known as one of the smallest protein modules with a triple-stranded β-sheet fold. Here, we present the solution structure of the second WW domain from the mouse salvador homolog 1 protein. This WW domain forms a homodimer with a β-clam-like motif, as evidenced by size exclusion chromatography, analytical ultracentrifugation and NMR spectroscopy. While typical WW domains are believed to function as monomeric modules that recognize proline-rich sequences, by using conserved aromatic and hydrophobic residues that are solvent-exposed on the surface of the β-sheet, this WW domain buries these residues in the dimer interface.