The WW domain is a structured protein module found in a wide range of regulatory, cytoskeletal, and signaling molecules. Its ligands contain proline-rich sequences, some of which show a core consensus of XPPXY that is critical for binding. In order to gain a better understanding of the molecular and biological functions of WW domains, we decided to predict their cognate ligands by searching databases for proteins containing the XPPXY consensus. Using several axioms that take into account evolutionary conservation and functional similarity, we have identified four groups of proteins representing candidate ligands that signal through known or unknown WW domains. These include viral Gag proteins, sodium channels, interleukin receptors, and a subgroup of serine/threonine kinases. In addition, we proposed that dystrophin and β-dystroglycan bind through the WW-XPPXY link and that interference with this interaction could result in muscular dystrophy. Our study provides guidelines for experiments to probe the molecular and biological functions of the WW domain-ligand connection. Should these predictions be proven empirically, the results may have important ramifications for basic research and medicine.
