A novel ligand for an SH3 domain of the adaptor protein Nck bears an SH2 domain and nuclear signaling motifs

Net is a small protein composed of Src homology regions (SH) 2 and 3, paralleling the adaptors c-Crk and Grb2/Ash, but its function remains enigmatic, To clarify Nck signaling, a human brain cDNA library was searched for targets of the SH3 moiety of Nck. A novel molecule detected therefrom (referred to as Nck-, Ash- and phospholipase C gamma-binding protein 4) contained proline-rich sequences and, through the function of one of them, interacted with the middle SH3 domain of Nck. A NAP4 fusion peptide exhibited an affinity for Nck, Ash and phospholipase C gamma in whole cell lysates. NAP4 also had an SH2 domain, which could bind to activated EGF receptor, These intermolecular interactions imply the intricacy of Nck-mediated signaling around the receptor protein-tyrosine kinases, In addition, NAP4 bore a putative nuclear localization signal and a Q-run/P-run composite, both characteristic of nuclear proteins, and might therefore relate to the presence of Nck in the cellular nucleus. (C) 1997 Academic Press.