REGULATORY INTRAMOLECULAR ASSOCIATION IN A TYROSINE KINASE OF THE TECFAMILY

THE T-cell-specific tyrosine kinase Itk is a member of the Tec family of non-receptor tyrosine kinases(1-7), and is required for signalling through the T-cell antigen receptor (TCR)(8). The role of Itk in TCR S ignalling and the manner in which Itk activity is regulated are not we ll understood. Substrate binding and enzymatic activity of the structu rally related Src kinases are regulated by an intramolecular interacti on between the Src-homology-2 (SH2) domain and a phosphotyrosine(9,10) . Although Itk also contains SH3, SH2 and tyrosine kinase domains, it lacks the corresponding regulatory phosphorylation site, and therefore must be regulated by an alternative mechanism. The proline-rich seque nce adjacent to the SH3 domain of Tec family kinases contains an SH3 l igand, potentially allowing a different intramolecular interaction. By using multidimensional nuclear magnetic resonance we have determined the structure of a fragment of Itk, confirming that these domains inte ract intramolecularly. Formation of this intramolecular SH3-ligand com plex prevents the Itk SH3 domain and proline-rich region from interact ing with their respective protein ligands, Sam68 and Grb-2. We believe that this structure represents the first example of an intramolecular interaction between an SH3 domain and a proline-rich ligand, and has implications for the regulation of Tec family kinases.