THE SOLUTION STRUCTURE OF ABL SH3, AND ITS RELATIONSHIP TO SH2 IN THE SH(32) CONSTRUCT

Background: The Src homology domains, SH3 and SH2, of Ab1 protein tyrosine kinase regulate enzymatic activity in vivo. Ab1 SH3 suppresses kinase activity, whereas Ab1 SH2 is required for the transforming activity of the activated form of Ab1. We expect that the solution structures of Ab1 SH3, Ab1 SH2 and Ab1 SH(32) (a dual domain comprising SH3 and SH2 subdomains) will contribute to a structural basis for understanding the mechanism of the Ab1 'regulatory apparatus'. Results: We present the solution structure of the free Ab1 SH3 domain and a structural characterization of the Ab1 regulatory apparatus, the SH(32) dual domain. The solution structure of Ab1 SH3 was determined using multidimensional double resonance NMR spectroscopy. It consists of two antiparallel beta sheets packed orthogonally, an arrangement first shown in spectrin SH3. Compared with the crystal structure of the Ab1 SH3 complexed with a natural ligand, there is no significant difference in overall folding pattern. The structure of the Ab1 SH(32) dual domain was characterized by NMR spectroscopy using the H-1 and N-15 resonance assignment of Ab1 SH3 and Ab1 SH2. On the basis of the high degree of similarity in chemical shifts' and hydrogen/deuterium exchange pattern for the individual domains of SH3 and SH2 compared with those of the SH(32) dual domain, a structural model of the Ab1 SH(32) regulatory apparatus is suggested. This model is in good agreement with the ligand-binding characteristics of Ab1 SH3, SH2 and SH(32). The binding constants for isolated SH3 and SH2 domains when binding to natural ligands, measured by intrinsic fluorescence quenching, do not differ significantly from the constants of these domains within SH(32). Conclusion: The solution structures of free Ab1 SH3 and Ab1 SH2, and the structural model of Ab1 SH(32), provide information about the overall topology of these modular domains. The structural model of Ab1 SH(32), a monomer, consists of the SH3 and SH2 domains connected by a flexible linker. Sites of ligand binding for the two subdomains are independent.