Structure and function of the SH3 domain from Bruton ́s tyrosine kinase

Mutations in the gene coding for Bruton ́s tyrosine kinase (Btk) lead to a lymphocyte differentiation block, which results in an extreme deficiency of B cells and plasma cells in the blood. These mutations are one cause of the hereditary immunodeficiency Xlinked agammaglobulinemia. Evolutionarily, Btk belongs to the Tec family of nonreceptor protein tyrosine kinases. Members of this family share a common domain composition with an N-terminal Pleckstrin homology domain, the Tec homology (TH) region, an Src homology (SH) 3 domain, an SH2 domain and a C-terminal catalytic (kinase) domain. The TH region, N-terminal to the SH3 domain in Btk, contains two proline-rich sequences that conform to the consensus sequence of SH3 ligands and which can recognize and bind to the Btk SH3 domain. The solution structure of the SH3 domain from Btk has been determined using NMR methods. Studies of a protein fragment containing the proline-rich sequences of the Btk TH region and the Btk SH3 domain revealed that the Btk SH3 domain interacts with the TH region in an inter-molecular manner and that experimental data are consistent with dimerization. Intermolecular NOEs between two monomers of a dimer indicate a normal polyproline type II helix conformation of the proline-rich sequences in the interaction with the SH3 domain. Activation of Btk by Src-family kinases leads to an auto-phosphorylation of tyrosine 223 within the binding site on the SH3 domain. It is possible that intermolecular interactions between SH3 and the proline-rich region within a Btk dimer play a key role for the regulation of Btk activity.