SOLUTION STRUCTURE OF THE SH3 DOMAIN FROM BRUTONS TYROSINE KINASE

X-linked agammaglobulinemia (XLA) is a heritable immunodeficiency caus ed by mutations in the gene coding for Bruton's tyrosine kinase (Btk). Btk belongs to the Tec family of tyrosine kinases. Each member of the family contains five regions and mutations causing XLA have been isol ated in all five regions. We have determined the solution structure of the Src homology 3 (SH3) domain of Btk using two-and three-dimensiona l nuclear magnetic resonance (NMR) spectroscopy on natural abundance a nd N-15-labeled protein material. The structure determination is compl emented by investigation of backbone dynamics based on N-15 NMR relaxa tion. The Btk SH3 forms a well-defined structure and shows the typical SH3 topology of two shea antiparallel beta-sheets packed almost perpe ndicular to each other in a sandwich-like fold. The N- and C-termini a re more flexible as are peptide fragments in the RT and n-Src loops. T he studied Btk SH3 fragment adopts two slowly interconverting conforma tions with a relative concentration ratio of 7:1. The overall fold of the minor form is similar to that of the major form, as judged on the basis of observed NOE connectivities and small chemical shift differen ces. A tryptophan (W251) ring flip is the favored mechanism for interc onversion, although other possibilities cannot be excluded. The side c hain of Y223, which becomes autophosphorylated upon activation of Btk, is exposed within the potential SH3 ligand binding site. Finally, we compare the present Btk SH3 structure with other SH3 structures.