Solution structure of the human BTK SH3 domain complexed with a proline-rich peptide from p120(cbl)

X-linked agammaglobulinemia (XLA), an inherited disease, is caused by mutat ions in the Bruton's tyrosine kinase (BTK). The absence of functional BTK l eads to failure of B cell differentiation which incapacitates antibody prod uction in XLA patients leading to, sometimes lethal, bacterial infections. Point mutation in the BTK gene that leads to deletion of C-terminal 14 aa r esidues of BTK SH3 domain was found in one patient family. To understand th e role of BTK in B cell development, we have determined the solution struct ure of BTK SH3 domain complexed with a proline-rich peptide from the protei n product of c-cbl protooncogene (p120(cbl)). Like other SH3 domains, BTK S H3 domain consists of five beta-strands packed in two beta-sheets forming a beta-barrel-like structure. The rmsd calculated from the averaged coordina tes for the BTK SH3 domain residues 218-271 and the p120(cbl) peptide resid ues 6-12 of the complex was 0.87 Angstrom (+/- 0.16 Angstrom) for the backb one heavy atoms (N, C, and C-alpha) and 1.64 Angstrom (+/- 0.16 Angstrom) f or all heavy atoms. Based on chemical shift changes and inter-molecular NOE s, we have found that the residues located in the RT loop, n-Src loop and h elix-like loop between beta 4 and beta 5 of BTK SH3 domain are involved in ligand binding. We have also determined that the proline-rich peptide from p120(cbl) binds to BTK SH3 domain in a class I orientation. These results c orrelate well with our earlier observation that the truncated BTK SH3 domai n (deletion of beta 4, beta 5 and the helix-like loop) exhibits weaker affi nity for the p120(cbl) peptide. It is likely that the truncated SH3 domain fails to present to the ligand the crucial residues in the correct context and hence the weaker binding. These results delineate the importance of the C-terminus in the binding of SH3 domains and also indicate that improper f olding and the altered binding behavior of mutant BTK SH3 domain likely lea d to XLA.