Identification and characterization of a molecule, BAM11, that associates with the pleckstrin homology domain of mouse Btk.

Bruton's tyrosine kinase (Btk) is required for normal B cell development and signal transduction through cell surface molecules, and its defects lead to X-linked immune deficiency in mice and X-linked agammaglobulinemia in humans. In this report, we will describe the identification and characterization of a molecule, BAM11, which binds to the pleckstrin homology domain of Btk. A sequence homology search revealed that BAM11 has 89% homology, at the amino acid level, to human LTG19/ENL, that was originally identified as one of the fusion partners involved in chromosomal translocations of 11q23, MLL/ALL-1/HRX, in leukemia cells. Deletion mutants demonstrated that the region of BAM11 required for binding to Btk was localized between amino acid residues 240 and 256. Forced expression of a truncated form of BAM11 (amino acids 246-368) inhibited IL-5-induced proliferation by 50%, whereas forced expression of full-length BAM11 in Y16 cells did not affect the IL-5 responsiveness. We have also shown that BAM11 (amino acids 246-368) inhibited the kinase activity of Btk. These results suggest that the binding of BAM11 to Btk plays a regulatory role in the Btk signal transduction pathway. A cell fractionation study and analysis using EGFP-fused Btk protein demonstrated that a proportion of Btk is present within the nucleus.