Mutation of the pleckstrin homology domain of Bruton's tyrosine kinase in immunodeficiency impaired inositol 1,3,4,5-tetrakisphosphate binding capacity

Bruton's tyrosine kinase (Btk), a cytoplasmic protein-tyrosine kinase, plays a pivotal role in B cell activation and development. Mutations in the pleckstrin homology (PH) domain of the Btk gene cause human X-linked agammaglobulinemia (XLA) and murine X-linked immunodeficiency (Xid). In this paper, we report that the PH domain of Btk functions as an inositol 1,3,4,5- tetrakisphosphate (IP₄), inositol 1,3,4,5,6-pentakisphosphate, and inositol 1,2,3,4,5,6-hexakisphosphate (IP₆) binding domain (K(d) of approximately 40 nM for IP₄), and that all of the XLA (Phe replaced by Ser at position 25 (F25S), R28H, T33P, V64F, and V113D) and Xid mutations (R28C) found in the PH domain result in a dramatic reduction of IP₄ binding activity. Furthermore, the rare alternative splicing variant, with 33 amino acids deleted in the PH domain, corresponding to exon 3 of the Btk gene, also impaired IP₄ binding capacity. In contrast, a gain-of-function mutant called Btk*, which carries a E41K mutation in the PH domain, binds IP₆ with two times higher affinity than the wild type. Our data suggest that B cell differentiation is closely correlated with the IP₄ binding capacity of the PH domain of Btk.