Physical and functional characterization of the genetic locus of IBtk, an inhibitor of Bruton's tyrosine kinase: evidence for three protein isoforms of IBtk

Bruton’s tyrosine kinase (Btk) is required for B-cell development. Btk deficiency causes X-linked agam- maglobulinemia (XLA) in humans and X-linked immu- nodeficiency (Xid) in mice. Btk lacks a negative regulatory domain and may rely on cytoplasmic pro- teins to regulate its activity. Consistently, we identified an inhibitor of Btk, IBtk, which binds to the PH domain of Btk and down-regulates the Btk kinase activity. IBtk is an evolutionary conserved protein encoded by a single genomic sequence at 6q14.1 cytogenetic loca- tion, a region of recurrent chromosomal aberrations in lymphoproliferative disorders; however, the physical and functional organization of IBTK is unknown. Here, we report that the human IBTK locus includes three distinct mRNAs arising from complete intron splicing, an additional polyadenylation signal and a second transcription start site that utilizes a specific ATG for protein translation. By northern blot, 5’RACE and 3’RACE we identified three IBTKa, IBTKb and IBTKc mRNAs, whose transcription is driven by two distinct promoter regions; the corresponding IBtk proteins were detected in human cells and mouse tissues by specific antibodies. These results provide the first characterization of the human IBTK locus and may assist in understanding the in vivo function of IBtk.