title: Physical and functional characterization of the genetic locus of IBtk, an inhibitor of Bruton's tyrosine kinase: evidence for three protein isoforms of IBtk.
abstract: Bruton's tyrosine kinase (Btk) is required for B-cell development. Btk deficiency causes X-linked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency (Xid) in mice. Btk lacks a negative regulatory domain and may rely on cytoplasmic proteins 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 location, 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 IBTKalpha, IBTKbeta and IBTKgamma 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.
authors.affiliation: Department of Experimental and Clinical Medicine, University of Catanzaro Magna Graecia, 88100 Catanzaro, Italy.
authors.forename: Carmen
authors.initials: C
authors.lastname: Spatuzza
authors.forename: Marco
authors.initials: M
authors.lastname: Schiavone
authors.forename: Emanuela
authors.initials: E
authors.lastname: Di Salle
authors.forename: Elzbieta
authors.initials: E
authors.lastname: Janda
authors.forename: Marco
authors.initials: M
authors.lastname: Sardiello
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Fiume
authors.forename: Olga
authors.initials: O
authors.lastname: Fierro
authors.forename: Marco
authors.initials: M
authors.lastname: Simonetta
authors.forename: Notis
authors.initials: N
authors.lastname: Argiriou
authors.forename: Raffaella
authors.initials: R
authors.lastname: Faraonio
authors.forename: Rosanna
authors.initials: R
authors.lastname: Capparelli
authors.forename: Ileana
authors.initials: I
authors.lastname: Quinto
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Scala
mesh_terms: D048868:Adaptor Proteins, Signal Transducing; D000818:Animals; D002352:Carrier Proteins / Q000032:analysis / Q000235:genetics* / Q000378:metabolism; D002460:Cell Line; D019295:Computational Biology; D019143:Evolution, Molecular; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins; D011401:Promoter Regions, Genetic; D020033:Protein Isoforms / Q000032:analysis / Q000235:genetics / Q000378:metabolism; D017434:Protein Structure, Tertiary; D012326:RNA Splicing; D012333:RNA, Messenger / Q000032:analysis / Q000737:chemistry
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D002352:Carrier Proteins; C437966:IBtk protein, human; D047908:Intracellular Signaling Peptides and Proteins; D020033:Protein Isoforms; D012333:RNA, Messenger