Pleckstrin Homology Domains of Tec Family Protein Kinases Michael P. Okoh* , † and Mauno Vihinen† *Institue of Medical Technology, University of Tampere, P.O. Box 607, FIN-33101 Tampere, Finland; and †Department of Biosciences, Division of Biochemistry, University of Helsinki, P.O. Box 56, FIN-00014 Helsinki, Finland Received August 17, 1999 Pleckstrin homology (PH) domains have been shown to be involved in different interactions, including binding to inositol compounds, protein kinase C iso- forms, and heterotrimeric G proteins. In some cases, the most important function of PH domains is tran- sient localisation of proteins to membranes, where they can interact with their partners. Tec family pro- tein tyrosine kinases contain a PH domain. In Btk, also PH domain mutations lead into an immunodeficiency, X-linked agammaglobulinemia (XLA). A new disease- causing mutation was identified in the PH domain. The structures for the PH domains of Bmx, Itk, and Tec were modelled based on Btk structure. The do- mains seem to have similar scaffolding and electro- static polarisation but to have some differences in the binding regions. The models provide new insight into the specificity, function, and regulation of Tec family kinases. © 1999 Academic Press Pleckstrin homology (PH) domain was initially dis- covered as an internal repeat in pleckstrin and later found from more than 150 different proteins having diverse cellular signalling and cytoskeletal functions, including protein kinases, both protein tyrosine ki- nases (PTKs) and protein serine/threonine kinases (PSKs), and their substrates, phospholipase C (PLC) isoforms, GTPases, GTPase activating proteins, gua- nine nucleotide releasing factors, and adaptor proteins (1–3). The function of the PH domain is still somewhat unclear, but in many cases it acts as a membrane localisation domain. Certain signalling molecules such as Src family PTKs are constitutively membrane- bound by N-terminal myristylation (4). The N-terminal half of at least certain PH domains binds inositol com- pounds thereby being possibly important for mem- brane localisation for proteins that have to be at least transiently close to membrane (5–9). Many PH domains have been shown to interact with subunits of heterotrimeric G proteins (10 –16). Certain PH domains can bind also to protein kinase C (PKC) isoforms (17–20). Btk PH domain interacts also with BAP-135 protein of unknown function (21). The three dimensional structure has been deter- mined for a number of PH domains including Btk (22, 23), dynamin (24 –27), PLC  (8), pleckstrin (28), SOS (29, 30), spectrin (7, 31), and -ARK1 (32). Although PH domains share very limited sequence identity, they have the same fold consisting of a -barrel formed of two -sheets and a C-terminal -helix that caps one end of the -barrel. The Tec family of cytoplasmic PTKs is formed of Btk (33, 34), Itk/Tsk (35–37), Tec (38), and Bmx (39). In addition to the Src homology 2 (SH2) and SH3 and the kinase domain, Tec group kinases have in their N terminus a PH domain followed by a Tec homology (TH) domain (40 – 42). The Tec family is thus far the only PTK family known to contain a PH domain. TH domain contains two parts. N terminal Btk motif is followed by a proline rich region (41, 42). The Btk motif contains HC 3 pattern (one histidine and three cys- teines) which binds stabilising Zn 2+ ion (22, 42). The proline rich region in Btk binds to certain SH3 domains (43– 45), possibly also intramolecularly to the adjacent SH3 domain (Okoh and Vihinen, in preparation) as in Itk (46), thereby regulating the function of these kinases. Btk is crucial for B cell development and mutations lead into an immunodeficiency, X-linked agammaglob- ulinemia (XLA) (33, 34). XLA is the best known signal transduction related disease with over 300 different mutations (47). Btk is expressed in hematopoietic lin- eage, except for T cells and plasma cells (40). IL-2 inducible T-cell kinase (Itk) (35) is expressed in thy- mus, NK cells and T cells. Tec appears in several alternatively spliced forms in which the termini are differently processed. It is expressed in T and myeloid lineages. Bmx has wider expression pattern including endothelial cells. These proteins are involved in many signalling events, but the signalling pathways are still largely unknown. Biochemical and Biophysical Research Communications 265, 151–157 (1999) Article ID bbrc.1999.1407, available online at http://www.idealibrary.com on 151 0006-291X/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.