Pt Mattsson

Redistribution of Bruton's tyrosine kinase by activation of phosphatidylinositol 3-kinase and Rho-family GTPases.

Brutons tyrosine kinase (Btk) is a member of the Tec family of protein tyrosine kinases (PTK) characterized by an N‐terminal pleckstrin homology domain (PH) thought to directly interact with phosphoinositides. We report here that wild‐type (wt) and also a gain‐of‐function mutant of Btk are redistributed following a wide range of receptor‐mediated stimuli through phosphatidylinositol 3‐kinase (PI 3‐K) activation. Employing chimeric Btk with green fluorescent protein in transient transfections resulted in Btk translocation to the cytoplasmic membrane of live cells through various forms of upstream PI 3‐K activation. The redistribution was blocked by pharmacological and biological inhibitors of PI 3‐K. A gain‐of‐function mutant of Btk was found to be a potent inducer of lamellipodia and / or membrane ruffle formation. In the presence of constitutively active forms of Rac1 and Cdc42, Btk is co‐localized with actin in these regions. Formation of the membrane structures was blocked by the dominant negative form of N17‐Rac1. Therefore, Btk forms a link between a vast number of cell surface receptors activating PI 3‐K and certain members of the Rho‐family of small GTPases. In the chicken B cell line, DT40, cells lacking Btk differed from wt cells in the actin pattern and showed decreased capacity to form aggregates, further suggesting that cytoskeletal regulation mediated by Btk may be of physiological relevance.

Conformation of full‐length Bruton tyrosine kinase (Btk) from synchrotron X‐ray solution scattering

Brutonss tyrosine kinase (Btk) is a non‐receptor protein tyrosine kinase (nrPTK) essential for the development of B lymphocytes in humans and mice. Like Src and Abl PTKs, Btk contains a conserved cassette formed by SH3, SH2 and protein kinase domains, but differs from them by the presence of an N‐terminal PH domain and the Tec homology region. The domain structure of Btk was analysed using X‐ray synchrotron radiation scattering in solution. Low resolution shapes of the full‐length protein and several deletion mutants determined ab initio from the scattering data indicated a linear arrangement of domains. This arrangement was further confirmed by rigid body modelling using known high resolution structures of individual domains. The final model of Btk displays an extended conformation with no, or little, inter‐domain interactions. In agreement with these results, deletion of non‐catalytic domains failed to enhance the activity of Btk. Taken together, our results indicate that, contrary to Src and Abl, Btk might not require an assembled conformation for the regulation of its activity.

Six X-linked agammaglobulinemia-causing missense mutations in the src homology 2 domain of Bruton's tyrosine kinase: Phosphotyrosine-binding and circular dichroism analysis

Src homology 2 (SH2) domains recognize phosphotyrosine (pY)-containing sequences and thereby mediate their association to ligands. Bruton’s tyrosine kinase (Btk) is a cytoplasmic protein tyrosine kinase, in which mutations cause a hereditary immunodeficiency disease, X-linked agammaglobulinemia (XLA). Mutations have been found in all Btk domains, including SH2. We have analyzed the structural and functional effects of six disease-related amino acid substitutions in the SH2 domain: G302E, R307G, Y334S, L358F, Y361C, and H362Q. Also, we present a novel Btk SH2 missense mutation, H362R, leading to classical XLA. Based on circular dichroism analysis, the conformation of five of the XLA mutants studied differs from the native Btk SH2 domain, while mutant R307G is structurally identical. The binding of XLA mutation-containing SH2 domains to pY-Sepharose was reduced, varying between 1 and 13% of that for the native SH2 domain. The solubility of all the mutated proteins was remarkably reduced. SH2 domain mutations were divided into three categories: 1) Functional mutations, which affect residues presumably participating directly in pY binding (R307G); 2) structural mutations that, via conformational change, not only impair pY binding, but severely derange the structure of the SH2 domain and possibly interfere with the overall conformation of the Btk molecule (G302E, Y334S, L358F, and H362Q); and 3) structural-functional mutations, which contain features from both categories above (Y361C).

Identification of phosphorylation sites within the SH3 domains of Tec family tyrosine kinases

Tec family protein tyrosine kinases (TFKs) play a central role in hematopoietic cellular signaling. Initial activation takes place through specific tyrosine phosphorylation situated in the activation loop. Further activation occurs within the SH3 domain via a transphosphorylation mechanism, which for Brutons tyrosine kinase (Btk) affects tyrosine 223. We found that TFKs phosphorylate preferentially their own SH3 domains, but differentially phosphorylate other member family SH3 domains, whereas non-related SH3 domains are not phosphorylated. We demonstrate that SH3 domains are good and reliable substrates. We observe that transphosphorylation is selective not only for SH3 domains, but also for dual SH3SH2 domains. However, the dual domain is phosphorylated more effectively. The major phosphorylation sites were identified as conserved tyrosines, for Itk Y180 and for Bmx Y215, both sites being homologous to the Y223 site in Btk. There is, however, one exception because the Tec-SH3 domain is phosphorylated at a non-homologous site, nevertheless a conserved tyrosine, Y206. Consistent with these findings, the 3D structures for SH3 domains point out that these phosphorylated tyrosines are located on the ligand-binding surface. Because a number of Tec family kinases are coexpressed in cells, it is possible that they could regulate the activity of each other through transphosphorylation.

Missense mutations affecting a conserved cysteine pair in the TH domain of Btk

Tec family protein tyrosine kinases have in their N‐terminus two domains. The PH domain is followed by Tec homology (TH) domain, which consists of two motifs. The first pattern, Btk motif, is also present in some Ras GAP molecules. C‐terminal half of the TH domain, a proline‐rich region, has been shown to bind to SH3 domains. Mutations in Brutons tyrosine kinase (Btk) belonging to the Tec family cause X‐linked agammaglobulinemia (XLA) due to developmental arrest of B cells. Here we present the first missense mutations in the TH domain. The substitutions affect a conserved pair of cysteines, residues 154 and 155, involved in Zn2+ binding and thereby the mutations alter protein folding and stability.

BTK, the tyrosine kinase affected in X-linked agammaglobulinemia.

X-linked agammaglobulinemia (XLA) is a heritable immunodeficiency disorder that is caused by a differentiation block leading to almost complete absence of B lymphocytes and plasma cells. The affected protein is a cytoplasmic protein tyrosine kinase, Brutons agammaglobulinemia tyrosine kinase (Btk). Btk along with Tec, Itk and Bmx belong to a distinct family of protein kinases. These proteins contain five regions; PH, TH, SH3, SH2 and kinase domains. Mutations causing XLA may affect any of these domains. About 200 unique mutations have been identified and are collected in a mutation database, BTKbase. Here, we describle, the structure, function, and interactions of the affected signaling molecules in atomic detail.